Carburetor tuning - a detailed picture

Dont read it - it is nothing but some tech rant, which will go over your head unless you love your bike and love getting your hands dirty...

I am posting it here to preserve it.

Before I start, one word of friendly advice - more often than not, carburetor is best left untouched. If you really want to play around with your carb, make sure you have the required knowledge, and tools. Carb is a very vital part of your bike, and properly tuning it is no Childs play.

 

Unicorns use a Keihin CV carburetor - in a CV (Constant Velocity / constant vacuum) carburetor the throttle cable is linked not to the throttle slide as in a conventional carb, but it activates a butterfly valve, and when you open the throttle, the air pressure difference between the sealed chamber above the vacuum slid and inside carburetor venturi forces the slide up and down. the slide is located in front of the butterfly valve. A CV carb gives good mileage, and adjusts better to altitude changes, but the throttle response is not as fast as of a conventional carb.

In a CV carb, the amount of air is controlled by the butterfly valve and the amount of fuel is controlled by drop in pressure, and the jets.

 

In a nutshell, to tune a carburetor properly, you need most suitable main jet, proper needle height / clip position and optimum float height.

 

The carburetor has a number of different jets, passages and orifices to control the precise amount of fuel and air flow into engine. Engine idle, part throttle, full throttle and acceleration uses different passages, and are referred commonly as 'circuit'. Practically, more than one circuit will be functioning at a time.

 

the basic circuits are - choke, idle circuit, midrange/main, high-speed/power enrichment and accelerator pump. Though these circuits overlap, all of these have their primary functions.

 

Choke helps is cold start and warm up.

 

Idle circuit controls the idle speed and some low end throttle response. The main function of the idle circuit is to control the idle circuit. It is controlled by slow speed jet and air fuel mixture screw.

 

Midrange - as the throttle is progressively opened, the flow through idle circuit gets reduced. In this range, the most effective component is the jet needle. Midrange is characterized by steady throttle and light acceleration. Generally the throttle opening will be from a quarter to three quarter of the full range. If you feel your mixture is rich when you are at a quarter throttle, and as you open the throttle further, if you feel the mixture is becoming lean, then you need a jet needle with larger taper. If the feeling is the opposite, then what you need is a small taper jet needle. If the mixture is lean throughout the range, then you need to raise the jet needle by lowering the clip position, and vice versa. From the moment the throttle is opened, till you open it really wide, it is the jet need which controls the flow.

 

High-speed/power - it is controlled by main jet. A rich mixture will provide more power, and will keep the engine cool. If you have tuned your carburetor lean, then don’t ride with throttle wide open much - it is not good in long run. so, the safe practice is to install a main jet which is one size higher than the one which offers best performance - it will help the engine in long run.

 

Accelerator pump - usually, this needs no adjustment. If you feel a stumble (a temporary lean condition when you suddenly open the throttle), you can widen the gap on the pump linkage, to delay the timing.

 

The simple and best way of setting a carb is to set the different circuits with ideal conditions for their primary functions. So, set the air fuel mixture and idle jet for best idle, set the needle for cruising at a steady speed (midrange) and set the main jet for wide open throttle.

 

Remember - carburetor tuning is and art, compromising lot of experience, lot of science and a bit of black magic... :)

 

(I express my gratitude to many websites which taught me the basics of carburetor tuning.)