how to tune royal enfield mikcarb vm24 carburetor / bullet carburetor tuning

Basic terminology used :
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.

Warning/Disclaimer : Work with soft hands,Do not over tight any screw otherwise you may damage the threads. I have taken full care while writing but I will not be responsible for any damage to your bike done during the steps. Follow at your own risk.

Locating the carburetor on bike , I have marked in red color in the image below.

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.

Click on the image above to see the exact location of screws.

Effect of these screws on Air-Fuel ratio and engine RPM.

1. Mixture adjustment screw : Rotating this screw clockwise makes Air-Fuel mixture leaner and rotating it anti-clockwise makes Air-Fuel mixture Rich.
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.

2. Tuning with role of auxiliary screw [as per royal enfield electra 5s service manual]:

Although I don’t use this method by one can follow and share the feedback with me.
In this method step 1 to 6 are same as explained above. Only following 2 are additional steps :
Step 7 :  Loosen the auxiliary screw by 1/2 turn (anticlockwise) , it will increase the RPM .
Step 8 : Loosen the idle adjustment screw to make RPM back to normal 1000 RPM.
keep on doing this exercise (step 7 and 8) until turning out auxiliary screw does not increase the engine RPM.

Identifying the optimum mixture by reading the spark plug insulator color.
1. This is how a new spark plug looks like :

2. spark plug color with optimum mixture : [Light brown to chocolate brown] , both are OK. First one is little bit lean.
 
3. spark plug color with rich mixture : [Black with lot of carbon deposit]

4. Following is the image from internet in case of lean mixture :[metallic white with metallic particles deposits ]

Hidden secret : Although everyone calls it tuning. If we speak technically then it is called setting up engine idling. Tuning is bit complicated process which involves upgrading jets . watch this space for more ....