Backfire

I have a 1994 kz1000 police the bike sat for 4 years trying to get it back on the road after doing a rewiring , it back fires under load will idle and NOT backfire setting
Bored out to 1100 with wesco 12.5/1 pistons, APE K410 cams, Dana ignition and coils
Had the head off to repair threads re degree the cams as per APE specs everything good. carbs were clean
HELP!!!

If backfire is through the carb(s), one suspected cause would be too tight intake valve clearance(s), because an intake valve's failure to fully close and seal against its seat (when the engine is at normal operating temperature) will allow combustion inside the cylinder to partially escape through the intake port and into the carb.

Good Fortune!

And would also assure that the ignition timing advancer is functioning correctly and not rusty or stuck in position.

With engine running, may observe the timing marks under a timing light to watch the advancer quickly move back and forth as the throttle is blipped to vary rpm.
The advancer may require lube, or both clean-up and lube.

The weight arm pivots may be lubed through the timing inspection window.

The advancer should not look like this:


Good Fortune!

it thru the exhaust and the ignition timing advancer is working