My Pontiac 400 just started to slightly miss under a load but with heavy throttle especially over 3K rpm (was running good before). Slight miss but I know it's not right and am losing power. Sometime you just know it's not quite right.
Slight throttle doesn't seem to invoke the problem. Engine idles fine with no noticeable miss. Cliff Ruggles tuned quadradrajet (no leaky vacuum caps) . I have mechanical fuel pump with an HEI (mechanical advance total timing 36 degrees) and MSD 6AL w/ dial rev limiter set at 5200 rpm. #13 iron heads Roller rockers, chrome moly pushrods, double valve springs, hydraulic XE-274 Comp cam. Double roller timing chain. Cap and rotor are relatively new. Taylor 8mm wires. I am ruling out vapor lock since it still does this when cold. It's seems to only happen under a load. Fuel filter is not blocked. Any suggestions? I hope I don't have a wiped cam lobe or bent pushrod. Seen anything like this? I have been told Air/Fuel/Spark but I obviously am overlooking something.