detonation / knock issue on 340

DON"T bother doing another thing after backing off the timing;
until the compression test results are in; else you may just be chasing your tail.



Exactly. If you know your timing is safe and your fuel is fresh and of the right octane and it’s still pinging ALL testing from this point forward should be with the engine shut off. Detonation is a killer of parts. When engines set this long anything can happen and tearing at least the intake off is cheap insurance. Gaskets are cheap.