pinging part throttle, not full throttle

Hi, I have a 84 Capri RS with a 5.0. Anyways, it has a problem where it pings when at about part throttle, but if you punch it to full throttle, it doesnt, but if you let off to part throttle it'll start pinging again. The carb is a 600 Holley vacuum secondary carb and has just been rebuilt and set. The bowls are set perfect, and the idle adjustment mixture screws are 1 and 1/4 screws out. The timing is set to 12 and I have tried to set it down to 10,8 and even up to 14 with no luck. I have the spark plugs gapped right too. Does anybody have a clue what it might be. The ignition is a full msd setup. Could it be the plug wires, spark plugs, burnt valve, rotor and cap? Im lost. It idles smooth and sounds great with no noticable miss, but if you take the air cleaner off and give it part throttle it sounds as if it pops through the carb and if your sitting in it going down the road and you try to give it any more throttle then needed but less then full it sounds like its pinging. There has also been a slight downfall in power since this has started too. Any ideas are welcome. Thanks

The timing is set at 12* right now and the spark plugs are just stock replacement motorcraft ones. I know its lean at part throttle and mid range, Im running the 6.5 powervalve, just like the carb came with and it worked great when it did. Any other ideas. How do you adjust the advance on the msd pro billet distributors?

I stopped by a shop today and told him the problem. I forgot that I changed the intake gaskets when I wrote this and this is what he believes to be the problem. Right at the time the problem started, I changed the intake gaskets because they were leaking oil out the front. Well he thinks they are leaking air and causing a really lean condition. So this weekend, im going to change the intake gaskets again and see if this fixes it. Your still welcome to give me more suggestions if wanted. Thanks, Chris

I changed the intake gaskets and still have a leak in the same spots. I put silicone on both sides of the gasket and made sure they were aligned. Could my intake be warped and can it be planed like a set of heads so its straight again? Ill take a closer look at it but all ideas are welcome.