how to tune royal enfield mikcarb vm24 carburetor / bullet carburetor tuning
carburetor : Basic functionality of the carburetor is to mix fuel with air in proper ratio and then feed it in the engine.
Lean Mixture : If Air-fuel mixture supplied by the carburetor has less amount of fuel than required it is said to be lean condition.
Rich Mixture : If Air-fuel mixture supplied by the carburetor has more amount of fuel than required it is said to be rich condition.
Tools : Only one screwdriver will be enough for the job.
Pre-requisites to the tuning : Take your bike for a short ride of 3 to 5 kilometers so that engine can warm up a little.
Understanding the different screws on the carburetor :
There are three main screws in the carburetor (explained from left to right in image below):
1. Mixture adjustment screw : This is the main screw for adjustment of Air-Fuel ratio.
2. Idle adjustment screw: This is to control the engine RPM at idle condition.
3. Auxiliary Air screw: this is part of emission controlling unit, to my greatest surprise most of the mechanics keep it close always.
Effect of these screws on Air-Fuel ratio and engine RPM.
2. Idle adjustment screw: Rotating this screw clockwise increases the engine RPM and rotating it anti-clockwise decreases the Engine RPM.
3. Auxiliary Air screw: Rotating this screw anti-clockwise increases the engine RPM.
There are two methods for tuning :
1. With no role of auxiliary air screw [Followed by me and most mechanics also]
2. With auxiliary air screw [as per book ]
1. Tuning with no role of auxiliary screw :
Step 1 : Start the bike and let it idle for some time (~40 seconds) to warm up .
Step 2 : Tighten the auxiliary air screw fully clockwise.
Step 3: Increase the engine idle RPM by 20 to 30 % of normal idling. Say around 1500 RPM. This can be achieved by slowly rotating idle screw clockwise i.e. closing it.Here you will have to use your senses like ears and feel the vibrations of bike since this particular method does not have tachometer.
Step 4 : Fully tight the mixture adjustment screw clockwise. It will make the mixture leanest possible and decrease the engine RPM to the lowest level. You will feel that bike vibrations are also decreasing.
Step 5: Slowly start rotating mixture adjustment screw anti-clockwise (1/8 turn at a time) . This will keep on increasing the engine RPM’s slowly and steadily , keep it opening until a point where engine RPM will become constant. Stop at this point , this is the idle mixture point. The bike is vibrating and humming well.
Hint : In my case this is between 1.5 turn to 2 turn.
[You have reached the optimum mixture of Air-fuel, if you will keep it rotating anticlockwise then it will make mixture richer and richer , and after 4 ,5 rotations you will find the RPM decreasing.]
Step 6 : Making the bike idle normal, rotate the idle adjustment screw anticlockwise slowly until bike RPM becomes normal say 1000 RPM.
Your tuning is complete. Test the throttle response now, it will be better. If you still feel some hesitation then repeat steps 1-6 again.
Hi I think you are wrong about the “mixture”screw. On the Mikcarb I’m sure, being next to the air filter side, it is an air screw. Therefore screwing it in, gives less air and a richer mixture? You state the opposite. Which one is true?
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I have old bullet lectra 350 but i faced carbourator problem so please give some information how to set it
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excellent explanation…closing the mixture screw should normally weaken the rpm but in my case it’s not happening… is that normal? or should closing the mixture screw completely cause engine to stop nearly?
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Kindly explain second method also.
I will try to adjust the carburetor setting tomorrow.
Thanks
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WHAT IS THE MODEL OF mikuni CARBORATOR FITTING FOR eNFIELD 1987
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nice and clear, thankyou
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