Quadrajet slight miss at idle

[Fairlane514]

I have the original quadrajet on my 84. I rebuilt it and put brass bushings in the primary throttle shaft.  The previous owner replaced the 305 with a Goodwrench 350. The motor has about 5000 miles on it and seems pretty solid.

There is a stumble or miss at idle, but once off idle it runs fine and has good power.  It has about 18-19 inches of vacuum with a steady needle and a Summit HEI distributor. I have been all over the place with the idle mixture screws. The acceptable range seems to be between 4 1/2 to 6 turns out, and not a lot of variation on the vacuum reading.

The carb does not seem to have a warped top plate and I can't find a vacuum leak anywhere. Plugs and wires are new..........one possibility is an arcing spark plug wire, but it isn't obvious.

[markus]

4 1/2 to 6 turns sounds quite a lot to me, chance that there is a blockage somewhere in the idle circuit so it gets to run a bit lean...

what type of quadrajet you're running? got the serial number at hand?

[bd]

4 1/2 - 6 turns out was not uncommon on some applications - 6 turns was about the limit - main thing is that the screws are adjusted the same for the smoothest running and highest vacuum at idle. 

I have the original quadrajet on my 84.  ...There is a stumble or miss at idle...

So, is it a stumble or miss?  ...or a misfire causing a stumble?  A stumble occurs upon acceleration.  A misfire can occur under nearly any circumstances and results from one or more cylinders dropping out. 

common causes of a stumble are a poor accelerator pump shot, very low float level, bad distributor vacuum advance, insufficient initial timing, or a misfire.

A misfire can result from spark not reaching the plugs, fouled plugs, spark leaking to ground along the wrong path, flat cam lobe, weak cylinder, and so on.  A misfire at idle is probably ignition related.  Check the "arcing plug wire" first.  Do you have more details?

[454Man]

Are there any spark plug wires touching the engine or any other metal parts?

[bake74]

So, is it a stumble or miss?  ...or a misfire causing a stumble?  A stumble occurs upon acceleration.  A misfire can occur under nearly any circumstances and results from one or more cylinders dropping out. 

     I read it as he means while it is idling and not accelerating, therefore I would go with misfire is that truly is the case.

[Fairlane514]

Carb number is 17084226.  It runs fine off of idle with no miss or hesitation, and smooth while cruising. I can rev it while under the hood and no bog or hesitation occurs.

So the issue is just at idle.  It sounds like a slight miss or burble  every now and then, I guess somewhat rhythmic.  I have 12 degrees initial mechanical advance with 10 degrees vacuum advance on full manifold vacuum. My Summit HEI has an adjustable vacuum advance. 

I will check again for arcing wires.

I started at 3 turns out on the idle mixture screws and it ran terrible. I hooked up my vacuum gauge and turned the screws out to 5 and it ran smoother with 18-19 inches of vacuum. 5 1/2 -6 may have helped some but not much difference in vacuum reading.   

May be possible the idle tubes have some dirt in them.   I tend to agree with the idle circuit being the culprit, since it seems to be fine off the idle circuit..............

[bd]

Have you tried hovering the carb to richen it slightly at idle?
If you think it's the idle circuit, alternate B-12 and compressed air to blow the idle circuit out again.  When you remove the cover recheck the float bowl gasket for tears, etc, and check the float bowl cover for flatness using a straightedge across the air bleeds.  Air horn warpage is common.  If an automatic trans, does the problem occur in gear or only in neutral/park?

[Fairlane514]

The problem is there in neutral and in gear (automatic). When you say hovering, do you mean placing my hand over the choke horn?  I have done this and it made no difference in idle speed.

[bd]

By hovering I mean a rag or other means to slightly choke the airflow and enrichen the mixture - I use a rag to help protect may hand in case of backfire.

"If" I'm picturing the problem correctly, I think your miss is not caused by carburetion - although fuel delivery might exaggerate it.  Nonetheless, you should verify there is no idle circuit problem.

[markus]

When you say hovering, do you mean placing my hand over the choke horn?  I have done this and it made no difference in idle speed.

then there may be a vacuum leak somewhere, leading to a lean condition and maybe cause misfires.

resuming possible causes:

• ignition related (is likely to occur also off- idle)
• clocked idle passages
• vacuum leak (not so likely since your vacuum readings are quite ok)
• low compression on one of the cylinders

attached btw a nice vacuum diagnose guide...

[Fairlane514]

I disassembled the carb today and put in the carb cleaner bath at a friend's shop.  Hopefully I can get rid of any blockage that may be causing the problem.

[codyC10kid]

I disassembled the carb today and put in the carb cleaner bath at a friend's shop.  Hopefully I can get rid of any blockage that may be causing the problem.

When you took the Carb apart,where you very carefull with the metering jets....i bent mine accidentally and full throttle was really bad no power dogs and chucks....i put an edelbrock on

[Fairlane514]

Put the carb back together and drove it this morning and the miss seems to be gone.  It still likes 5 1/2 turns out on the idle mixture screws.

[codyC10kid]

Put the carb back together and drove it this morning and the miss seems to be gone.  It still likes 5 1/2 turns out on the idle mixture screws.

well thats good,at least you had better luck than i did

[Fairlane514]

Yes, one of those rare occasions.....................I probably just jinxed myself