Back firing out the exhaust

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Here's the history 1973 351c rebuilt in 1990 ,,drove around till about 2004 (tires are stamped 2003 with very little wear) probably sat in a garage till i got it in 2018 new gas tank 15gal gas high test..timing set at 16* i set the carb adjustments too,, starts up and runs fine (but first time starting it it ran bad ran it for a while then started to run good) idles fine rev it up a little fine,, floor it ,,it back fires out the exhaust loud, i have not driven it yet,,could it be bad jets in the carb?

 
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1971 Mustang Sportroof M code
If it is actually coming from the exhaust and not the carburetor that means fuel is building up in the exhaust system and then igniting. Likely culprit is bad or fouled spark plugs. Backfiring through the carburetor can be an indication of a lean condition, caused by plugged passages in the carburetor.

I would start with checking the spark plugs and rebuilding the carburetor. When they set and the fuel dries up it forms varnish.

Did you blow out the fuel line? The same varnish likely formed in it, too.

 
Ok some great info thanks guys ,,one varnish can be still in the lines or carb,,,it has new spark plugs ,,so thats out,,and exhaust manifold does have a leak cause of a broke bolt ,,an old guy told me once fix the first thing you see wrong,,

 
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