Since my original carburetor got eaten up by the dustmeister, I bought a new VM30 and I'm about ready to put it on the bike since it's almost ready to start up. Minus cleaning the gas tank, that I decided I would look at the main jet. I find that the main jet on the new carburetor is labeled 280. It is an original Mukini, it's not a Chinese knock-off. The original carburetor has a main jet marked 250.

Is this something that I should swap over from the 280 to the 250 since my bike originally ran with the 250 jet? Or because it's a newer carburetor all be at the same number and practically the exact same thing in all likes and sizes as the original. Should I stay with a 250 or should I keep the original 280 in it? I am mixing my gas and oil so it shouldn't affect the gas fuel mix at all?

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The jets work as a set.  So unless the new carb has the same needle, needle jet, slide cutaway, and pilot jet, you never want to just match one jet from the old carb.  (Cutaway and pilot jet are not as relevant to the main jet as the needle and needle jet)


Basic rule of carb tuning is you give the engine what it needs.  In my experience, new carbs generally run decent with the jets they come with.  I'd run it and make adjustments as needed.  

Always remember that the circuit you need to adjust is dependent on the throttle opening, NOT RPM.  For example, if it's rich at 1/4 throttle at high RPM, it would be a mistake to change the main jet as the main jet primarily affects 3/4 to full throttle.  

Mikuni VM tuning guide is in the tech library here or can be found online