Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Idle Problem

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runnerguy:
Hello,

This is my first post, but have visited this forum many times. The problem with my Q-jet is when I accelerate hard enough to activate the secondaries and then slow down and come to a stop. The engine will idle @ around 1400 RPM. I can turn off the engine and back on immediately and the engine will idle normally around 750 -800 RPM. I am very stumped with this problem. I bought an SR kit for this last year. And Cliff's book. The carb is stamped 07029207. I don't know the history this carb. or the background of the carb number. I bought it off ebay last year.

This Q-jet has 68 main jets, 37B primary rods, power piston spring measures 1.030" in length with 10 coils, BG secondary rods, K hanger. Tonight I replaced the secondary air bleed tubes (the originals were bent slightly at the tips) and drilled them out to .036. I have not measured the other restrictions.

Addition background is my car is a 58 Chevy Delray, 40 over 327, with flat top pistons, 882 heads, Comp Cam 265 DEH, Edelbrock Performer RPM Q-Jet intake, 1 5/8 headers, M-20 Muncie, 3:55 open rear end, and 235-60- 15's. It also has a point ignition, Total timing is 38, 14 base, 12 Mech., 12 Vac.

I have narrowed this problem down to the carb. My 600 Holley never had this problem. I really like the performance of this quadrajet except for this problem. Any advise would be greatly appreciated.

Thanks.
Dave

TommyK:
It sounds like either the primary or secondary butterflies may be hanging up for some reason. The next time it happens and before turning the ignition off, check to see if the there is a gap between the curb idle screw and the stop on the primary shaft. If the primaries are returning to the correct idle position push on the secondary linkage to see if the secondary butterflies are hung up.

runnerguy:
I don't believe they are hanging up, but I'll check. Because, I can turn the engine off, then on without touching the throttle and the engine will be back to normal idle.

I am under the impression that the carb is getting excess fuel. Because last week when this happened I was able to pull out on the choke pull off, allowing me to manually open the secondary air flaps, while the engine was idling high. When I opened the air flaps, allowing more air into the engine, the RPM had dropped. So, that's when I decided yesterday to change out the secondary air bleed tubes and enlarge them as indicated in the book to .036. with no luck on this problem. The bent tubes were the only thing I can tell was wrong this this quadrajet.

Dave

Shark Racer:
Engine vacuum can take a throttle blade that feels normal with the vacuum off and make it hang with the vacuum on.

Will the primary throttle return to completely closed via the spring action of the accelerator pump alone?

runnerguy:
Thanks Shark racer

I will have to get the engine to do this and look at the throttle position more closely later this afternoon. When I had the carburetor apart the throttle shafts felt tight, by that I mean not loose and worn out. When the engine idles high I have stepped on the throttle quickly to see if they are stuck but it doesn't idle down only turning off and back on the engine will make it idle normally. I have also checked the vacuum advance to see if it is getting a signal and it is not, I have checked and made sure the mechanical advance weights are moving free and not sticking.

Thanks,
Dave

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