I have a 1978 chevy small block 400 with 11:1 compression, forged pistons with the cylinders bored and heads milled. I run a 421/283 cam with gas supplied to a Holly 670 street avenger cam. I run av gas and only recently developed detonation with the motor. Ive checked the timing and it is at factory specs. Even backing the timing off doesn't do much good. What other factors could be causing my motor to become a ticking time bomb? Any input is much appreciated.  

High compression needs higher octane. Try 93 octane gas with 104 octane booster and see if that helps.  

He's running AV gas, that's 115 octane last time I looked. At the least it's well over 100 octane

I think the problem is in that you're using AV gas. The additive package for aircraft gas is different from gas for cars.

Since it just started recently, sounds like you've got some carbon build up problems. You might try some top engine cleaner.

Another trick is to switch to a colder spark plug.

Also, what ignition system are you running? You said you checked the timing, did you check to make sure the ignition wasn't getting too much advance?  

I mis read above comment. I am unfamiliar with aircraft fuel. I actually thought you meant average fuel(87oct). I am wrong.  

Carbon buildup does sound like a likely cause...

Does this engine have any noticeable blow-by?  

Sorry for delay in replying. My computer crashed taking down all aboard. But I will break it down into a few smaller posts as opposed to a larger one as I have lost a few big ones and retyping sucks. I have changed the plugs  -again. There was build up on almost all of them. But let me tell you what transpired around the time it started detonating badly. I met a dude at auto zone who worked there. He told me to change the weights in the dist. for quick revving. We changed the weights and springs,
but caused the motor to backfire. It backfired so loud I thought the power valve blew out. I changed it out, but I could see no difference in the old one vs. new one. I went one size bigger as it the plugs told me it was a little lean..  

I have since put the old weights back in,but left the bigger springs on the dist.
The av gas is 100 octane. The only time I run pump gas is when I have Hy-per Racing additive. That stuff is the real deal when it comes to Octane boosters. Runs like crap on anything else. The timing is at 4 degrees on the thing. What kind of additive would you recommend to clean out the deposits?? Your help on this is very much appreciated.  

one other thing. There is blow by, but nothing to wild. One other thing that is significant. The exhaust has one of those valves that open as temperature goes up. It appears to have been stuck closed. I have wired it open, but it is still detonating.  

4° initial timing is not NEARLY enough for av gas in a high compression engine. This is definitely the reason why your plugs are dirty. A vacuum advance connected to manifold vacuum is beneficial here, as this makes it easy to get even 20° initial, but 12-15° is a good starting place. Not having enough total timing may also be a problem since I highly doubt there is enough mechanical advance in the ignition to compenstate for such low initial timing. If this engine has non-vortec OE iron heads I'd say it may want as much as 38-40° total.

The rate at which advance is coming in has little to no affect if the advance isn't of the right amount. If you're worried about detonation at cruise speeds it may be a good idea to tone down the speed at which the advance comes in because too much advance coming in too fast is definitely going to be means for pinging/preignition etcetera.

Eliminate that EGR (exhaust gas recirculation) valve, it only heats the mixture and adds contaminants back to the burn which makes the engine more prone to detonation.