1981 k1500 stalling....

[Frogman]

1981 GMC K1500, 305 engine with Edelbrock 1406. this has been used as a plow truck around our property for the last 15 years. horribly neglected. it got to a point where it really didn't want to run, so I decided to dive into it. so far, new plugs, new cap and rotor, new ignition module, new coil, new fuel pump, carb cleaned and rebuilt, new vacuum hoses, new PCV valve. I've got it to the point where it starts right up and idles beautifully on high idle, once it drops down to normal idle, it will stall in 1 to 2 minutes. Does not sputter, just stops. Starts right back up. tried adjusting mix on the carb, have both Advanced and retarded timing, sprayed carb cleaner all over looking for vacuum leaks. looking to the group wisdom for something that I haven't thought of. thanks for any help, and for letting me be part of your group.

Cheers.
Rick

 

[Sad Sack]

Have you used a vacuum gauge yet to check actual running vacuum?
And took a mechanical reading of fuel pressure with a fuel pressure gauge at idle and say 1500 rpms?

 

[Ricko1966]

What transmission does it have. If it runs good at fast idle,then doesn't run off of fast idle. Okay fast Idle the choke is on and the throttle plates are held open pulling fuel from the transition circuit. To me that indicates you are probably lean at idle. Clogged Idle passages,intake leak,ruptured vacuum modulator. Etc.

 

[Frogman]

Have you used a vacuum gauge yet to check actual running vacuum?
And took a mechanical reading of fuel pressure with a fuel pressure gauge at idle and say 1500 rpms?

Thanks Goober. I have not checked vacuum or fuel pressure. If I can get the gauges needed, what numbers should I be looking for?

 

[Frogman]

What transmission does it have. If it runs good at fast idle,then doesn't run off of fast idle. Okay fast Idle the choke is on and the throttle plates are held open pulling fuel from the transition circuit. To me that indicates you are probably lean at idle. Clogged Idle passages,intake leak,ruptured vacuum modulator. Etc.

Thanks Rick. Not sure what trans is in it. How can I tell?
How can I diagnose the idle passages, etc.?

 

[Sad Sack]

Thanks Goober. I have not checked vacuum or fuel pressure. If I can get the gauges needed, what numbers should I be looking for?

On a stock engine if I can recall, vacuum at warm idle MAYBE around 21hg steady and fuel 4-6 pounds at carb. Others will hopefully add their thoughts on this question.

 

[Ricko1966]

Thanks Rick. Not sure what trans is in it. How can I tell?
How can I diagnose the idle passages, etc.?

Is it automatic or manual. If automatic what does your dash say for forward gears 3 or 4 321 vs. Od 321

 

[Snoots]

Clogged cat.

 

[Frogman]

Is it automatic or manual. If automatic what does your dash say for forward gears 3 or 4 321 vs. Od 321

Automatic. 3 forward gears.

 

[Ricko1966]

Automatic. 3 forward gears.

Okay that has a vacuum modulator in the transmission. Pull the hose off at the transmission,see if the hose has transmission fluid in it. If so the diaphragm in the modulator is ruptured and you coukd be sucking transmission fluid. Plug the line and see if it runs better. As for plugged Idle passages,that should have been caught during the carb rebuild stage. Do the mixture screws affect your Idle? Is the throttle stop screw cranked in or barely in. If it is screwed in to far you are being on the transition circuit,bit the Idle circuit. You can pull the carb and look at the transition circuit. If you can see more than a square of it with the throttle shut you are cracked in too far.

 

[Craig Nedrow]

Did you check the fuel level, though if it starts right up?....

 

[jcperformance]

Sounds like it’s dying when it comes off choke. Check idle speed and mixture again, and make sure the idle circuit isn’t clogged. Also verify fuel pressure and float level.

 

[Frogman]

Thanks for all the help guys. Apparently the problem was the new ignition control module was crap. Replaced that and all is well.