I need some sage advice from this crew. I have an 81 with the original L81, 4 speed, and Quadrajet. I purchased it last fall and spent 100's of hours this winter working on it. (It ran OK when I drove it home). My plan was to rebuild the carburetor before Spring, but I ran out of time. I did replace the wires, cap, rotor, timing chain, timing sprockets, harmonic balancer, oil pump, rear main, valve cover gaskets, water pump, heater core, radiator, all engine vacuum lines (not the headlight lines nor the heating system vacuum lines - I tested them when the dash was apart to replace the core and they held vacuum without issues) - as well as refurbing or replacing the entire rear suspension. Cleaning the crud off the parts i was working on was the longest part of the process.
I adjusted the TPS to 0.56V, set the timing to 9° (based on various threads in this forum) and, earlier this Spring, I fired her up - she started and ran great. It started like champ. I let it warm up thoroughly and it drove great. Since then I have driven it many times with no issues - it always starts right up and runs well (i always warm it up a while before i drive it).
The last time i wanted to go for a ride, it fired right up as usual, but it would die as soon as the shot of fuel from the accelerator pump burned out. I could get it to fire back up immediately by priming it with the accelerator pump - and i could sort of keep in going by pumping the throttle, but it would die as soon as i stopped pumping the gas.
I have checked the choke and it operates properly. I rechecked the TPS and it was right on. The Mixture Control solenoid has the proper resistance and clicks when the ignition is turned on. Disconnecting the MCS does nothing. The car still fires right up and then dies. The solenoid plunger moves up and down freely.
I am running out of ideas - Can anyone point me in a logical direction? I really appreciate the help of this group!

 

Have you checked the fuel filter located in the fuel line inlet housing on the carburetor?
Have you done a fuel pressure test?

 

It's getting fuel as the accelerator pump functions. Plugged up low speed circuits in the carburetor?

 

i replaced the fuel filter this Spring. probably only 50 miles on it.

 

Potentially, but with a new filter and going from driving fine to instant problems i don't know how the low speed circuits could both clog that fast.

 

Again, perform a fuel pressure test. Low pressure will provide enough for a pump squirt but the float level will be too low to permit the engine to run.

 

Simple enough to check fuel pump output. But, how does it go from running good to a circuit plugged up instantly? Old scale formations on the walls and floor of the float bowl break loose. Instantly get sucked into the low speed or intermediate jets. Blocking it.
Bottom line, sounds like you have a fuel problem. I think you know where to look. Unhook fuel line from carb. Point line into bucket. Have someone crank the starter for 5 seconds. Lots of fuel?

 

Not saying this is your problem ( no indication of a backfire - lean condition) but maybe worth looking at, my 76 L-48 was running great one day then next day total loss of power with engine dying, only way to keep running was to pump accelerator pedal. Would also occasionally shoot a nice flame up through the carburetor as well with the air cleaner removed. After extensive troubleshooting and deciding to remove the carburetor to have a look internally I found the carburetor base gasket had become brittle and broke apart causing a huge vacuum leak. Good Luck.

The large section missing broke off while I was scraping it off the manifold but there was a small piece missing....

 

Epilogue - at least for now.

Thanks for all the suggestions!!!

I went over the vacuum lines and fuel lines today and found nothing wrong. I checked the carb gaskets and did not find any leaks. As a final effort before pulling the carb for a rebuild, I reset the two idle mixture screws: turning them both in to lightly seat and then turning them both out 31/2 turns. The car started right up and ran great. I know it doesn't mean the problem is totally solved, but I thoroughly enjoyed the ride today. I'll take it one ride at a time!

 

That's the starting setting. Did you then use a tachometer or vacuum gauge to set best lean idle?

 

I set the timing and used a dwell meter to set the idle mixture screws when the car was running about a month ago. There is clearly something wrong with the carb. I could not get the dwell to move off of about 20. meaning it would not move at all no matter what i did to the mixture screws or the air bleed. At that time i just set the idle mixture screws and the air bleed to the startign settings and it ran alright.
Sadly, the car would not run again today. I am working with Quadrajet power to get the rebuilt parts and ordering a new fuel pump. we will see where it goes.

 

The dwell meter was set to 6 cylinder when attempting to set the air bleed to 30?

 

Yes. not sure if the MCS is bad. it has the correct resistance.

 

I had the carburetor in my 81 rebuilt by a - reputable- carb business out of Oregon and the car just didn’t run right. I looked everywhere for the problem,went so far as to isolate all vacuum ports on the carb with plugs. When setting the idle air mixture screws with a dwell meter the adjustments made no change on the dwell meter. I finally found the O ring IDLE AIR VALVE was worn out. You would think that a very reputable carburetor shop would have replaced the O ring. The car still runs a little rough when cold at first start up but does run a lot better. I got burnt out fighting this issue so I haven’t messed with it for awhile. If anybody is thinking about sending your carburetor to some hack in Oregon,might want to second think that.

 

It's pretty easy to rebuild the carb , I did mine a few years ago replaced everything inside after a good cleaning. My opinion if you have any carb issues do a rebuild instead of trying to adjust everything the carb is old it needs it. A new fuel pump is a great idea. Another thing I did was replace the computer , auto zone got me one for cheap money.

 

This is a great insight- thanks. Every little bit of experience that others provide gets me closer to getting this car more reliable so I can enjoy being in and not under it.

 

I am super surprised that Autozone got you a computer for your car. I would replace mine in a heartbeat if I can get a replacement ( not a used, untested, probably burned out one from EBay). Can you let me know the part number or other identifying information so I can work with my local Autozone guys to order one. I need to give them a direction to look because they have tremendous difficulty finding basic parts for the car when I go down there unless I come in with their part number. It would be Christmas morning if I could get a new (rebuilt) ECM from the Zone!

 

I found the ECM on the Autozone site. Unfortunately- it is unavailable to even order. Now that I know there may be rebuilt ones available, I will dig deeper to find one.

 

Try spraying carb cleaner into the carb, If the problem goes away, then I would check the fuel pump. If the check valve in the pump is leaking, you'll lose fuel pressure. How many miles on it?