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Old 12-04-2003, 16:36   #1 (permalink)
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confused Carb backfiring on hard acceleration!

When I floor it, my carby backfires.
Would an untuned carby cause this? It has just been rebuilt.
What else should i check to fix this?
Should i get the dizzy regraphed? I think it was running straight gas before.
Im unsure of the condition of anything on the engine and the engine itself as it is all second hand.

Thanks a lot,
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Old 12-04-2003, 17:01   #2 (permalink)
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Re: Carb backfiring on hard acceleration!

Well if timing is way out, and it andvances too far on full throttle would cause it, so a dizzy regraph may be the go, or is the dizzy shaft etc, just worn out.
Have you checked its not dumping way too much fuel on full throttle (only slight possability of this being the reason)

BTW - what motor, what carby, and what car we talking about.

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Old 12-04-2003, 17:41   #3 (permalink)
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Re: Carb backfiring on hard acceleration!

If it happened after changing the carby, its most likely the carby. A backfire throught the carby means its lean. Exactly when does it backfire? Immediately when you floor it?
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Old 12-04-2003, 17:50   #4 (permalink)
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Re: Carb backfiring on hard acceleration!

Screw the accellerator pump screw in Or work on more acc- pump.. May have to change acc cam on carb. Too lean main jets. Leaking inlet some where ???
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Old 12-04-2003, 21:07   #5 (permalink)
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Re: Carb backfiring on hard acceleration!

Too many variables.

Narrow it down to what you have changed/done for it do start doing this.

List as much info as you can.

Does it do it under light acceleration as well?

Timing retarded or advanced too far can give similar symptoms to what you have.

Although if you have just had the carby apart and reco'd it. My guess would be that it is running lean and as suggested, guessing its a thermoquad, your accelerator pump needs attention.

post more details
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Old 12-04-2003, 21:10   #6 (permalink)
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Re: Carb backfiring on hard acceleration!

Its doing it as soon as i floor it.
Its in a 351 cleveland with the thermoquad. It was doing it before as well. It seems to only happen before the car really warms up.
It still has a flat spot and I think that is due to the air door for the secondaries. I read that on a website that shows how to adjust them. It is hard to adjust cos you need a special tool to do it.
Does anyone know a good place to take it to to get tuned on a dyno? Someone that wont tell me to get a holley.
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Old 12-04-2003, 21:53   #7 (permalink)
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Re: Carb backfiring on hard acceleration!

too lean in 2 words.
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Old 12-05-2003, 01:20   #8 (permalink)
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Re: Carb backfiring on hard acceleration!

You don't need a special tool for the rear air door. I have done it with 2 blade screwdrivers.
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Old 12-05-2003, 01:45   #9 (permalink)
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Re: Carb backfiring on hard acceleration!

Check for intake leaks, eg: back of carby, hose that goes to rocker cover pvc valve not on or cracked or hose perished, look around to make sure all vacum hoses are connected.
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Old 12-05-2003, 04:43   #10 (permalink)
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Re: Carb backfiring on hard acceleration!

Quote:
Originally Posted by DansEdgli
Its doing it as soon as i floor it.
Its in a 351 cleveland with the thermoquad. It was doing it before as well. It seems to only happen before the car really warms up.
It still has a flat spot and I think that is due to the air door for the secondaries. I read that on a website that shows how to adjust them. It is hard to adjust cos you need a special tool to do it.
Does anyone know a good place to take it to to get tuned on a dyno? Someone that wont tell me to get a holley.
Get a Holley... Seriously. Having mucked around with the Thermoquads myself and later changing to a Holley I never looked back.. The problem you describe is from a lean out from the secondaries, mine used to do the same thing and I had to run less than optimum timing as a result to stop it pinging because of the lean condition at WOT.

If you went to a Holley set up properly you would get gains all round especially by optimising the timing. The 750 3310 square bore vac secondary is cheap and works well, 650DP also good on a stocker but more expensive but would also be punchier and better throttle response but more fuel around town.
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