Ever since I did my tune up back in Mar-Apr timeframe I've had small issues, well I think I'be uncovered the likely culprit and now I need a solution. I'm experiencing detonation while pulling mountain passes or when the engine is in a full load situation. I've already backed the timing WAY off, like 0 degrees and tried premium fuel to reduce the problem, no luck. The tuneup parts were all stock level replacements and I replaced them all since the problem started, cap & rotor, points, wires & plugs, fuel pump. No other engine parts have messed with. So what else could cause detonation, limited fuel supply, dizzy that is failing in some manner, timing issue that isn't related to the basic setting?

Thanks,
Kirk

Hook your timing light up and disconnect the vacuum advance. Rev it up to see how much mechanical advance your getting.

Did you remove the dist? Check to verify thast your wires are properly attached and in the correct order. And, as stated, check you advance, w/ out being hooked to a vacuum source.

Kirk you could definitely have an issue in the dizzy. Zoso's Bronco had issues where the mechanical added too much and he had some severe detonation. That's what it looks like you got too.

I pulled the vacuum (hooked to a ported carb inlet, so it only kicks in when the carb opens up) and revved it up and I've got 20 degrees mech advance, sound about right?

Sound good to me. Now do it with the vacuum advance hooked up and see what you get.

Same numbers, no diff hooked up or not. So maybe the vacuum can has failed.