Bike backfiring after it warms up.

[seabass]

My friend needed to get new cases for his bike. It ran fine before it was rebuilt. Now that it is back together, it starts up right away and sounds good. After about 30 seconds it starts stuttering and backfiring. From what the guy that put it together says, as best as I understand, it has spark but after it warms up, he uses an OHMS tester and no spark. It changes from cylinder to cylinder? I'm thinking a ground or something to do with the wiring. The TORS is removed and no airbox, just pods. He tried all different parts from another bike, from flywheel to cdi box. They tried spraying the reed cages and no leaks there. Maybe the head or cylinders not torqued correctly? I'm trying to help him out and figured you guys would have a good starting point for him. He'll be joining HQ soon, so he can elaborate a little more in this post after he registers, but sounds like a fairly easy fix. Any suggestions for him are greatly appreciated and thanks in advance for your replies.

Edited August 15, 2009 by seabass

[seabass]

bump.

[AKheathen]

do some research on this site, build a leakdown tester, to check for leaks. ohm the stator and ground/power circuit. have you replaced the plug caps/wires/coil? the coil is a pretty common one, so if you have a good used one off a sno-go twin, that should work. the plug caps are cheap, and do go out, acting funny in different ways. are you jetted properly? any other recent mods?