1984 GMC back fire when you let go of the gas

Tiny
GMCONYX
  • MEMBER
  • 1984 GMC SIERRA
1984 GMC Sierra Classic 150,000 5.0 engine

The old carburator blew the catlitic converter and muffler I changed the carburator no I have a handme down holley straight form another engine and the truck still pops loudly when I release the gas going don the freeway, it sounds like cannon fire. Also when I rev up the engine in the drive way and release the gas it also pops loudly. I don't want to change the mufflers yet otherwise it could blow them again. Please Advise me on what it could be.
I just finished changing out the motor this past weekend it was smoking and it wasn;t going to pass the emmissions test. It was popping before the motor change and is still popping after the motor change. I installed a short block.

Thanks for you help with this. !
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Friday, March 9th, 2007 AT 10:57 AM

4 Replies

Tiny
MHPAUTOS
  • MECHANIC
  • 31,938 POSTS
Hi there,

It sounds like that the engine may have a vacuum leak and it is running very lean, check around the carby with a small hose held to your ear & inlet manifold. You will soon hear a leak. (Keep away from fans and belt) also chek all vacuum hoses for condition, they should be soft not hard & cracked also check ignition timing, and also the carby float levels are not to low, check ignition leads for cross fire, but my money is on a vacuum leak.

Mark (mhpautos)
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Wednesday, March 14th, 2007 AT 12:01 AM
Tiny
JD329
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  • 16 POSTS
Sounds to me like it could be a firing problem. Check all of the components under the distributor cap.
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Friday, April 27th, 2007 AT 1:56 PM
Tiny
BLANTON1276
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  • 1 POST
Is the belt still on the air pump? If it is take it off.
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Tuesday, July 24th, 2007 AT 5:26 PM
Tiny
THATONEGUYWITHTHETRUCK
  • MEMBER
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My moneys on the distributer cap as well. Could be a leak in the vacuum lines, I had the same problem with my truck for a while, same year and model and make, check your lines, and it could be a firing problem
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Monday, December 20th, 2010 AT 7:40 PM

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