[voodoochild]

Well, there are quite a few threads/posts dealing with carb tuning for various motorcycles, but I'll post again for the Pacco/Jikov carbs used on the Yezdi/Jawa.

A brief description of the carbs:
Pacco: These have just two screws, one spring loaded and one not. The spring loaded screw is the idle screw, the other is the AIR screw. The Pacco P43 is similar to the Jikov, as described next.

The Jikov (and Pacco P43): Three screws: two facing sideways, one facing down. The spring loaded screw facing sideways is the AIR screw. The spring loaded screw facing down is the IDLE screw. The topmost screw without spring is to be screwed ALL THE WAY IN and left that way.

NOTE: These carbs have an AIR screw (NOT MIXTURE screw). Screw IN = less air = RICH mix. Screw OUT = more air = LEAN mix.

Now basic procedure to get a decent running motor. Turn the idle screw in to get a sufficiently high idle (so the engine wont die when you are playing with the air). Turn IN the air screw fully (do NOT overtighten). If the motor dies, add some more idle and start again. Now turn out about 1.5 turns as a basic starting point. Now start turning out the air screw fractionally (in steps of 1/8th to 1/4th turn). Wait at every step, let the revs settle. You will notice the revs getting higher at every step. At one point, the revs begin to drop. Leave it at this stage, get back to the idle screw and set a good rhythmic idle (approx. 1000rpm).
Now if you would like a troublefree cold start, close the air screw 1/4th turn. Basically a richer mixture allows easier early morning starts.

This process can be followed for almost all carbs. For the ones with a MIXTURE screw instead of an air screw, wherever I've said "screw IN", just screw OUT. Identifying if it is an air or a mixture screw is already explained previously.
PHEW!

[Sankar]

Originally Posted by black12rr

The carb in the picture is BS29 ? , then tighten the screw (clock wise ) will make it run rich and loosening it will make it lean .

The carb in the pic above what's marked in the pic is right.
Clockwise = tightening the screw = cutting the fuel supply = lean.
Anticlockwise = loosening the screw = opening up the fuel supply = rich.

If the mixture control screw is on the engine side its a fuel control screw and if its on the airbox side then its an air control screw.

[black12rr]

sankar , I have same BS 29 carb on the pulsar . Tightening the screw is rich and loosening the screw is lean . I was talking about only this carb .

When you say engine side , engine side is both left and right side , so its confusing to others . We are talking only about the carb shown in the first post . If you are mentioning about any other carb , please post respective pic of it .

[voodoochild]

black12rr, I guess Sankar meant to use the words Upstream and Downstream. The AIR screw will be upstream of the fuel entry point. Air screw will meter air before the fuel enters it. If it is a fuel screw, it obviously will be downstream of the fuel entry.

In other words, if the screw is between the float bowl and the engine, it is a fuel screw. If it is between float bowl and air intake, it is an air screw. Hope its clear now

Cheers,
Rahul

[Sutripta]

I think Headers wants to know how to setup a carb, rather than how to set idling.

@Headers: Why this sudden interest in carbs?

Regards
Sutripta

[adrian]

Originally Posted by black12rr

sankar , I have same BS 29 carb on the pulsar . Tightening the screw is rich and loosening the screw is lean . I was talking about only this carb .

When you say engine side , engine side is both left and right side , so its confusing to others . We are talking only about the carb shown in the first post . If you are mentioning about any other carb , please post respective pic of it .

Sankar is mentioning about the BS29 carburetor itself. There are two screws available for adjustment when viewed from the left side of the bike. Idle adjustment screw and fuel screw which is closer to the engine. Spot it just below the choke lever.
Engine->Fuel screw-> Carburetor.
Tightening the screw (clock wise) is lean
Loosening the screw (anti clock wise) is rich (Turn it further anticlockwise and the screw will come off with the o-ring and the spring)
Regards Adrian

[headers]

Originally Posted by Sutripta

I think Headers wants to know how to setup a carb, rather than how to set idling.

@Headers: Why this sudden interest in carbs?

Regards
Sutripta

Google pointed me to TBHP - I just wanted to understand the various carburettor types and tweaks available.

However, I am unable to get more elaborate information.

Nobody talks about jet sizes, reference as to how to choose the jet, understand various methods and effects in various parameters of tuning a carb setup etc. How to read the car settings without opening the carb etc!


Everybody talks about lean and RICH settings

[Sutripta]

^^^
Cars or bikes? Or in general.

I think the VM setup guide is available somewhere on the net.
So too for the Webers, esp. DCOEs.

Do you have any specific questions?

Regards
Sutripta

[Sankar]

Originally Posted by Sutripta

I think Headers wants to know how to setup a carb, rather than how to set idling.

Mixture control (not idling) screw is also a part of setting up a carb. It affects the pilot circuit but not much influence on mid or wot.

Originally Posted by headers

Everybody talks about lean and RICH settings

Thats what the jet sizes too achieve isn't it? And that's all that there is to setting up a carb - lean and rich settings.

There are a lot of helpful info on setting up a carb over the Internet, starting from the very basics. There's a lot to read too - different circuits and at what throttle openings they operate etc. Getting the fueling right is not tough but it sin't easy either if one doesn't know where to start and what to read. One has to understand the engine - the way it responds, sounds, heats up, at various throttle positions and change the circuit coming into play at that particular throttle position as required. Plug chop is also necessary. Its safe to start rich and go poor (lean), its safe for the engine that way.

The last carb i set up was a 40mm mikuni flatslide on my Karizma in 2008/9. It took some days to get it to work on the Karizma and then it took couple of weeks to dial it in. The changes made were, jets-pilot&main/float level/needle In the end it was almost perfect and a lot better compared to stock CV, the bike was much faster with this one change. The techniques used were-engine behaviour/engine sound/plugchop/enigne hotness. The details of the TM are in my Karizma thread. I couldn't fine tune it as my biking career ended shortly there after.

[Sankar]

Originally Posted by Sankar

The last carb i set up was a 40mm mikuni flatslide on my Karizma in 2008/9.

Correction: It was a TMS32.
http://www.team-bhp.com/forum/motorb...-ideas-20.html

Still a huge carb for an otherwise stock Karizma engine. When i got that carb i wasn't sure i would be able to make it work, but dialed it in finally.

[Sudipto-S-Team]

Originally Posted by headers

I just wanted to understand the various carburettor types and tweaks available.

Certain things are best learnt in person from a guru in the good old traditional style. I think carb tweaking is one such art. Don't try to learn it from the internet.

[headers]

Originally Posted by Sutripta

^^^
Do you have any specific questions?

For Guruji - Questions:

Basics
1. Pictures of various types of carburetors ?
2. Primary and secondary jets pictures?
3. How to clean the jet? Pictures ?
4. How to Tune the carburetor?
5. How to find out if the tune up is satisfactory?

Advanced:
1. How a particular carb is decided for a particular engine - Any calculations?
2. Why certain carbs are better than equivalent EFI engines?
3. How carbs handle near AFR of 14:7:1?
4. Why carbs are loosing market share [This one is easy and the answer is not power ]

[Sutripta]

Originally Posted by Sudipto-S-Team

Certain things are best learnt in person from a guru in the good old traditional style.

I agree. But still posts from inhouse hands on experts would be illuminating.

Eagerly waiting.

Regards
Sutripta

[rajeev k]

Just accidentally bumped into this thread and read Mr DB's post and sorry to note that Mr DB did not get proper advice. Is it because Mr DB is an auto expert? Hope the problem is solved by now and would like to hear from him on how the snag was fixed.

[ananthkamath]

I'm not an expert but can speak from experience, having worked on pretty much every single popular carb out there ranging from the Mikuni VM and BS series to Keihin flatslide to the Solex and SU carbs found on Fiats and Dolphins, the Jikov and Pacco found on Jawas, racing carbs on karts and formula cars etc.

Carb tuning is not a simple subject and requires lots of theoretical knowledge as well as excellent feel for what your particular engine does under different conditions (by this I mean how it sounds, how it runs, how it pulls, transient response, how it slows down, how crisply it revs in neutral etc). I would say that tuning a performance engine for the street is among the toughest things one can do, even more difficult than tuning it to run on a racetrack!

I won't go into specifics, but almost always you have to start with good spark plugs that are properly gapped (preferably new), the best quality fuel you can get, and good quality two-stroke oil mixed at the correct ratio, if the engine is a two stroke. The air filter has to be clean as a whistle - if you don't take care of these things first, any misfires or other poor running can never be traced to its true origin, and this may not necessarily be caused by the jetting.

Next thing you should do is read up very well on carb theory. There are many good automotive engg. books that explain this very well. Reading on the internet is spotty because any idiot can write anything he wants - takes a bit more effort and credibility to publish a book!

After that, the important thing is to get hold of a workshop manual for your bike or engine and read the heck out of it. You need to understand EXACTLY what each adjustment does. Generally every adjustment on the carb is for a specific range of throttle opening. The idle and air screws that are on the outside of most motorcycle carbs, together with the pilot jet inside the float bowl control the first 25% approximately of the throttle opening. The needle position, needle taper and the shape of the slide control about 25 to 75%. The main jet controls 75% to WOT (wide open throttle).

Before beginning tuning, the important thing is to open and clean the carb thoroughly. Here in the US it is easy because you can buy pressurized aerosol cans of carb cleaner - just point and spray in any hole and you're done. If this is not available you can just use clean petrol and a toothbrush to get rid of most deposits - then blow well using your lungs. The workshop manual usually has a procedure to set the float height which is extremely important because it controls the mixture throughout the full range. Learn how to set this properly using the manual - many times it will call for the use of a special tool but they usually also give a measurement from some surface on the body of the carb - its easy to measure using a ruler. Also check that the float needle valve is not worn out and leaking. In race engines we do different things with the float level to suit the track but this is not required most times for a street engine.

Its very important to make sure that the carb is completely restored to the same condition it came from the factory.

The typical tunable parts that you can have on a motorcycle carb are:
Main jet
Pilot jet
Needle
Emulsion tube (the part that the main jet screws into)
Slide
Venturi (if removable)

You can get to 95% of the perfection in jetting with just proper selection of a pilot and main jet, needle, needle position and adjusting the mixture screw. Most of the guys here will probably not experiment with jetting to a level where a slide or venturi change is required.

A two stroke is a lot more sensitive to proper jetting and therefore much easier to read. You can tell a lot just looking at the color of the plugs and the way the engine picks up and drops revs. A typical "plug chop" involves running the engine at the throttle opening that you are trying to adjust (i.e. main jet adjustment needs WOT runs) and then shutting off the engine and coasting to a stop. Pull the plugs and see the color around the outer electrode (called the land) and of the insulator. If it is oily or black, you know its rich. If its white and has specks of aluminum you probably melted something inside the engine and need to richen it out. If its a mild to medium brown or golden color, you're doing it right. On a street engine that will see different weather conditions, different qualities of gas, different loading etc. I would not go too lean even though the engine feels awesome when it is on the edge and close to a complete meltdown.

Different people do different things but my preference is to use whatever it takes to get the engine started and running decently (not great but decently). Then do the main jet calibration. You have to find a deserted piece of road to test full throttle acceleration. Start big and then keep reducing until the engine pulls hard, but you have to be careful: do a plug chop everytime. Multiple sets of plugs will be required during main jet adjustment.

Once that is done, 50% of the work is over since you dont have to take the float bowl apart everytime to change the main jet (some carbs have separate fittings on the bowl to make this easier). Next, move on to the needle. Needle selection is not easy, but it is if your choice is limited . Start with the needle in the highest position (i.e. lowest slot) and keep reducing until you get very crisp acceleration at between 1/2 and 3/4 throttle. You have to play with mixture screw (the one that's on the outside of the carb) adjustment at the same time as needle position adjustment, in order to get the crispest response between 1/8 and 1/3 throttle.

Next is the off-idle and idle adjustment which is again an iterative process between the mixture screw and idle speed screw. Generally the crispest throttle response for spirited driving is not obtained at the manufacturer recommended idle speed - they do it for emission and fuel economy reasons. You may have to go a little higher and when you are done, the vehicle will take off very crisply from off-idle and transition to 1/2 throttle in a very smooth manner.

Sometimes the pilot jet size needs to be finely adjusted if the engine response is not as crisp as desired. It takes a lot of experience to reach this stage unless the carb is from a completely different engine.

So like I said in the beginning, it takes a lot of time to learn carb tuning skills but its not hard. It just takes a lot of experimentation and discipline on your part (i.e. keep notes of every adjustment you make, and the results) to achieve anything useful. Take it to the mechanic if you don't want to put in the time!