mikuni jetting


In a few days i will recieve a new mikuni carb for my 650R instead of the stock keihin.

I have uncorked the bike, put on a twin-air airfilter, removed backfire screen, drilled six 2cm holes in the airfiltercover. and the exhaust is a fully open White Brothers E-2.

Now I need good advice about jetting the carb, please help :thumbsup:

My Mikuni TM40 Carburetor -> 140 main, 27.5 pilot, 1.2 air jet, Y-4 needle jet, 4th pos. on the 90JY1 needle

The air jet is the jet that causes a lot of problems. It is usually a 0.9 or 1.o used for a Harley but, our bike needs a 1.2 air jet.

Most of the TM 40's have a Y-8 or Y-6 needle jet but, I found the Y-4 to work best for me. 140 main and 27.5 pilot should be to rich for most but, my bike is modified.


I'm at sea level, (Florida) with a stock uncorked bike. I run a 25 pilot, 1.2 air jet, and a 140-145 main (haven't fine tuned the main jet, I just know it's rich enough). With a free flowing exhaust I have better luck with the y-6 needle jet and the 9DJY4 needle 3rd clip. With a quieter, more restrictive exhaust I use the y-4 needle jet.

My TM40 was shipped VERY lean, so lean the bike wouldn't even start. Be aware how your's is jetted, don't just bolt it on and run. If you are at 1000ft+ elevation then you will probably want to go with a 20 or 22.5 pilot and maybe a 135-140 main jet and the y-4 needle jet.

Be sure to check the air jet (in the mouth of the carb), you will want a 1.1 or 1.2 in there. Mine shipped with a 1.0 air jet. Good luck.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now