1972 Pontiac Firebird backfire

THETHERAPYDOCTOR

MEMBER
1972 PONTIAC FIREBIRD
V8
2WD
AUTOMATIC
350 MILES

Newly rebuillt olds 455 big block with edelbrock performance package, heads, intake, 750 carb. The car ran great for two days. Now car backfires thru carb and exaust. Mostly exaust. It only backfires after engine is warmed up and I step hard on the gas. Also when I get my first backfire after engine is warmed up, car will stall when I come to a stop and is really hard to start again, it will spit and pop thru carb when trying to start it. When car sits over night it starts up great and runs great until engine is warmed up. Please help.

3 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Check the ignition timing, also could be wrong heat range of spark plugs, lean or rich mix, hot valves and carbon in the chamber. Also check the carb's accelerator pump. When you pull the throttle back you should see a stream of fuel out of the Acc, Pump-

Also check the fuel pressure-

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Tuesday, June 3rd, 2008 AT 3:09 AM

THETHERAPYDOCTOR

MEMBER
2 POSTS

Thankyou for your advice. I checked everything you told me to. It still backfires the way I explained. The only thing I didnt check, is what you said about a hot valves and carbon in the chamber. I do not fully understand what you mean about that. The engine only has 350 miles on it. Do you mean that a valve could be sticking. I dont understand why engine runs good until warmed up. Please help again. Thankyou.

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Thursday, June 5th, 2008 AT 3:08 AM

RASMATAZ

MECHANIC
75,992 POSTS

Do you mean that a valve could be sticking-could be

Have you checked the fuel pressure? If not do so-Check the ignition wires resistances/make sure its routed properly and inspect the distributor cap internally for hairline crack-

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Thursday, June 5th, 2008 AT 3:20 AM