Is it "popping" through the carb? Or backfiring through the exhaust? If it is then i would check out the cam and springs like Dave said, It will backfire through both of them if springs are broken or bad,weak. The engine will run ok low RPM but when it try's to climb higher on RPM the springs cant handle it.Now this is only if you have done all what the other guys have told you to do.

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1936 1.5 wrecker,1937 gmc coe,1939 Chevrolet coe,1942 4x4 coe, 1942 coe,1946 dump,1947 2 ton dump,another 1947 2 ton dump,1950 coe,1967 c30,1937 cat 22,1936 Chevrolet 5 window foremans coupe, 1914 ford speedster.