Carburettor tuning advice. Review on Tips on Carburettor Tuning

Carburettor basics: This is the device which is attached to the engine which is responsible for mixing air and fuel in a particular ratio and feeding the engine.

If choke is used this air fuel ratio would be theoriticaly 1:1 and in normal running condition 1:16(parts of fuel: parts of air). The carburettor can be tuned to alter this above said ratio of fuel and air as per the user needs. This ratio is technically termed as AirFuel ratio.

The ratios can be classified broadly as

1. Rich mixture - Fuel part is more than the amount if in ideal setting. In this case air part would be less than ideal setting.

   
   




2. Lean mixture - Fuel part is less than the amount if in ideal setting. In this case air part would be high than ideal setting.

   
   




3. Ideal setting

   
   

Rich Mixture:

A rich mixture setting will be used by people who need raw power high pickup etc. This is a belief. But in essence since more fuel is supplied to overall engine the efficiency would be lesser than ideal setting. The fuel will not be properly burnt due to lesser amount of air in this setting. This can result in excess Carbon deposit in engine cylinder, spark plug etc. If this kind of setting is used for longer durations engine life would be reduced. But if you love to enjoy the power, pickup you can have rich mixture setting. In this case things to be considered would be.

1. A high flow air filter

   
   




2. Engine porting(which would increase the inlet and exhaust port diameters)

   
   




3. High lift Cams(These increase the opening of the valves in enhancing fluid flow in and out of the engine)

   
   

To achieve modifications like the above said requires good mechanic with experience and an eye for engineering.

Lean Mixture:

In this setting the amount of fuel is less and air part is high.This aides in excellent combustion of fuel inside the engine cylinder. Since very less fuel is used and lot of air are available, due to very good combustion there will be very less carbon deposit in engine, spark plugs, piston etc. Nowadays many bikes like Apache, TVS flame, XCD DTSSi, Honda etc. come with this setting(although completely not lean).

If the air fuel ratio is made completely lean it results in some undesirable things.

1. In this case fuel quantity is very less and air is very high.

   
   




2. Due to very less fuel supply to the engine the power generated would be very less requiring driver to downshift gears frequenly to accelerate due to less torque produced by engine.

   
   




3. Excessive opening of throttle by driver happens to maintain speed which again increases fuel consumption.

   
   




4. Due to extremely good combustion and above said points engine tends to overheat very easily which can cause problems.

   
   

This setting is good if.

1. The vehicle will run on less load(in the case of bike if only driver is riding without pillion or total payload is less).

   
   




2. The vehicle will run lesser distance

   
   

How to tune the carburettor.

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The carburettor has two principal controls which normally a mechanic uses to tune the engine.

1. Idle screw(to change the engine idling speed)

   
   




2. Air screw(to adjust the amount of air entering engine). Since most of carburettors used nowadays like the c.v or constant velocity types have constant mass flow rate. Adjusting air quantity automaticaly has effect on the fuel quantity. So the air fuel ratio can be controlled by this one screw.

   
   

The first step is to identify where these screws are in the engine(i hope you can take ur mechanic's or vehicle manual help on this).

lets get into some action!

First step(Make sure engine is cold - I mean engine hasnt run for atleast half an hour)

Remove the spark plug. Clean it with cloth, sand paper can be used softly so that the carbon deposit on the plug can be removed completely. Now the plug tips should be shining like silver. Once the cleaning is done put the spark plug back in place .

Now before touching the air screw start the engine

If the engine doesnt idle(run without throttle) adjust the idle screw so that the engine idles or runs without throttle.

Now screw in the air screw(Note how many turns you make in a paper). As you turn the screw clockwise direction(screwing in) the engine runs faster. Now completly screw it in till the end. Normaly engine runs in extreme speed.

Now screw out(anticlockwise) the air screw slowly to 1.75 turns(1 turn is 360 degree rotation). Now if the engine is running at excessive speed adjust the idle screw so that normal idling speed is achieved(engine speed just to maintain the engine running and nothing more than that). The best tuning range is in between 1.75 turns to 2.75 turns.

Now take your vehicle and drive for around 10 kms at the way you normally drive(like around 60 kmph).

Now switch off the engine. Once the engine is cold, remove the plug and notice the plug tip colour. If its

BLACK - RICH MIXTURE

SILVER - LEAN MIXTURE

BROWN - CORRECT TUNING

Best wishes. But if you dont get it right keep the air screw setting at the original one which you would have noted down initially.

Note; Use fuel from a reputed petrol pump. The high octane fuel to my knowledge helps only for high compression engines like turbocharged engines which is not relevant to Indian bikes. So normal petrol is more than sufficient.