[ringoism]

Originally Posted by Sutripta

Mine was also selfmade.

You do know how to trim jets?

I thought you said you were having problems even in Manali ~ 7000 ft.

On a single cam engine, the valve overlap remains the same. Whether the engine runs better, worse, or about the same depends on too many factors. Can't be predicted. Truth only after instrumented testing! (On Marutis, one tooth out, and the engine would run badly!)

More later, if you're keen.

It's been very interesting interacting with you on all this. Always keen to discuss tech matters and glean from others' experiences. A bit OT, but you must have a fascinating history in the automotive realm. Is that outlined elsewhere in Tbhp? You certainly have acquired an impressive wealth of worldwide automotive / technical knowledge - as one more example, I doubt there are many others in the subcontinent (or even the esteemed Tbhp community) with whom I could discuss a Holley carburetor! Okay, it's "my" thread so I can hijack it: To answer your earlier question, I had a Holley on only one of my cars, (650cfm spread-bore - ran GREAT!) but sold it after just a little bit of experience with a tuning kit (various accelerator pump cams and "shooters", power valves, jets, etc) - a very "configurable" and high-performance carb (that also used a lot of fuel when driven hard, on account of its twin accelerator pumps). In those days there was no Internet, and info on theory was a little hard to come by - so I can't say I understood the fuel circuit(s) much at all.

Most of my tuning experience was with the Rochester Quadrajet (a mostly unloved GM exclusive), which while completely different in physical design from CV's and SU's, would seem to be similar in some ways re: their secondary barrel side, which had butterflys AND vacuum-actuated top plates that actually controlled (dare I say metered?) flow through the bores. The possible setups / combinations / applications were almost endless. Q-jets, like the SU's, were largely misunderstood carbs, but I managed to get a number of them set up nicely for both high-performance and economy-type applications (V8's from 4.5-7.5 litres)- had a full range of jets / metering rods (tapered needles moving up and down with the vacuum-actuated opening of the secondary flaps, kind of like CV's) & their hangers, accelerator pump actuators, etc, etc, sourced from - of all places - the local scrapyard!!! I don't doubt that people have observed large enough differences in "identical" uncalibrated jets to produce noticably differing plug colors - just never experienced it myself - so truly I guess I'm either color-blind, "lucky", or... well we'll get to that later...). Fact is, these were great-running cars. A street-race against a friend's 429-powered 1970 Mustang is extremely memorable - in that I beat him soundly with a 350-powered Oldsmobile that probably weighed a lot more and had probably a third of the money invested...!

Re: the slide carb vs. CV/SU thing: Would be helpful if you could refer me to some specific good website or other source such as you indicated would clarify their specific operational differences, especially as it regards the slide. I'd like to be finally straightened out on this, if indeed it's not just semantics (For one thing, I'm wondering why the needle on a typical slide carb generally has five adjustment slots, where a CV's only has three - or even none). Anyway, everything I found (whether SU/CV) was either non-explicit with respect to slide / butterfly functional definitions, or in a couple cases made statements similar to the one quoted from Wiki, which you refute. Might be a misconception (that slide functions are basically the same between the two apart from actuation), but seems to be held even amongst certain of the "experts" (admittedly, "ex"= a has-been, and "spurt" = a drip under pressure"!!). One very practical idea that might settle it would be to wire a CV's butterfly wide open, attach a throttle cable through the diaphragm cover directly to the slide (first blocking the vacuum port, of course), and see whether it might indeed function pretty much just like a regular slide carb! I don't have time for this, but imagine it could be rather fun. Anyway... won't go further on that.

Re: the camshaft, right - overlap cannot possibly change. I understand that less overlap would increase pressures and more would decrease it - which is why, for example, wildly-cammed engines (my Cutlass W31 had 108degrees of overlap, if I remember right) need high static compression ratios to run well (witness drag-racing cars running 13.5:1).

But since all valve events are moved forward when a cam is advanced, the intake valve is also opening (and closing) earlier, which (according to high-performance aftermarket cam manufacturers themselves, on the topic of advancing/retarding cams with fixed profiles) typically does have the effect of increasing cylinder pressure (at least at lower rpm's?). It's also pretty well established that dynamic cylinder pressure decreases with altitude (reflected in the fact that you'll find it in all the formulas / online calculators). You're absolutely right in saying there are many variables, so I'm not suggesting that the result would be totally predictable - just that it would seem a technically sound and simple attempt towards compensating for the lower air pressure here in the mountains - or even towards simply lowering the peak torque rpm, which could also be helpful in the high ranges (UNLESS, of course, that's what HERO already did when they created this "high torque" version of the 150 - the advertised torque peak is at least 500rpm below that of the other Hero/Honda bikes, @4,500 - need to compare some OE cam gears, I guess!).

BTW, for anyone else thinking about doing this mod, a single tooth would probably be far too much advance increase with most engines. Something in the region of 4-6 degrees seems to be generally cited in efforts towards raising the cylinder pressure (for either high altitude / lower static-compression-ratio engine setups - both of which I'm dealing with here!) and lowering the torque peak rpm. I don't know how many teeth the Hero (or Maruti) cam sprockets have, but even there were as many as 36 (think it might be less than that), one tooth would equal ten degrees. Besides running badly, you might run into intake valve / piston clearance issues there and end up with serious damage. Usually offset keys or else multiple keyways cut at various alternative degrees advance/retard in the crankshaft drivegear are utilized. Don't know if the Honda engines have a separate key at all (Pulsars don't, as I recall), so I have to look into my options once I get inside it. I don't feel that modifying the cam gear would be difficult - just grind out the existing stamped lock, and weld/braze in a little material elsewhere that can be filed to the same shape, but at a few degrees advanced, with a new corresponding timing mark.

Sorry for any confusion re: "problems at Manali" - that was a little vague on my part. To be clearer, a Hero technician friend noted symptoms (missing) on his own Impulse just a few km north of here at Solang (around 8,000?), and I noted the same on an uphill link road a few km south of Manali, of unknown altitude, but well above that of the main market once I got halfway up and noted the missing. Anyway, it is a question of the severity of the problem. Our local experience here around Manali was perhaps not unlike your own at similar 7,000-8,000ft altitudes - I could still pull uphill all right, despite the sooty plug. The missing was apparent at certain throttle positions/rpm's, but disappeared at others. More than an actual "problem", it was the troubling awareness that: 1) I'd never before seen actual tuning-related missing on any other new/well-maintained bike or car in the Manali vicinity, and 2) that if I was seeing this here, WAY below what we were going to have to climb en route to Leh, symptoms were likely to be very severe higher up - an accurate prediction, as it turned out.

If possible, I'd like to try and clarify some personal objectives here - and I think these would reflect what a number of other Impulse owners are after: The difficulty is that on the Manali-Leh ride, we're dealing with altitude variations of more than 10,000ft (It's funny to get on forum threads from abroad and have people complaining about poor performance at a mere 3,000ft!!!). Anyway, under Manali-Leh conditions, no matter how a vehicle is jetted, tuning (A/F ratio) is only going to be SPOT-ON at some particular sections along the way that are at a correspondingly ideal altitude for that setup. Tune it for perfection at the More Plains, and it'll be lean and running a little hot up the Manali side of Rohtang. Etc. So like with most other things in the motor vehicle realm (and life!), there some compromises involved. The general goal for me is to get something that runs acceptably okay most of the time between 10,000-17,500ft, which covers me from half way up Rohtang to Thanglang-La. I can re-jet when I get home, but don't want to have to do it at various points along the way. Now, inevitably, that "happy medium" is going to put me somewhat lean at Manali and a somewhat rich at the passes. I don't mind if my plug is sooty up there and a little light here. What I do NOT want is for the bike to be misfiring so badly up high that I'm spending all my concentration just trying to keep it moving, or overheating noticeably enough down low to risk internal engine damage.

If I was on a racing circuit (or drag strip) at fixed altitude, there might be a much greater call for precision - so yes, trimming jets (think I read something about that way back but have never done it) and testing them with a flow comparator and/or buying tested/certified units, etc, etc might be in order (though a lot of weekend amateur racers have done fine just hand-drilling theirs as I have) - because the closer I am to absolute perfection, the more likely it is that I can win a race (I would still have things like humidity / temperature variations to throw it off as much or more than an un-trimmed jet might, but anyway, I agree that eliminating variables is a good thing).

I feel it's a little different situation here with Himalayan riding, where IMO we're just trying to have a basically enjoyable riding experience over these several hundred dramatically variable kilometers. If my deshi jet size reads 102 and matches my 102 drill, but it's flowing more like a calibrated 100.5 or 103.5 should, that's not really a problem for me, because it's clearly still going to be flowing a lot less than the 110 that was in there earlier. Maybe the tuning will now be "perfect" at 12,000ft instead of the 12,500 or 11,500 it technically should be at. Whatever. Whether my #102 is a crude hand-drilled junkyard unit or a Swiss-made masterpiece, neither is going to be "right" at either 10,000 or 17,000ft. It'll be "perfect" somewhere between 11,500 and 12,500 maybe (depending on a LOT of other variables both known and unknown, controllable and uncontrollable) - and somewhat compromised everywhere else. So I'm looking for something approximate, not exact, because exact is usually pretty hard (and usually a lot more expensive) to come by, and never worth it if the situation doesn't actually demand it. This general approach / mindset has worked acceptably well pretty consistently in past tunings of several other machines up here. Could be luck of course... (except I don't believe in luck ... so it must be those "mystical" influences).

The most perplexing question here is why the Hero seems to require so much more re-tuning than other bikes. Many have done the ride on (among the list others mentioned - like the stock Pulsar 150 - also my KB100 and a couple iron Bullet 350's I rode with) without re-jetting at all and only minimal power-loss / difficulties. And these last few had slide carbs, vs. the "better compensating" CV's. Moreover, My KB125 never required more than a few sizes (say, from a 106 to a 103, or maybe a 101) to get it running in full health again (with good plug color) when I changed jets at places like Pang (and it never had the serious missing, just some power loss), where the Impulse is requiring ten or even fifteen sizes (from 132.5 to 118 or maybe 115) to get it running acceptably). All this points me to the possibility that painstakingly idealizing the Impulse's carb jetting is still going to leave me wanting. There is something that's just making it a lot more sensitive / less tolerant / less flexible to changes in environment than other, very similar (indeed, almost identical) bikes. Hence my broader search for other potential causes / solutions.

My thinking on the plug here is related to this inevitably "compromised" situation - I KNOW the bike will be running too rich at the passes if I want it to be tuned to be anything like ridable (i.e., not extremely lean) at lower altitudes. Is it possible that the Honda/Hero engine was for some reason (perhaps with the added backpressure of a catalytic converter in this case?) found to be a bit detonation-prone, and engineers decided to run a cooler plug for the hot-climate Indian market? Mmmm... probably not, because the U.S. market bikes with that engine use the same plug! Is it a restrictive intake tract? Mmmm... not likely, because I've had it all opened now and can't see anything restrictive about it (excepting the rubber grommet at the airbox intake, which following the Hero tech's lead, I'd already removed in the first couple weeks).

Still, neither the U.S. nor Indian (regrettably) marketeers were thinking about 15,000+foot passes, and a hotter plug will help burn off more of that over-rich soot up on the passes, by keeping the plug in its specified range of operating temperatures with the cooler air-fuel charge and lower cylinder pressures inevitably experienced. I will not have an ideal A/F ratio, nor maximum power, but MAYBE it will help prevent it from actually missing, which is really the killer. The NGK tech pages say a hotter plug is indicated for lower cylinder pressures and colder ambient temperatures to prevent misfire. Multiple sources confirm that (dynamic) cylinder pressures are lower at higher altitudes, and we all know how cold the passes can be. So it seems pretty reasonable. How much hotter of a plug would be required? Let's see! Just found a second-hand Pulsar unit today that's two ranges hotter. Going to clean it up and if it works, will take it along on my next ride to compare any difference. It could turn out to be a fruitless exercise, of course... but as I said, only a few rupees and minutes of my time.

I will digress here in conclusion:
In working through this problem in the context of the forum, it's also got me thinking a bit about humankind...

Sutripta, your inputs could ultimately help lead to the final solution(s) here, and I'm eager to glean any additional insights / suggestions that you can share. So please bear with me in my being more of a "seat of the pants" tinkerer than a rocket scientist. I do have a (mechanical) engineering degree and worked in the automotive field for some years (apart from lifelong hobbyist pursuits); At one point did quite a bit with bringing in the initial ISO certifications for our company, so was quite familiar with calibrations, documentations, standardization and repeatability of processes, etc - also did test equipment design / r&d for a few years; so can fully appreciate any considerations along these lines of calculation / precision / documentation / instrumented testing, etc.

But not spouting qualifications here (believe me), just sharing my personal path (my downgrade from engineer to tinkerer, as it were) and THIS is where, for the time being, I've ended up: In my own (limited) history, I found that in so many cases I've (also "we've", re: the engineering dept back then) faced, the innumerable number of variables involved (quite often unseen / unanticipated) often made efforts at calculating / measuring / predicting with precision considerably more difficult and time consuming than simply (utilizing a little common-sense analysis, rules-of-thumb, the wealth of others' past experiences, manufacturer / aftermarket / supplier recommendations, and sometimes sincere prayer) - engaging in a bit of educated guessing and trial and error - which can also be quite enjoyable... for me, at least

Another aspect is this: I've been amazed in my years in India to find that the best of the traditional Indian skilled labor community have developed amazing ways of getting things done "right", with almost none of the supposed benefits of the high-technology (or even advanced hand tools) that the West has been developing and spending immense amounts of money on for decades. Very impressive. One way to say it is that there's a kind of "art" to all this, a certain finesse and skill and "sixth sense" that isn't typically taught in engineering programs. Not seriously saying I possess it. But I have seen it in action and believe it can be developed. Combined with some kind of solid technical grounding, it can be powerful. And that is the strength of forums like this one.

Mind you, not suggesting jugaad will quite cut it when it comes to things like big dams/bridges or aeronautics... But I'll bet the right kind of it can get us over the Himalayas quite satisfyingly in many cases (Sincerely wish Hero would've made at least that much of an attempt in the first place, being that Impulse commercials showed riders on and off the Ladakh roads)!!!

Come to think of it, I myself could have done quite a lot of jugaad-level tinkering out there in the time it took to sit and write this in here... so I'd better stop now and close with this:


I've got a friend who will spend weeks and weeks on end working out a single complex math problem - just for fun, as a hobby; while for my part, I cannot think of a more unpleasant - and potentially dangerous - pursuit... (my head would seriously just explode). That only underscores the amazing and beautiful variety and complexity with which the Creator formed us all. It certainly "takes all kinds to make the world go 'round".

So sincerely, many thanks. Any further advice (from anyone) is truly appreciated. Please note: if I at any points resist any masochistic temptations to engage in activities that could ultimately prove threatening to my own (or others'!) life and health, I hope you'll understand that it is for the sake of my family (I've got three-year-old and nine-month-old sons and need to hold onto my limited brain functions for awhile yet) and that it will not be construed as any defiant rejection of good advice!

Sorry to go on so long here -

Warm Regards,
-Eric

[Sutripta]

Wow! That's quite a long (and interesting) post.
On TBhp we are all peers. Think of me as a layman (but not an idiot), and we'll get along just fine.

Originally Posted by ringoism

(650cfm spread-bore - ran GREAT!) but sold it after just a little bit of experience with a tuning kit (various accelerator pump cams and "shooters", power valves, jets, etc) - a very "configurable" and high-performance carb (that also used a lot of fuel when driven hard,

Nature of a spreadbore.
What intakes? Edelbrocks?

Fact is, these were great-running cars.

Slight differences in jetting in a street setup (esp if set slightly rich) will hardly register on a dyno, let alone on butt dyno. More likely to show up in behaviour when transitioning from one circuit to another (esp if set lean).

Re: the slide carb vs. CV/SU thing: Would be helpful if you could refer me to some specific good website or other source such as you indicated would clarify their specific operational differences, especially as it regards the slide. I'd like to be finally straightened out on this, if indeed it's not just semantics (For one thing, I'm wondering why the needle on a typical slide carb generally has five adjustment slots, where a CV's only has three - or even none).

I'm old fashioned (result of being no spring chicken) bricks and mortar/ library/ lab/ shop floor/ workshop kind of guy. My view: the internet is great for opinions. Not so much for (peer reviewed and generally accepted) facts. (Guess reason why $100+ textbooks still sell).
SU used to have some pamphlets.
In general there used to be fairly regular articles in the magazines/ special interest/ trade papers of the day. And not just SUs. (I remember one on Fords sonic idling circuit!)
Can't recollect if Bell's books had anything on SUs.
David Vizard (I think) had a book on tuning the SU. (He cut his teeth modifying and racing Minis before moving on to American V8s)

Anyway, everything I found (whether SU/CV) was either non-explicit with respect to slide / butterfly functional definitions, or in a couple cases made statements similar to the one quoted from Wiki, which you refute. Might be a misconception (that slide functions are basically the same between the two apart from actuation), but seems to be held even amongst certain of the "experts" (admittedly, "ex"= a has-been, and "spurt" = a drip under pressure"!!).

Could you point out exactly which Wiki article. Would be interesting going through its edit history.

Would you call the mass flow sensor of the K-Jetronic and L-Jetronic systems an air metering devices? (Come to think of it, the SU carb itself can act as a crude mass flow sensor!)

One very practical idea that might settle it would be to wire a CV's butterfly wide open, attach a throttle cable through the diaphragm cover directly to the slide (first blocking the vacuum port, of course), and see whether it might indeed function pretty much just like a regular slide carb! I don't have time for this, but imagine it could be rather fun. Anyway... won't go further on that.

Yes, it will work as a normal slide carb. Not very different from the Amals and Bings of yore. You will need a stronger return spring. And play around with the needle profile.
But if you have to ask that question, doubt it will settle any arguments!

Re: the camshaft, right - overlap cannot possibly change. I understand that less overlap would increase pressures and more would
...
I don't feel that modifying the cam gear would be difficult - just grind out the existing stamped lock, and weld/braze in a little material elsewhere that can be filed to the same shape, but at a few degrees advanced, with a new corresponding timing mark.

We have a long discussion ahead. Cams after carbs shall we say. Basically cam timing is a function of the torque curve wanted. Interacts intimately with runner and header designs.

Quite a few tuners on our forum. Some use engine simulation software. Essentially a question for them.

Sorry for any confusion re: "problems at Manali" - that was a little vague on my part.

OK.

The rest of your post touches on quite a few points. And some of the most important, in my view, are not technical. And should make for interesting reading. (That is what the internet is for - free flow of opinions!) Before jumping in, if it is OK with you, would like to wait for a couple of days to see if others have anything to add.

Regards
Sutripta

Just noticed I've been thanked a 1000 times. Now I can retire!

[ringoism]

Originally Posted by Sutripta

The rest of your post touches on quite a few points. And some of the most important, in my view, are not technical. And should make for interesting reading. (That is what the internet is for - free flow of opinions!) Before jumping in, if it is OK with you, would like to wait for a couple of days to see if others have anything to add.
Just noticed I've been thanked a 1000 times. Now I can retire!

Sounds like a good suggestion for my part, too (I should be getting some sleep at night anyway). And congrats on the 1000 - I did notice you were at 999 when I'd sent that.

Having got back to tinkering (vs. typing), will post some photo-based notes on the air-intake system, since I think I've pretty fairly analyzed it (and can't see potential causes there for Impulse-ive misbehavior).

-Eric










Don't actually expect that this will help things much at altitude... unless the stock paper element was just WAY more restrictive than it appeared it should be (the missing was there when the filter was brand new, incidentally). It was mainly my way of eliminating that unknown (since some claimed the trouble had mainly stopped when the filter was removed entirely) - and regularly saving Rs170 (?) on replacement Hero filters.

[ringoism]

Briefly re: plugs:

Design-wise, plugs made to run hotter either have more projection of the center electrode (placing it further into the combustion chamber and further away from the "heat sink" of the body of the plug & cylinder head, etc), OR a thinner ceramic surrounding it (again, less mass, so heats up faster and transfers less heat away from the electrode), or BOTH. Plugs designed to run cooler, exact opposite. Read the descriptions here from RIGHT TO LEFT (sorry) if you want them to make any sense:



Can't remember where I found the chart below. May or may not be totally accurate in translating the heat range numbers between brands, but probably within one number or so. Parts interchange lists seem pretty approximate and variable.

Attachment 1288213

I'm trying to order an NGK CPR6EA9 (heat number two ranges hotter than stock) from Delhi, just to be sure. Meanwhile, with plans for travel to high passes firming up towards months' end, have scavenged various other second-hand units of various numbers from here and there.

The Champion, which according to some comparison lists would seem the coolest, definitely ran worse than the stock NGK plug on the Ladakh trip back in July (note its blackness - it was missing badly). The Bosch UR6DC should be the hottest, and I'll compare it to the stock plug on this upcoming trip if the new and hotter NGK isn't in my hands by then.

On another line, a mechanic I know told me today that he knows the Impulse's problem to be the air filter. Despite earlier such assertions, that had always seemed hard for me to believe, but ripped my old one apart when I got home for more analysis. Huge surface area, of course, on account of its folded form - but almost impossible to blow through any part of that paper (even the clean parts!). Very tight / restrictive. My bike seems to have had some lean-surge develop after adding my foam element. If so, it points to the original element choking the system (i.e., being freer now, more air comes in, and the engine is asking for correspondingly more fuel than what the current 118 jet is providing it).

Remembered another detail from the Ladakh ride, which is that when the missing was worst, if I'd shut the bike down for a minute and let it cool, it would run considerably better on re-start, till it was run under load a couple minutes again, whereupon the misfiring would again start (not the fuel filter / float level here [classic symptoms], if that's what you're thinking, because that would've manifested at low altitudes, too).

Gets me suspecting that catalytic converter and/or small diameter exhaust. A blocked exhaust pipe causes rich running that has to be compensated for with smaller jets. Catalytic converters are restrictive, but more than that, they run hot. If I remember right, flow of hot gases through a pipe is slower than that of cool gases.

Could it be that as the cat / pipe heat up on those long uphill stretches, the exhaust backpressure increases, mixture richens, and a need for leaner jetting manifests itself? And that when it cools off again, jetting seems temporarily closer to where it should be?

Also noticed that the Impulse's stock exhaust pipe header is considerably smaller than that of the CBZ/Unicorn/etc. Don't know why it would've been designed that way, but it's certainly more restrictive than it could be. Aftermarket pipes for the same-engined CRF150F are 1-1/8" (29mm) diameter in the header section and 1-3/8" further back (U.S. market, no metric here). This Impulse's header is more like 3/4" (19mm).

More later,
-Eric

[Sutripta]

Originally Posted by Sutripta

The rest of your post touches on quite a few points. And some of the most important, in my view, are not technical. And should make for interesting reading. (That is what the internet is for - free flow of opinions!) Before jumping in, if it is OK with you, would like to wait for a couple of days to see if others have anything to add.

Looks like it is going to be a dialogue, rather than a group discussion! So here goes.

Originally Posted by ringoism

If possible, I'd like to try and clarify some personal objectives here - and I think these would reflect what a number of other Impulse owners are after: The difficulty is that on the Manali-Leh ride, we're dealing with altitude variations of more than 10,000ft (It's funny to get on forum threads from abroad and have people complaining about poor performance at a mere 3,000ft!!!). Anyway, under Manali-Leh conditions, no matter how a vehicle is jetted, tuning (A/F ratio) is only going to be SPOT-ON at some particular sections along the way that are at a correspondingly ideal altitude for that setup. Tune it for perfection at the More Plains, and it'll be lean and running a little hot up the Manali side of Rohtang. Etc. So like with most other things in the motor vehicle realm (and life!), there some compromises involved. The general goal for me is to get something that runs acceptably okay most of the time between 10,000-17,500ft, which covers me from half way up Rohtang to Thanglang-La. I can re-jet when I get home, but don't want to have to do it at various points along the way. Now, inevitably, that "happy medium" is going to put me somewhat lean at Manali and a somewhat rich at the passes. I don't mind if my plug is sooty up there and a little light here. What I do NOT want is for the bike to be misfiring so badly up high that I'm spending all my concentration just trying to keep it moving, or overheating noticeably enough down low to risk internal engine damage.

If I was on a racing circuit (or drag strip) at fixed altitude, there might be a much greater call for precision - so yes, trimming jets (think I read something about that way back but have never done it) and testing them with a flow comparator and/or buying tested/certified units, etc, etc might be in order (though a lot of weekend amateur racers have done fine just hand-drilling theirs as I have) - because the closer I am to absolute perfection, the more likely it is that I can win a race (I would still have things like humidity / temperature variations to throw it off as much or more than an un-trimmed jet might, but anyway, I agree that eliminating variables is a good thing).

Agreed with your goal. And general statement. Not with the methodology.
If someone has worked out (accurately) the setup needed for a particular application (altitude in this case) then some deviation (production tolerance) is to be expected, allowed, and not a big deal.
But when working out the best setup (which is what you are doing, being the first) one needs much more.
Say you are trying to zero in to the best setup (converge to a solution). And you decide you need a slightly leaner jet. But the jet you fit flows more than the one you are replacing. (Which is a distinct possibility.) So what do you do then. After going around in circles, you decide that something is 'good enough'.

I feel it's a little different situation here with Himalayan riding, where IMO we're just trying to have a basically enjoyable riding experience over these several hundred dramatically variable kilometers. If my deshi jet size reads 102 and matches my 102 drill, but it's flowing more like a calibrated 100.5 or 103.5 should, that's not really a problem for me, because it's clearly still going to be flowing a lot less than the 110 that was in there earlier. Maybe the tuning will now be "perfect" at 12,000ft instead of the 12,500 or 11,500 it technically should be at. Whatever. Whether my #102 is a crude hand-drilled junkyard unit or a Swiss-made masterpiece, neither is going to be "right" at either 10,000 or 17,000ft. It'll be "perfect" somewhere between 11,500 and 12,500 maybe (depending on a LOT of other variables both known and unknown, controllable and uncontrollable) - and somewhat compromised everywhere else. So I'm looking for something approximate, not exact, because exact is usually pretty hard (and usually a lot more expensive) to come by, and never worth it if the situation doesn't actually demand it. This general approach / mindset has worked acceptably well pretty consistently in past tunings of several other machines up here. Could be luck of course... (except I don't believe in luck ... so it must be those "mystical" influences).

Your views on the dimensional accuracy (we'll ignore surface finish for now) of making holes in metal by the common processes like punching, drilling, boring, reaming, broaching, honing.
(BTW, a Swiss lathe is a standard high volume production machinery)

The most perplexing question here is why the Hero seems to require so much more re-tuning than other bikes.
...
There is something that's just making it a lot more sensitive / less tolerant / less flexible to changes in environment than other, very similar (indeed, almost identical) bikes. Hence my broader search for other potential causes / solutions. [/b]

As I said, I'm intrigued.

I will digress here in conclusion:
In working through this problem in the context of the forum, it's also got me thinking a bit about humankind...

And mine was thoughts on good enough/ juggad. And of our readiness to push everything into the 'accept things which I cant change'!

I do have a (mechanical) engineering degree and worked in the automotive field for some years (apart from lifelong hobbyist pursuits); At one point did quite a bit with bringing in the initial ISO certifications for our company, so was quite familiar with calibrations, documentations, standardization and repeatability of processes, etc - also did test equipment design / r&d for a few years; so can fully appreciate any considerations along these lines of calculation / precision / documentation / instrumented testing, etc.

Good, no make that great to know that TBHP + US does not necessarily mean a laptop slinger!

But not spouting qualifications here (believe me), just sharing my personal path (my downgrade from engineer to tinkerer, as it were) and THIS is where, for the time being, I've ended up: In my own (limited) history, I found that in so many cases I've (also "we've", re: the engineering dept back then) faced, the innumerable number of variables involved (quite often unseen / unanticipated) often made efforts at calculating / measuring / predicting with precision considerably more difficult and time consuming than simply (utilizing a little common-sense analysis, rules-of-thumb, the wealth of others' past experiences, manufacturer / aftermarket / supplier recommendations, and sometimes sincere prayer) - engaging in a bit of educated guessing and trial and error - which can also be quite enjoyable... for me, at least

Discussions over a cup of coffee. Someday. Maybe.

Another aspect is this: I've been amazed in my years in India to find that the best of the traditional Indian skilled labor community have developed amazing ways of getting things done "right", with almost none of the supposed benefits of the high-technology (or even advanced hand tools) that the West has been developing and spending immense amounts of money on for decades. Very impressive. [b] One way to say it is that there's a kind of "art" to all this, a certain finesse and skill and "sixth sense" that isn't typically taught in engineering programs.

Feel pretty strongly about this. Unfortunately, I'm on the other side.

Re: your post on plugs, do look up 'projected nose plugs'.

Regards
Sutripta

[Ajaybiz]

Very nicely captured details Ringoism, very few are into so much details when it comes to carb tuning specially for 4 stroke bikes, you ignited my hidden will to get one impulse even though I am almost out of small two wheels now.

[ringoism]

Originally Posted by RishikeshK

I met one gentleman...in Leh who rode to Leh from Manali side on an Impulse...air-filter removed... it solved pretty much all the power issues. He mentioned that the bike was very smooth and could do 100kmph with ease.

This is (surprisingly, to me) seeming much more believable after the last (air cleaner) mod I documented... with the foam element, went from the now lean 118main jet I'd installed for Ladakh, back up to a 125, and it's still lean enough (too much air, vs. fuel) to surge beyond 3/4 throttle (which is primarily where the main jet comes into play), and plug shows absolutely white. Going to have to go back to the stock 132 jet and see what happens. Would be terrible if the Impulse failed in the mountains only on account of having the wrong air filter for the application. But maybe not quite that simple:

Originally Posted by vishwaschettri

I took my unicorn (same 150cc engine as your Impulse) quite a few times to leh all stock with pillion and luggage and it used to match bullets on the moore planes....came tuned pretty lean from the factory with a 115 mainjet for fuel efficiency. I had added an oiled K&N filter to make it lean further for the passes.

Now here's something quite fascinating I discovered today when I finally opened up a Unicorn for inspection: the engine is not actually quite the same, nor will jet sizing translate, and here's why: The Uni utilizes a much smaller bore 25mm Keihin (CV) carburetor, vs. the Impulse's 28mm (CV) Keihin. Odd, being that the power output is within 0.1bhp. Impulse has more torque (peaks at a lower rpm, too), but that's something more often associated with smaller carbs, not bigger ones. Looking around, it seems 28mm is quite oversized for a 150cc. Newer Pulsar 150 DTSi has a 26mm, even while making more bhp than the Impulse. As mentioned earlier, the Honda CRF150F's (roughly same bhp) have 24mm's (non-CV), and the 28mm is recommended only for the 225cc big-bore kit (BBR Motorsports). Heck, even 350 Bullets were using 28mm (round-slide conventional) carbs. We might finally be onto something major here. One drag-racing source suggested that on high-altitude tracks, racers tended to run smaller carbs, to keep air velocities up and compensate (somehow or the other) for the lower atmospheric pressure. If so, then whoever spec'd out the Impulse went the wrong way, if they had Himalayan touring in mind. So here's an idea: The 28mm flows TOO MUCH, creating too little vacuum in thin air. Too little vacuum on a CV carb means the slide won't raise properly. Slide doesn't raise properly, air/fuel flow doesn't increase proportionate to throttle opening. Main jet only comes into play when slide is nearly raised to the top - so since slide is simply not reaching the top, whichever smaller main (high-speed) jet was installed is only of limited value. Practically speaking, on the outbound journey (before the needle mod) I had to slide the clutch and rev the thing like mad to build rpm's and presumably get the slide up - at which point it actually would finally sort of find a sweet spot and fly very robustly up the passes. As I said in the first post on this topic, the power was "there" - it just wasn't easily accessible.

I'm going to see if I can scrounge a junk Unicorn carb locally and try it out. It's clear enough that you don't need a 28mm carb to make 13.4bhp - or even 14+bhp, as a number of other bikes do that with much smaller carbs. Think the P180/200's used a 28 or 29mm. Hmmmm.... Could also be part of the reason for the lower FE of the Impulse vs. most of the "same engined" 150cc HH/Honda models.

Originally Posted by Ajaybiz

I am almost out of small two wheels now.

If I could find the spares and afford to feed them, I'd be running something like one of your former RD's... wonderful bikes. Almost bought one more than once over the years (they were famous in my college days - two guys had beautiful RD400's, someone else ran a heavily modified RD250 engine (ported, big carbs, alcohol) on a racing go-kart!). Before they became classics here in India, they could be had for as little as Rs5,000 (saw one such in Vellore around 2004). Still was a forlorn-looking unit sitting unused next to an old church in Calcutta, last time I was there two years back. Lady shop-owner in Karol Bagh, Delhi sold fully (and beautifully) restored units for 40 grand less than a decade ago... now up to 1.5lakhs... Seriously considered an offroad-modified RX135 in lieu of a new Impulse. And now seriously suspecting, in light of all the time I'm spending trying to make the Impulse work, that it would've been a much better choice.

Originally Posted by Sutripta

Discussions over a cup of coffee. Someday. Maybe. Re: your post on plugs, do look up 'projected nose plugs'.

Actually, the "P" in the original NGK plug's alpha-numeric code (CPR8-EA9) indicates a projected insulator. Unless the topic had temporarily changed to swim accessories ("nose plugs" ), I'm assuming we're talking about the same thing. At any rate, local mechanics here seem to be pretty arbitrary about what sparkplugs they install, so long as the thread is right. I'm sure that none of them know anything about heat ranges or other construction features (projection, etc). Seems projection would start the ignition marginally further into the CC, otherwise can't see any difference. Have any info/experience on this?

And any personal thoughts on the effects of plug heat ranges, or the carb sizing issue above, as it relates to high-altitude conditions / problems?

Might be in Kolkata this winter, and if so a chat over coffee sounds absolutely great, if you're up for it.

Thanks, and more later,
Eric

[Sutripta]

Originally Posted by ringoism

Now here's something quite fascinating I discovered today when I finally opened up a Unicorn for inspection: [b]the engine is not actually quite the same, nor will jet sizing translate, and here's why:
...

Carbs are sized according to required power per cylinder (assuming no overlapping suction strokes).
One can normally oversize a CV without major penalty.
A list of all current (and immediate past) Indian bikes/ carbs would be an useful compilation.

Actually, the "P" in the original NGK plug's alpha-numeric code (CPR8-EA9) indicates a projected insulator. Unless the topic had temporarily changed to swim accessories ("nose plugs" ), I'm assuming we're talking about the same thing. At any rate, local mechanics here seem to be pretty arbitrary about what sparkplugs they install, so long as the thread is right. I'm sure that none of them know anything about heat ranges or other construction features (projection, etc). Seems projection would start the ignition marginally further into the CC, otherwise can't see any difference. Have any info/experience on this?

Best way of describing the characteristics of a projected nose plug would be
Single grade oil : multigrade oil :: normal plug : projected nose plug.
It is the insulator design inside the shell which determines heat range, not that which is projecting out.
Interesting titbit: See if you can lay your hands on old KLG 2 piece spark plugs.

And any personal thoughts on the effects of plug heat ranges, or the carb sizing issue above, as it relates to high-altitude conditions / problems?

Need a feel of whats going on. Otherwise only commonsense approach. (I would give the slide return spring a very hard look.)
Speaking of plugs and carb tuning, check out something called the Gunson Colorplug. (Be warned though that these were (and are) expensive, and fragile.)

Might be in Kolkata this winter, and if so a chat over coffee sounds absolutely great, if you're up for it.

My pleasure.
For Christmas, I take it?

Regards
Sutripta

[vishwaschettri]

Originally Posted by ringoism

Now here's something quite fascinating I discovered today when I finally opened up a Unicorn for inspection: the engine is not actually quite the same, nor will jet sizing translate, and here's why: The Uni utilizes a much smaller bore 25mm Keihin (CV) carburetor, vs. the Impulse's 28mm (CV) Keihin. Odd, being that the power output is within 0.1bhp. Impulse has more torque (peaks at a lower rpm, too), but that's something more often associated with smaller carbs, not bigger ones.

Hmm.. I dint knew Impulse has a larger carb. What about Achiever / CBZX / Hunk? Do they have larger carb as well? My bike used to run rich with a 125 jet unless I plonked in a K&N.

Though most of the posts here have gone above my head, I will still go out on a limb here and say that any larger jet / carb will be an overkill for this engine from my experience.

Between I got my unicorn engine opened up for overbore last week. Piston is absolutely same as Achiever. Infact Mahale piston kit had Unicorn / Achiever written on it. Valves are different from the Hero family and stems were visibly shorter. Also I remember reading somewhere that CBZX cam provides longer valve lift though was not able to check personally.

So bottom line being that inside the engine too there are some subtle differences.

Originally Posted by ringoism

I'm going to see if I can scrounge a junk Unicorn carb locally and try it out. [b]It's clear enough that you don't need a 28mm carb to make 13.4bhp - or even 14+bhp, as a number of other bikes do that with much smaller carbs.

You can buy a complete unicorn with papers under 10K in Delhi. How about trying that swingarm with monoshock in Impulse. It begs for a monoshock rear.

cheers
Vishwas

[abhinav.s]

Originally Posted by vishwaschettri

You can buy a complete unicorn with papers under 10K in Delhi. How about trying that swingarm with monoshock in Impulse. It begs for a monoshock rear.

If I am not wrong, Impulse rolled out of the factory with monoshocks at the rear! If you are meaning to say that the Uni monoshock set up is better than the Impulse stock monoshock set up, I cannot comment on that since I have used neither.

[vishwaschettri]

Originally Posted by abhinav.s

If I am not wrong, Impulse rolled out of the factory with monoshocks at the rear! If you are meaning to say that the Uni monoshock set up is better than the Impulse stock monoshock set up, I cannot comment on that since I have used neither.

After reading your post I searched images of Impulse and stared at them so hard hoping for two shockers to appear magically on either side of the wheel but alas.

I have seen Impulse a thousand times and briefly ridden one as well. I just dont know why it was stuck in my mind that it had a conventional setup at the rear.

You sir are absolutely right, it always had monoshock at the rear. Kindly excuse me I have to go and put my foot in my big mouth .

cheers
Vishwas

[ringoism]

Managed to get out to Lahaul for a couple easy-going days... accompanied by an easy-going videshi friend on his easy-going CL500 rental.


No drama at Rohtang; occasional light surging, but no heavy missing and plenty of reserve power. That with original #132 jet, afore-described aircleaner mod(s), and one mod to the exhaust system which for the moment shall remain unstated... Plug color was perfect at Mardi - didn't check on top.




at the guesthouse at Sissu (lovely place)


on a little link road towards Telling village (never get tourists up there).


sacks of the famed Lahaul potatoes awaiting transport.


somewhere along the way...


pristine and untouched, high above the main road...



The bike... which with its - eh - mature (yet completely fearless) 56-year-old rider, completely left me in the dust, though I was pushing my Impulse as hard as it could be, and myself to the point of feeling foolhardy. This is (was?) a TVS/Suzuki Fiero. Now with Impulse forks, Unicorn rear suspension, RX carb, custom fabricated pipe and many other bits. Extremely healthy machine. Kind of left me wishing I'd done something like this instead of buying the Hero... especially now that I've spent just about as much time fiddling around trying to make it run right as he must have spent modifying that humbler and much lower-priced but still capable older unit. He restores vintage bikes in Calcutta, and will be competing in the Raid-de-Himalaya for the fourth time next week. Amazing man whom I hope to meet again.

...as for older quotes and ongoing technical investigations...

Originally Posted by vishwaschettri

You sir are absolutely right, it always had monoshock at the rear. Kindly excuse me I have to go and put my foot in my big mouth.

Blunder it may be, but it serves to bring to mind one distinct advantage of the Unicorn's monoshock: that it is adjustable (for preload), whereas the Impulse's (oddly/sadly) is not (and really, really should be). The Unicorn seems to enjoy a plusher ride quality overall, as well, while still not being very prone to bottoming out the suspension on big bumps.

Originally Posted by vishwaschettri

What about Achiever / CBZX / Hunk? Do they have larger carb as well? I will still go out on a limb here and say that any larger jet / carb will be an overkill for this engine from my experience.

...So bottom line being that inside the engine too there are some subtle differences.

I'd be truly interested to discover what all those differences are - cam profiles and advance specs? Valve sizes? Compression ratios? ??? And do these other bikes unmistakably, consistently run better than the Impulse up high?

Achiever has a small carb, probably same as Unicorn's; will try and get a look at the CBZX/Hunk's if I can.

Originally Posted by Sutripta

One can normally oversize a CV without major penalty.

For Christmas, I take it?

Indirectly, yes. We fly to in-laws in Mizoram (where Christmas is a huge celebration) via Kolkata's Dum-Dum, as it's a lot cheaper than from Delhi. And I mess with old wristwatches a bit, too, which Lal Bazaar is good for. And we just love the street food, the history, and the friendliness / helpfulness / diversity of the people.

Yes, I'd heard that CV's are supposed to be quite forgiving that way, at least in theory. I may or may not be on the right track. Heard just before going from local Hero tech that he'd tried a smaller Achiever carb on the Impulse up over Rohtang (13,500ft), and still experienced some "light" missing (which is what I also experienced, I guess, with the bigger carb). Then I met fellow bhpian Bharath at the Koksar checkpost back-side of Rohtang - he's running a Mikuni BS32 (P220) carb on his Impulse now - which seems grossly oversized to me - but says it runs great in the plains. Interested to see how it fares during the Raid rally next week... I'm guessing the slide might not be able to open far enough (based on low demand from the small 150cc displacement engine) to actually make the main (high-speed) jet completely take over from the needle / needle jet; might by chance run fine, but suspect it would make proper tuning more difficult to achieve for WOT operation.

For now, might take some respite from fooling with the Impulse. Got house / car work that needs done before winter, and not likely any more riding beyond the Pass till Spring.

Kind of feeling I'm close enough now that I could put a 15 or 16-tooth drive sprocket (vs. orig 17) on it, a slightly smaller main jet (125-129range) and drop the needle another 0.5mm, and have it just about the perfect compromise for the Manali-Leh road, that should run great at 11-12,000ft and fair enough to get over the 17,000+ ones.

But only time will tell.

Now I really have to get the steering bearings (cones) worked out. A problem from day one. I tighten them up (I DO know how to do this properly) and in as little as 30km on rough roads, a little play develops, and I start hearing a lot of noise / feeling a bit of looseness in the front end. I must have done this at least ten times already. Not like any other bike I've owned, and a real pain in the neck. Can't imagine what keeps increasing the clearance there. Bearings themselves can't be that soft, can they? Are the flanges that hold the races receding into the frame (sounds crazy, but...).

Anyone with insights / experience there?

Regards, Eric

[Sutripta]

Originally Posted by ringoism

Will keep my eyes open re: your wishlist... though the names are not at all familiar to me.
-Eric

Auto parts/ Accessories. Not watches.

Regards
Sutripta

[Suresh Stephen]

Dear Eric,
This is one thread I was following with lot of interest. Iam seriously considering buying an impulse bike. Don't loose heart. With you technical appreciation, Iam sure you can make the bike work the way you want. All the best
Suresh

[naveenroy]

Ha, good to see an informed Impulse thread. Planning the engine upgrade. Let me see when I can do it.