Q-Jet rough running in cold weather

[madmike]

I have a 1979 K20, 350 ci 4 bbl QuadraJet carb, SM465, 153,xxx miles on it. The engine and carb appear to be all stock, unmolested and unmodified. For its age, the truck runs well. I recently replaced the distributor cap, rotor, wires and plugs. It starts and shuts off fine, but has a rough running spot just off of idle. The colder the weather gets, the worse the problem.

When starting the truck, it will run rough (miss) for a few minutes, and then smooth out and go into it's high idle mode. After the truck fully warms up and the idle speed drops down to normal, the choke is fully open and ready to go. That seems to be the point when you give it a little gas to move forward, that it wants to bog out, unless you feather the gas somewhat. This problem fades a little bit, especially if the truck is driven for a longer period of time. I have sprayed the carb with cleaner, while the engine was running, in an effort to find vacuum leaks, but there weren't any. The spark plugs appear to be slightly colored due to oil (high mileage), not overly rich or lean from gas.

Other than rebuilding the carb, is there some external adjustment or part that can be replaced to fix this rough running problem? I hate to pull the carb and rebuild it, I have never had a fully successful Q-Jet rebuild work perfectly.

What do you Q-Jet experts think?

 

[vortec]

there are two idle mixture screws way down on the front of the carb. i believe cranking them in richens the mix (can't swear to that. been a while since i messed with it). they won't help midrange or high rpm, but idle should improve and probably help the off-idle stumble

my q-jet runs fine year-round. a bit rich in the 100 degree summers and just a tiny bit lean in the winter. your winter is a different beast than mine, though. it's been dropping into the 20s around here lately and people are acting like they'll die any second. i just toss a sweatshirt on and try to avoid wind.

 

[madmike]

C'mon, I know there are some Q-Jet experts out there...!

Vortec, I am aware of the idle mixture screws, but I don't (think) this is a situation where they should be messed with much...

 

[rcamacho]

Other than the idle mixture, you might want to check to see if the primary throttle shaft is worn. Since the primaries are so small on Q-jets they tend to be very sensitive to any leaks past the shaft. If worn, you'll need to or have the shaft re-bushed. Also, check to see if the accelerator pump plunger is still sealed - they sometimes leak as well.

http://www.quadrajetcarburetors.com/
http://www.tinindianperformance.com/Qjet parts and services.htm
http://www.supercooltools.com/

 

[dyeager535]

Do you have any emissions components on it that are thermally switched?

It doesn't sound like idle mix to me, and doesn't sound like choke either, because you say it's wide open when up to temp.

On my '83, the primary throttle shaft was so worn that idle was never the same RPM, unless I intentionally monkeyed with the gas pedal to get RPM's the same. Just an indicator of a problem there if it's really bad.

 

[madmike]

By spraying carb cleaner around the primary throttle shaft, and not detecting a vacuum leak, I figured the bushings there were probably OK. Maybe something in the accelerator pump circuit is leaking by.

I don't think I have any emissions equipment that is thermally switched (none come to mind), but I'll have to double check that as well.

 

[Hogback Fabrication]

For the cold start rough running issue:

That can be typical of a carburetor with a choke, it depends on how cold it is and how bad it runs.

If it runs pretty bad the choke could be misadjusted.


For the stumble on acceleration (once the motor is warm):

Sounds like part of the accelerator pump is leaking and a rebuild is in order.

Total rebuild should cost you ~$40 (get a new float too, make sure you set the float level and choke according to the instructions in the kit)