Just got my engine back together (again) and now I am getting detonation in the manifold just below the carb. Tried moving the dizzy around for timing but it doesn't seem to help much. What could be the problem?

You might have a bad intake valve or broken spring. What was the problem the first time when you got the engine back together that you had to take it a part again?

Blew out the back rubber gasket on the manifold.

BTW it only does it under load not when it is just running (at idle).

What year car and engine ?

1969 RS, 5.7l , dual plane manifold, flowmaster headers, 4b edelbrock 650 carb.

Carbs don't detonate. To blow out back intake manifold seal ( if thats what i understood) you had a back fire, pressure from the cly got past the rings thus went to crank case, thus over pressured and blew out gasket,, but that musta been a big one to do that. Yes, as someone mentioned, you could have an intake that is not sealing and leaking fuel into an exhaust port , thus ignition. Do some checking,

I didn't have a backfire, I just didn't seal it properly when I replace it and the pressure found the weak spot. Is it possible that I have the dizzy off a gear? If I did would it still run yet have issues under load?

OK, to drag this out just a little more....I ensured the dizzy is on the one wire and it is at TDC but the timing was off so after checking... the vacuum advance isn't working. Going to replace it and see how it works out. Thanks for the advice!

Ok, this is going to drive me nuts! I replaced the vacuum advance but it still isn't advancing the timing. Do I need to rebuild the HEI?