I have a AH 100-6. Under load I get some valve chatter. When I put in high test, it goes away. Anything I can do with timing/ mixture to limit it with going the $ fuel route?

I would guess you're talking about engine knock as opposed to valve chatter if higher octane cures the sound. Check your timing, it may be advanced from spec.

I agree with GregW, accelerate hard you get chatter, mark your distributor position, move it a bit and try again - chatter goes away or gets worse, worse move it the other way. Crude,,,but it will identify what your issue is.

Johnb

Oh! Then time it right.