OK, here is the truck...1973 Blazer with a 383, Stage II Quadrajet, mild cam and a 4 speed, mechanical fule pump and HEI. The issue is that my truck backfires when I have the RPM's above 3000rpm and then suddenly let my foot off the accelarator. Backfire, meaning a huge fireball shoots out of the exhasust (according to witnesses). This generally does not happen in 1st and 2nd gear, only 3rd... 1) I checked the timing first, and it is fine, running at 9 degrees, which I think should be fine... 2) Looked for a vacuum leak, but all is well there... 3) Checked the plugs and they were caked with carbon... Conclusions: a) Just getting too much fuel into the cyclinders and when the carbon heats up, blame there are detonations even after my foot comes off the accelator. b) Timing chain is stretching so that when I let off th eaccelarator from under a load, the slack doesn't close the valve all the way and blame there is detonation.. I am open to any suggestions, as my brain hurts.