abhisheklakhanpal

So ,
i figure most of us are still riving around carburetted bikes and at some point of time , some dumb ass mechanic will screw it up for you (like a Bajaj Auth Service for me ) , or some drastic altitude changes will leave you feeling powerless. This goes to say , that your carburettor will be out of tune .

So , while i have kinda figured the trick , i m sure none of you wanna be slaves to these headless mechanics waiting to tune a blunder for you .


A little Guide to tune you 4 stroke CV carburettor for the optimal ( Not power oriented ) air fuel mixture.

There are mainly two screws used for the tunning of a carburetor

1.Fuel screw(4 stroke) or air screw (2 stroke)
2.Idle Rpm screw

The main difference between two stroke the fuel screw is that for the fuel screw(used in pulsar etc) Clockwise tightening makes the mixture lean , and anticlockwise loosening makes the mixture rich.

For two stroke bikes using an air screw the setting is exactly opposite.

The idle rpm screw is what sets the warm engine idle rpm . The greatest significance of the idle rpm or pilot setting would crop up at the point that it affects 1/8th of the initial throttle response. After that , it’s the butterflies and other that take over . But this initial turning is critical so as to ensure a smooth pickup as well as control knocking or over racing.

Judging the air-fuel mixture for the bike can only be done on a warmed up engine.

Warming up the engine

1.Drive for atleast 10-15minutes before trying to adjust any of the carburetor setting.
2.Especially for CV carburetors, used in the pulsar, use 1/2 throttle on an uphill for 1 minute. ( alternative )

Setting the mixture

1.Set the idle screw at 3000rpm
2.Now adjust the fuel screw for the leanest possible mix without stalling the engine.
3.Now after adjustment is made the rpm becomes constant
4.Now , adjust screw to tune within the lean range
5.Decrease idle rpm to about 1100
6.Check the throttle for a quick and crisp response. The crispiness is important
Also check for instant start


After warming and adjusting , drive for a while to check the spark plugs for the following signs
1.White Deposits : Lean Mixture
2.Black deposits : Rich mixture
3.Sandy side of chocolate brown : Ideal mixture.

Please note that the fuel mixture is always checked by driving between 1/4 th and 3/4th of the total throttle twist. This also goes to imply that the throttle body must have a uniform response to the same twist at all levels and is working perfectly.
The other , not so accurate way to check the correct mixture is the silencer or exhaust fume . Place your hand at the exhaust as you rake the engine after 1/4th of the throttle .

1.The sign of the optimal mix is that your hand feel slight warmish moisture as the throttle is increased a bit . Then check the same sign at constant throttle.
2.The very heat or temperature of the fumes at the upper limit of the power band should not feel too much to keep the hand there for about 10 seconds.


A few signs for your carburetor out of tunning
1.RPM rises fast but reduces slowly : Mixture is not optimal , usually on the leaner side.
2.Grunting sound (too much torque) : Rich mixture.
3.Engine knocking at low rpm without power : Lean
4.Not enough power , overracing at high rpm for same speed : Clutch setting req . Mix is a little lean



A Lean mixture is usally more harmful than a richer one . This is because , the engine is knocking at a higher rate for the same power . Thus the high compression damages the pistons causing holing(hole in the top center of the piston).It also jerks the entire combustion chamber which may loosen the o-rings as well as cause uneven wear.
For this reason it is necessary that you always try to keep the needle in the powerband for medium and overdrive gears, and also never try and keep the bike in the same gear for thought of better efficiency.

For those of you who think that a richer mixture is a gateway for more power, it may be noteworthy of a mention that even a richer mix is damaging and causes faster wear and tear.

Hence engine life is reduced though it will function better than a len mixed carburettor. In case performance is what you look at , Jetting of the carburettor is the right way to go. It is basically increasing the fuel pipe radius as well as the air pipe, to allow for a greater fuel flow rate. The most significant increase of jetting is felt at the top end , where the limiting fuel flow is increased, thought a difference is also felt throughout the entire RPM range.

However , once you drive a day or two , you will figure if your setting is correct or not . If the mixture is the optimal one then just a single anticlockwise turn ( richer) or maybe two will give you a reasonable boost in the power , though naturally you are sacrificing the Fuel effiency , although it wont matter much . But , repeat , test the optimal setting for atleast two days , before switching to the optimally rich setting . Coz knowing what is right is the only way to know , know what is beyond right.





Towards biking Independence......

Cheers!

Aryan

Universal Thread moved and Approved.

@abhisheklakhanpal: Do mention the source of the information if you hace copy-pasted it from somewhere. Also, such large font sizes are not really much legible. Do correct it.

Thanks.

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L.P.

Nice piece of Information Abhishek, But it would have been better if you could illustrate it with the pictures as people might not know which is the air screw or the Fuel screw.
and what to do..

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velociraptor13

Good info,some pics would be welcome.

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abhijeet080808

Good initiative there! I am sure it will help us a lot! Some pics are needed!!

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abhisheklakhanpal

Dear aryan , most of this is really what i knwo of the thing by a bit of toddling. Yeah a bit comes from anodder article , but i have made changes and modifications to the things. In fact it is a small part of the entire thing.
FONT - KEep in mind.

abhisheklakhanpal

OK bro . Will post em in a while !!

moeed

Good info there!

fiery1

Hi there abhishek! my bike is a unicorn 2008(met. silver with 3d emblem), and i guess the instructions u have provided goes about well with my bike too.
I seem to have most of the conditions that you have specified in the case of a poorly tuned carb. Do u have anything specific for the Uni (2008)?
BTW where are u put up?

abhisheklakhanpal

Hey there .

Well , as for the carb brother,

1. Honda carbs ar e, in my personal view on the sensitive side. Dunno why , but a buddy ov mine. However , the difference in performance gained by tunning his carb optimally as compared to ssay a pulsar (that i ride), was very little.

2. No specific honda instructions.
quick tip: Try adjusting the settings to extremes and then gradually fine tune them (you will figure the settings only as you do ride , not at idle speeds)
IMHO, if i owned a unicorn , if u want to save effort and time , let an experienced Honda mechanic do it. My friend still hasnt gotten his tunning right.

3. Check for signs(black or white deposits) . clean after checking. If they dont appear again, that means your carb is all gud to go. Usually , the optimal point is hard to detect always (esp true for teh unicorn , atleast my friend claims so ...lol)

Hope this helps.