Maybe i over read this but did you say your only using 30deg total ign timing? shouldnt you be about 36Deg min? ive also had similar issue before on a 340 with the carb i kept jetting up and the prob never got better just stayed the same with a miss at 6500 rpm but i did everything but lean it down some and when i did it was a new beast and revd clean to 7200rpm.
