with nyrocs exhaust mod and k&n air filter. below 1600 feet, should i have to rejet. if so what size jets should i go with

 

142 front, 145 rear (kehin sizes)

But make sure your needles are shimmed or you use DJ needles on 5th notch. If you shim the needles you may have to drill and/or file them to make them fit and make them smooth. Try #3 or #4 washers. Someone I read used #3 nylon washers and drilled the hole larger for the needle. Put the washers under the needle so that you have at least .040 thick of washers to shim the needle away from the bore. Go up to .070 or more if you need it.

If you drilled larger holes in your exhaust, you may need bigger jets than 142/145.