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How to Tune 4S Motorcycle Carburetor

October 27th, 2010 Leave a comment Go to comments
Tuning a motor cycle carburetor could not be as easy as it could get.  All one should know for doing the tuning oneself is to guess the speed of the engine motor (the rotations per minute).  Have you ever wondered about the sound a motor makes when it starts and stops ??  I am meaning the MMRRMMRRMmrrmrrmrrmmm….. sound 🙂  If you know about that you can tune your motorcycle yourself.


The Basics

The motorcycle carburetor is meant for atomizing (converting the liquid to gas state) the petrol and send the gaseous fuel into the engine cycle for combustion.  So, a carburetor gets the petrol fuel from the petrol tank, mixes that with atmospheric air (after filtering it using air-filter) to atomize the fuel in to a fuel charge using the venturi float setup.  The fuel charge is fed into the engine cylinder for combustion.  The quality of combustion is a function of the air-fuel mixture ratio.  The air-fuel mixture is the ratio of quantity of fuel to the quantity of air.  When there is too much air in the charge, the mixture is called “lean mixture“, and when fuel is more than air it is called “rich mixture“.  For complete combustion, there should be enough air for combustion.  Complete combustion enhances mileage.  For pickup tuned bikes, the fuel is kept little higher than air.  Also for cold starts, one needs rich mixture for quick firing up.

Modern day bikes are equipped with Constant Vacuum Carburetors which ensure best air-fuel mixture by adapting to conditions.  The carburetor adjusts the air flow automatically based on the load on the engine despite the accelerator position.  This is done by the baloon and back-pressure setup in the CV carburetors.  The CV carburetors come with two adjustments, a) Idling b) Air-Fuel mixture.  The Idling setup is trivial, it is just a offset setting of the accelerator wire.  Adjusting the idling screw (the screw with a spring) is just like you accelerating a bit.  The air-fuel mixture setting is just a screw near the idling screw, most likely the screw with have dirt on it (I mean more dirt). The air-fuel mixture screw controls the air-fuel ratio.  Full tight means low or no air flow and Full loose means more air.

The Method

  1. Set the idling screw for little more throttling, meaning the engine should rev faster than before.  Typically, 10% more than the idling speed that you are used to.
  2. Set the air-fuel mixture screw to full close (tight, don’t tight it too hard). You should feel that the engine speed has reduced a lot now.
  3. Open the air-fuel screw slowly and observe that the engine speed is increasing.
  4. There will be a point, which if you cross by opening it further, the engine speed will start to decrease.
  5. You will have to adjust the screw to find the point where your engine speed is higher. Consider the following graph for better understanding.

The graph (indicative) shows the trend of the engine speed for various air-fuel screw positions and various idling speeds.  The green line is the locus of all the peak engine speeds.  To the left of the green line, you see the enriched mixture condition and to the right of the green line you can observe lean mixture condition.

Pre-Conditions

  1. Don’t tune the engine when it is cold (cold start conditions). You may rev it for a while and tune it.  Basically, the engine oil has to pass through all the chambers and gears (otherwise too much friction is offered).  The dynamics of the engine are different when it is cold and hot.  If you tune when it is cold, you may supply lean fuel  mixture when the engine gets hot. Because while cold start, the engine needs enriched fuel charge.
  2. Set the idling to a reasonable value when you tune.  Too low or high idling can get you local maxima conditions, where you would not get the best peak point (refer graph).
  3. Check whether the air-filter is in good condition.  If the air-filter is blocked, tuning will not help.  Likewise, engine oil level and viscosity should be good.

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