sbc 350 rapid backfire

This is a 1977 Chevy p/u 350 with quadrajet that has a rapid backfire thru the carb when it accelarates. It idles fine, but when it revs it pops. Any advise greatly appreciated. :banghead:

Check timing 1st.

that's the first thing I did. It doesn't matter where the dist. is positioned it still does it.

update; found the problem; it was in the carb, the plunger with the two metering jets was frozen, did a rebuild and all problems cured.