who did the valve shim adjustment?
are you sure timings on now? how were the intake valves? usually those are the ones that need the most attention. when you pulled the plug was it dry or wet? if dry then your not getting gas with a backfire out the carb i would guess your timing is off and if the timing is off i would guess its not looking good for your valves. i would pull your plug on the mag side put the T in the little notch and pull the valve cover and check your timing marks. i wouldnt try cranking on it until you make sure timing is on 1 tooth will crush a valve into a piston and toast a head.
04 Polaris predator 558 Flat track quad
04 Polaris predator 500 trail build(in the works)
started life as a replacement frame so a ground up build
Build predator and outlaw lowering kits