there should of been a paper that came with your yoshi slip on that says what they recommend for jet size and if you remove the air box lid I would go up 1 size more
and I think on that carb you have to remove the float bowl to get to the main jet. I have a FCR carb on mine and its easy to do unscrew the bottom plug and there it is
and as hondahero53 says BUY A CLYMERS MANUAL
that book will help you alot on jetting and how to do it
go here
carb click year then z400 then carb
look at #18 thats the main jet

 

I don't know much about a Z but I think they have a 135mj stock so a 150 might be a good starting point.

 

Im at about 600 ft elevation also. When I had my pipe and airbok lid removed I was runing a 160 Mikuni main, stock pilot, stock needle (it ran great). I was close to 3 turns out on the pilot screw so I bought a 25 pilot. I also had a dynojet needle, but I took it out because I would get a burble around 1/3 throttle.

I would guess a 160 to 165 for a starting point. I have used both, the 160 felt better to me.
Or call yoshi and ask them, they should be able to give a good starting point.

I agree. It may run ok by just puting a jet in, but not at its peak performance.

It is easier to get at the carb after pulling the plastic. I loosten the front and rear clamps and tilt carb to the left side of the quad. Its tight, you have to watch the throttle cable. When pulling the bowl off to change the mainjet, make sure you have a good screwdriver or impact screwdriver fore the 4 screws, there in there tight.

Good luck

 

It runs perfect now, with lid on and the mods, so if i get a 160 main i should be good?