stock exhaust with baffles. no holes. plugs are new and look good.
I'll need more than "look good". Describe the deposit patterns.
Is the air filter stock or modified?
Backfire on decell is almost always too lean,
Here is why:
The engine RPM is driven by rear wheel on decel, while the throttle is closed and providing minimum fuel and air. (Mechanical slide carbs.)
If the mixture becomes too lean, it will not fire at all on some power strokes and be pushed into the exhaust system unburned on the exhaust stroke. It can do this for several cycles accumulating enough air AND fuel to support combustion, an ember then ignites it and you get pops on deceleration.
To cure this, the idle mixture is enriched, so even during decell, combustion take place on every combustion stroke.
Decel popping can also happen with exhaust system leaks and accumulated fuel in the exhaust. Extra oxygen from the leaks mixes with unburnt fuel from the incomplete power cycles, again creating a combustible mixture. Here comes that carbon ember again, and pop.
If you are satisfied that the exhaust header gaskets and system are air tight. Check the Pilot mixture screw position compared to book value.
Today's modern shops are taught to adjust them to minimum positions, or as lean as possible, which is not correct for the SOHC4 carbs without accelerator pumps.
Cheers,
