1969 Pontiac Firebird Part-Throttle Stumble

1969 PONTIAC FIREBIRD
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ANDYJOE
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  • 2 POSTS
about the car:

Pontiac 350
orig 2 bbl now 4bbl (Rochester Quadrajet)
HO heads
single exhaust 2.25 (sadly)
TH 350 Auto
2.56 open diff (sadly)

My problem is an acceleration issue.
either rolling or from a stop
when I give it between .25-.75 throttle,
the engine stumbles, pops out the exhaust, hiccups, and makes a scene.
But, give it full throttle and it runs like a top.

It has a new fuel tank, filter, and carb.
the fuel pump is probably original

The HO heads put the engine right on the verge of detonation and I've retarded the timing a little as a saftey precation.
I'm thinking that the problem is with the single exhaust and maybe the fuel pump. I'm hoping it's got nothing to do with detonation as a higher octane fuel is not readily available to me.

can someone confirm my diagnosis and/or give me any other ideas?

Thanks
Feb 7, 2006 at 8:26 AM
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CHEVY350GUY
  • ✓Experienced Mechanic
  • 109 POSTS
what did you change your timing to?
Feb 7, 2006 at 10:59 AM
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ANDYJOE
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  • 2 POSTS
I think the stock timing is between 9-12 degrees and I bumped it back about to about 5 btdc.
Feb 8, 2006 at 3:39 AM
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CHARLEY DION
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  • 8 POSTS
I think your problem is in the carb. First check for vacuum leaks especially at the throttle shaft. If that seems fine that remove the air cleaner and just in front of the air cleaner stud that is a larger hole. Take a small screw driver and find the power piston and gently push down. If it moves it most likely is OK but I suspect it is stuck. When you full throttle the carb you are into the secondary's and at 1/2 throttle you are still running on the primary circuit which requires the power piston to work.
Mar 10, 2006 at 2:12 PM
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Suggested Repair

Check for vacuum leaks, especially at the carburetor throttle shaft. Remove the air cleaner and, in the larger hole just ahead of the air cleaner stud, gently press the Quadrajet power piston with a small screwdriver. If it does not move, it is likely stuck and not enriching the primary circuit at part throttle; free or repair the power piston. Full throttle uses the secondaries, which explains why wide-open throttle runs normally.

Conversation Summary

A 1969 Pontiac Firebird with a Pontiac 350 and Rochester Quadrajet stumbles, pops from the exhaust, and hiccups at about one-quarter to three-quarter throttle, from a stop or while rolling, but runs well at full throttle. The owner has a new tank, filter, and carburetor, suspects the original fuel pump or restrictive single exhaust, and has retarded timing to about 5 degrees BTDC because of detonation risk with HO heads. An expert attributes the part-throttle stumble to the carburetor and recommends checking for vacuum leaks at the throttle shaft and inspecting the power piston, which must operate the primary circuit at partial throttle.

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