Hey, i have a 2005 Z400, it is currently ripped apart getting a 434 athena kit put on it. It also has a UNI air filter and a FMF slip on exhaust. What size of main jet do you think i should try with the athena kit installed?

 

JUST GO UP 5 SIZES FROM WHERE YOUR AT. START THERE.

 

Buy several jets - they are only about $2.00 each. Tune your engine to peak at WOT (wide open throttle). When your sparkplug shows the correct jetting (medium brown in color) then go one size larger on your jet. Lessens the chance of a melt down from being too lean. As in life, it is a better to be a little on the rich side.

Start your engine and don't run at idle and as soon as it will run without the choke take the choke off. You do not want the pilot or starter jet to influence the color of your spark plug. Or you can start your engine and let it warm up and then change sparkplugs and then do the WOT.
Run WOT and shut engine off and shift into neutral at your top speed (5 seconds at top speed will do it) and then pull the spark plug out and check it. (don't let it come down to an idle before shutting it off).
When you change jets you should put a new spark plug in but you can wire brush it so that it is silver again.
As a final check you can do a spark plug chop. (cut the side of the threads off with a hacksaw and the base of the white porcelen should be a medium to chocolate brown)