I have the following mods:
FMF Fatty Pipes
Pro Flow
Air Box Lid Off

The question is: What jets should I be running?

I have 270's and the needle raised one notch from stock. Also, the airscrews are 1.5 turns out.

The bike seems to run great but, others in this discussion group have indicated that they are running 300-320's. The same set-up with the airbox lid on ran so rich that it would not peak out.

 

i got a 99 banshee and have a cool head, unifilter,boysen reeds, boost bottle,reed spacers,fmf fattys,power core 2 silencers,and i only have 280 jets and diff needles and mine runs fine and on top end and low end i dont have no problem


joe jute 99 banshee and 99 warrior

 

Reading your plugs is going to be your best bet on knowing if your jetting is right. No 2 bikes are alike. they may be close, but not identical. Its hard to compare on here unless you find someone that lives in the same area or at the same altitude. If your plugs look good, and the bike runs good, stick with the 270's.

 

With regards to the airbox cover: How many jet sizes between cover on and cover off?

 

You will be better off to run without the air box lid. It will breath better. You shouldn't have to jet more than 2 sizes at the most. Keep a selection of jets on hand.
The cooler it gets out, the larger the main you will need. Cold air is more dense than warm air. Jets are alot less expensive than a rebuild due to seizing it. Keep an eye on your plugs and make adjustments accordingly.