If you have a TM 40 Mikuni carb (flat slide) then you should have 520 main jets in there as stock unless someone has changed it. Basics for stetting carb jetting is to first set the slide height (read your manual, it tells you how to do this and you can download one for less than $10.00 from many sites), then turn your Pilot screw clockwise until it gently seats, then back it out 1 1/2 turns. Now reassemble your carbs and airbox and take it for a spin. You need a good smooth straight away for some WOT (wide open throttle) runs. Once the sled is fully warmed up take 3-4 WOT runs of hopefully 20 seconds or so, this will burn off any excess oil build up on your plugs and piston tops. Now do a final WOT run and after 10-15 seconds of WOT (and without letting off the gas) hit your kill switch and ride it to a stop, this is a throttle chop test. Now pull the plugs and see if you are running rich, lean, or just right using this chart (
http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html ) as a reference. On the 700 and 800 ZX's there is also enough room to stuff your head in there and if you have a small flexible shaft flashlight like the ones the go on the end of a mini-mag then you can check piston wash, just rotate your primary clutch until each piston is at the bottom of it's stroke and you should be able to get a good look at the piston wash. Ideally you want to see about a 1/3 inch semi circle patch of clear piston top on the intake side ports and a consistent and dark wash over the rest of the piston. Adjust your main jet as indicated by either the plugs or the piston wash going up or down in jetting as needed. This sets the Main Jet orifice only which controls from 3/4 to WOT but it is the first step in setting you jet sizing. Sometimes this takes a couple of times out, pulling your air box and carbs to change the jets until you get it right but that's basically how it's done. You can aslo do this in the parking lot of your local ridnig rea as it's easy to pull the airbox and carbs and then you can get it all done in an hour or so. Your MPEM should adjust the air fuel mix for altitude and environmental changes from this base line setting. If you still have a mid range burble after setting your Main jet correctly then this tends to indicate a slightly rich mid rang mix and you can try shimming your needle but if I remember right your carb only has one slot in the needle so you can't just move it up one clip position, you have to shim it. You can also buy the next thinner needle taper but I don't know what that number is, check with you dealer or on the Mikuni site, or heck call Mikuni and ask, I did and they were super helpful.
Remember, Main jet controls 3/4 to WOT. Needle taper controls 1/4 to 3/4 throttle but it is also related to the Main jet orifice size, and Pilot jet covers from idle to about 1/4 throttle, or basically up to just after track engagement speed. If you have changed your Pilot jet 3 sizes then something probably isn't right there, I am running a 1.25 Pilot Jet and 490 Main jets on my 01 Summit 700 at 5000- 8500 ft but I have some Rage cages and a snow stuff Rumble Pack can.
Oh, Mikuni did something funky when they went to jet sizing over 500, the 520 is supposed to be the next size up from the 500 but I couldn't tell the difference, the 490 definitely dropped it down some for me, don't buy the N series jets as they are a different sizing/calibration system entirely.
Best of luck
Hope this helps :thumb:
