ok so on my 99 formula z 670 thy is there 2 different jet sizes? i belive the one side is a 420 and the other is a 390, why the difference? should i go to one size like a 400 or what? any ideas?

Most engines will run odd size jetting with the larger on the PTO for cooling as that's the cyl which develops most heat. Some triples ran 3 different sizes.

What he said. I dropped jetting on my 98 MXZ 670(same engine) and the plugs are chocko-right. I ride across the lake in Vt.

You need to remember that your twin is really 2 motors. Each "motor" cylinder is different from the other due to the way the pipe configuration is. If they are staggard then leave them staggard.

Bombi specs will show that is correct.

doo suggests staggard jetting on my rev 440 but i will run the same jets on both sides, probably 320's.

I can't remember which way the Rotary Valve spins, but it opens
from the bottom of the port on one side and from the top of the
port on the other side. One of the cylinders needs more fuel because
of the way it atomizes in the carbs and goes through the port.