Im gonna say 260-280 should be closer to where you should be on an FMF. Its a mid pipe. FMF jetting is odd. They use different needle taper and possibly different pilot tuning than something say like.. a Toomey.
Heres the deal. If I were you, Id buy a 300 and a 280. If it runs good on the 290, Id put a 300 in there. If it runs good on the 300, Id check plug and call it. If its boggy on the 290 Id put the 280 in.
See what Im getting at? If youre too rich, youll break up on the top end of the rev range. You want to be jetted so youre ALMOST too rich, but just right. Easiest way to tell if youre just right is go a jet size higher, if it breaks up on the jet size higher but runs good back where you started, youre good. Likewise, if you break up where youre at, but a size lower cleans it up, thats a good place to be also.
FMFs need more fuel in the low/mid than the mid/high area. So youll prolly get something by lowering your needle clip (that RAISES the needle up) so it richens you up in that area.
And of course, jet by feel, and sound. You can tell when youre too lean cause it will get boggy and sound like its dying out. If youre too rich it will sound like its loaded up and sputtery.
And of course, if youre too lean, you may also hear detonation which sounds like a little hammer banging on your cylinder by the exhaust pipe. If you hear that, definately richen it up.
