[SunnyBoi]

Originally Posted by ashwin.terminat

4 stroke engines come with a Fuel Screw. This screw, if turned anti-clockwise makes the mixture richer and turning it clockwise makes the mixture leaner. The screw controls the amount of fuel that you are adding to the mixture, hence the name, Fuel Screw. Opening up the screw lets in more fuel and closing the screw lowers the amount of fuel going in.

2 stroke engines on the other hand come with an Air Screw. This screw, if turned anti-clockwise makes the mixture leaner and turning it clockwise makes the mixture richer. This screw affects the amount of air going into the engine, hence the name Air Screw. Opening it out lets in more air and closing it in lets in less air.

Quite simple, yes?

I'm sorry but you're incorrect.

Poh-tay-toh, Poh-tah-toh. Both are the same. If the screw thread is reversed, the effects will be reversed.

Air screws have a taper at the end which control the amount of air flowing into the pilot circuit. Deeper the needle goes in, greater the taper, more blockage hence less air thus richer air mixture. As the needle comes out, taper decreases, less blockage, more air thus leaner mixture.

Rather than confusing about 2 strokes, 4 strokes, air screw, fuel screw, just observe the movement of the screw in relation to clockwise or anti clockwise movement. If screw goes in on turning clockwise, AF mix will get richer, counter clockwise will make it lean. Vice versa.

PS : My ZMA Keihin carb's air screw leans as I turn it counter-clockwise and my RX's Keihin PWK does the opposite.

[ashwin.terminat]

Originally Posted by SunnyBoi

I'm sorry but you're incorrect.

Poh-tay-toh, Poh-tah-toh. Both are the same. If the screw thread is reversed, the effects will be reversed.

Air screws have a taper at the end which control the amount of air flowing into the pilot circuit. Deeper the needle goes in, greater the taper, more blockage hence less air thus richer air mixture. As the needle comes out, taper decreases, less blockage, more air thus leaner mixture.

Rather than confusing about 2 strokes, 4 strokes, air screw, fuel screw, just observe the movement of the screw in relation to clockwise or anti clockwise movement. If screw goes in on turning clockwise, AF mix will get richer, counter clockwise will make it lean. Vice versa.

PS : My ZMA Keihin carb's air screw leans as I turn it counter-clockwise and my RX's Keihin PWK does the opposite.

Oh, I always thought it was the way I had mentioned!

That's even how I'd read it to be on a few pages when I was looking for tips to tune my carb.

Thanks a lot for correcting me, SunnyBoi!

[saket77]

Parag: Yes, I remember myself too fiddling with the carb setting on my Fiat 1100D. Now it is difficult to find a good tuner. MPFI systems have made that art extinct. At times, I feel that its a good riddance, but then differently tuned cars gave each one of them their own character which lacks in MPFI cars. But may be I am nitpicking!


Ashwin, mostly yes, but not all 4 strokes come with similar arrangement. You can determine if the screw controls air or fuel by its location on the carb. The rule is if the screw is near the air intake hose coming from air filter, then it is an air screw. And if the screw sits closer to the engine intake (the output hose of the carb) then it will be controlling fuel.
Some 4 strokes, esp. older models came with air screws as they probably used carbs originally designed for older 2 stroke engines.

When you tune the engine, the RPM will reach at the peak at one point. Further tuning in the same direction will not increase the RPM (and may not decrease it much too). The art is to get the feel of the engine and to look for a crisp exhaust note. Further tuning will make the exhaust note dull while the RPM may not change much.

Regards,
Saket

[pathik810]

I have replaced the BS-26 (26 mm output) carb in my Royal Enfield Thunderbird AVL with a UCAL UCD-33 (33mm output) carb, the one which is used in Pulsar 200NS (probably same as used in Enfield Bullet 500). Now I am so much confused for the tuning purpose. Some say:

Turning the screw clockwise = Lean mixture
Turning the screw anti clockwise = Rich mixture

Where as my rookie mechanic says,
Turning the screw clockwise = Rich mixture
Turning the screw anti clockwise = Lean mixture.

Plus a guy in this forum says, in order to detect, if the screw is towards the engine side then it is a volume screw, and if it is towards the air filter side then it is air screw. The screw in my newly fitted carb is towards the engine side and as per the mechanic, it is the Air screw. So the theory again gets complicated.

Please refer to the attached picture. Say it BS-33 or UCD-33, but can any one help me out if this screw is Volume screw of Air screw? And turning it which way will make it lean?

[scorpian]

I believe anti clockwise will make the mixture rich as the screw will be moving outwards from the hole there by increasing fuel supply.

I think a simple way is to open the screw 1 full rotation then another half rotation start the bike and if by closing the screw full the bike should go off then the above mentioned theory holds true.

In the karizma the screw is engine side and anticlockwise will make it richer.
The same will be for Bs carb.

[pathik810]

Yes, the engine turns off when I tighten it fully.

[aditya.bhardwaj]

Guys, i have a doubt with regards to my carb fitted onto my impulse. Firstly, can anyone please tell me whats the model number of the Keihin Carb. I know that it is a 28mm but is it PE or PWK ?

Now lets go on to the issues i am facing. I bought my brand new impulse around a month ago and i have covered 1500 kms since then. Few days back i observed that some amount of fuel was leaking from the lower side of the carb. Showed the same to my mech. He opened the bottom most chamber where the float sits, told me that the gasket is blown ( As the bike was sitting for 3 years ) and that a new gasket is very hard to source. So he put the silicon gasket bond and refit the whole thing and then there were no issues till yesterday.

In the morning, before starting the bike, i saw a pool of fuel underneath the carb. Please note that i switch off the petrol tap whenever the bike is not in use ( Later got to know that the petrol T is busted ). I could see that the fuel was leaking from the overflow pipe outlet whenever the bike was shaken.

Showed the same to my mech again and he said that the needle inside is the culprit and i need to change the carb and put the one from the 200NS. Now i surely know this cannot be such a major concern, so i am planning to open the carb, clean the bottom chamber and put everything back again . I feel that the float is not seated properly.

My friends from the Tbhp delhi group also suggested me to do the same.

Now i hope there is nothing seriously wrong with the carb.

Any help or advice would be surely appreciated.

Thanks

[abhishek46]

The last time i touch the carb myself on my bike, it resulted in heavy smoking and had to re-bored.
These things are best left to experts or the service center.

Also, non-trust worthy road side garage amateurs should be avoided like plague. A wrong setting can lead to heavy carbon build up in the engine internals, which is difficult to get rid of in a 4 stroke mill.

[Sutripta]

Originally Posted by aditya.bhardwaj

He opened the bottom most chamber where the float sits, told me that the gasket is blown ( As the bike was sitting for 3 years ) and that a new gasket is very hard to source. So he put the silicon gasket bond and refit the whole thing and then there were no issues till yesterday.

In the morning, before starting the bike, i saw a pool of fuel underneath the carb. Please note that i switch off the petrol tap whenever the bike is not in use ( Later got to know that the petrol T is busted ). I could see that the fuel was leaking from the overflow pipe outlet whenever the bike was shaken.

Very unlikely you'll have to change the carb.

Possibly the fuel pipe is perishing and shedding material, or maybe rust from the fuel tank is preventing the needle from seating properly. In any case, a minor problem.

Drop a PM to forum member Ringoism.

Regards
Sutripta

[Mr.Boss]

Originally Posted by aditya.bhardwaj

Showed the same to my mech again and he said that the needle inside is the culprit and i need to change the carb and put the one from the 200NS. Now i surely know this cannot be such a major concern, so i am planning to open the carb, clean the bottom chamber and put everything back again . I feel that the float is not seated properly.

Which needle he is speaking about? If it is the float needle, yes there are possibilities and replacing the same is not an costlier affair. But in most cases I came across, the culprit being the small plate in the float unit that actuates the float needle (mostly bend) Try sourcing the carb repair kit before you open up.

[aditya.bhardwaj]

Originally Posted by Mr.Boss

Which needle he is speaking about? If it is the float needle, yes there are possibilities and replacing the same is not an costlier affair. But in most cases I came across, the culprit being the small plate in the float unit that actuates the float needle (mostly bend) Try sourcing the carb repair kit before you open up.

Hi, i changed the float, gasket and the float needle as well. But the problem still exists. Got all the parts before hand as i had done it before. I am still figuring out what the problem exactly is !

[eko]

Setting of Electric Choke in Kinetic Honda to Achieve Good Fuel Average

Hi,

I had mentioned in my introduction about the setting of electric choke in Kinetic Honda scooter to achieve good fuel average. There was a reply asking for more details. So here we go..

The Kinetic Honda was very notorious for excess fuel consumption. It was considered lucky if you got 40 km/l. The main reason is that the vehicle has a tricky electric choke which Kinetic never bothered to calibrate at manufacturing stage. They also never trained their dealer nor mentioned in their service manual about choke setting. Even the great Honda did not recognize or rectify this problem. Honda service manual mentions a method for testing the choke (not setting) but the method is not fool proof.

Let me explain what the Honda manual says about the choke testing. Sorry I do not have any illustrations now. "Dismantle choke unit from carburetor. Connect the 2 leads to 12 V battery. You can see the plunger in open position. After few minutes you can see the plunger moving down.Check, if is any visible gap". There are 2 inaccuracies with this statement. An air passage cannot be tested only by saying "see for any gap". It is better to check valve in service. The manual does not say the action to be taken in case of visible gap.

The work with choke involves 2 parts. First is the testing, then the setting. I am talking about the mid eighties and there was no internet to run to for help. I did some thinking and came up with 2 options to check the electric choke. The first one can be done by anyone. Second by DIY or mechanics.


Electric choke testing:

1. Start the Kinetic Honda when cold. The idle RPM should be above normal with electric choke in open position. As the choke warms up after few minutes (usually takes 2 to 4 minutes) the idle RPM will go down to normal. If the idle RPM is above normal at cold start and go down after warming, then your choke setting is correct. But have you observed any Kinetic Honda behaving in the manner, I described? Chances are : NO. This is the reason, I mentioned that the choke is not calibrated at factory. If not done at factory, it should be done at dealer during PDI. Or at later service? But this problem was neither recognised by Honda or Kinetic in India and the vehicle was notorious for fuel guzzling. All it needed was an hour to correct the problem.

2. Dismantle the air inlet side of carburetor. Note the position of choke air passage. Keep a cotton bud ready to block the air passage. Start the vehicle and keep it idling for 5 min. With the cotton bud block the air passage to choke. If the engine cuts of when you block passage then that means choke is not closing well. Means your engine is running all the time with choke on. This is causing the excess fuel consumption. If air passage blocking has no effect on engine, then it means the choke is well closed. (This method can be used to test any choke.)


Now the choke setting part:

I dont have ready a picture of choke. But I will explain. The plastic choke body has a slot for spanner. Hold the round end of choke with plier (with some rags around for protection) and insert spanner and remove the half by rotating anti clock wise. Inside you will find screw mechanism to adjust the choke plunger. You will need to lower the plunger setting for it to close well. You may need trial and error method to reach correct setting. When setting is correct, the idle RPM will be higher at cold start and will go down when warm, indicating that the correct setting has been achieved. Choke correction alone will bring about 10 to 15 kmpl difference in consumption figures.

[eko]

Now the choke setting part:

I dont have ready a picture of choke. But I will explain. The plastic choke body has a slot for spanner. Hold the round end of choke with plier (with some rags around for protection) and insert spanner and remove the half by rotating anti clock wise. Inside you will find screw mechanism to adjust the choke plunger.

Hi,
I do not remember when was last time I did this choke setting. Must have been 15 years ago. My memory has played tricks and there is an error in the above quoted part.The correction is as follows.

The choke has a 2 part body. The upper part containing the electric coil and wax bag. The lower part (collar) fixes to the choke body. The upper and lower part is joined by thread. The adjustment of choke is done using this threaded portion. Screwing the collar inside lowers the choke plunger and vice versa, taking care of the adjustment. If your problem is with choke not closing, then you screw in the plunger by a turn and check. Repeat till the setting is correct. (There is no need to dismantle the collar as described in my last post). Sorry for the error.

[eko]

Originally Posted by aditya.bhardwaj

In the morning, before starting the bike, i saw a pool of fuel underneath the carb. Please note that i switch off the petrol tap whenever the bike is not in use ( Later got to know that the petrol T is busted ). I could see that the fuel was leaking from the overflow pipe outlet whenever the bike was shaken.

Hi,

There is leak definitely at the float needle valve . This is the reason for overflow. It can be due to the following reasons. Keihin carburetors have rubber tipped valve needle and do not overflow at all.

1 ) The float pivot may be sticky since the bike was sitting as you said. Check if float is free on pivot. If not polish the pivot or replace. The float has to move free on pivot for valve to close well.
2 ) Inspect the rubber tip of float needle for wear . In case of wear replacement is the only remedy.
3 ) The valve seat molded to the carburetor body needs to be checked and cleaned.There could be foreign particles adhering to the valve body, preventing full seal. Lap the valve body with a conical metal rod or wooden rod to clean.

Regards,eko.

[Crankpin]

On a CV carburetor, the screw toward the engine side of the carburetor will be a fuel screw and it will en-rich the fuel flow if turned anti-clockwise.
However on most newer scooters and two-stroke mopeds/ bikes, the carburetor is a simpler round slide type one which has an air-screw which mostly sits closer to the carburetor venturi or the air-box side. This air-screw will richen the mixture if turned clockwise and lean it out if turned anti-clockwise. Most factory settings are 1&1/2 turns anti-clockwise after turning the air screw fully in( clockwise).
Care should be taken not to tighten these screws too much as they are made of brass and sit in an aluminium body both being relatively soft metals. Use a small screw driver.

(The image on the left is an Air screw ( blunt end) and the one on the right ( pointy end) is the fuel metering screw)

These screws mostly meter the pilot jet i.e. 0-1/8 throttle positions ( initial pick-up speeds) after which the circuit moves on to the main jet inside the carburetor which is metered by the carburetor slide needle.