I was interested in the article in the news letter about a 4410 carburetor backfire at 2000 to 2500 rpm when the owner floored the gas in third gear.The writer fixed the problem he said was a lean condition back fire by installing a 8.5 power valve with 66 primary jets, the card had vacuum secondaries.I would diagnose the problem as a rich situation not lean, by using a 8.5 power vale that actuates at 17 lbs of vacuum ( 8.5 x 2= 17 this is how you determine the size power valve )it sounds like he corrected the problem by eliminating the power valve altogether and is now running a very lean 66 jet only.the size of the cam is unknown, he also does not seem to have checked his max vacuum at idle which is how you determine the size of the power valve ( 13 pounds of vacuum a 6.5 power valve. )to have 18 lbs of vacuum would be a very tame stock cam. I am not a carb wizard so any input on why this not the case would be appreciated.
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