Engine troubles (carb)

[Mendo70C20]

I have a 70' K20 with a Blueprint 383 stroker and Holley 750 carb. Truck was running fine, and then recently, i can't keep it running. I pump the gas pedal twice, crank it over and she fires right up, i then keep the RPM's at 1500 or 2000 to let it warm up. If i let the RPMs drop below 1500 she'll start to stall out and i can't catch it, or stop it by giving more throttle. Then once she's stalled, she won't start again. Just cranks. I've tried keeping it floored, pumping the gas, you name it, won't start back up. First thought was a plugged needle thing thats below the float bowl adjustment screw, so i pulled that, blew it out and reinstalled. No luck. Then i thought maybe a vacuum leak, so i checked all my vacuum hoses, and they all look good. If the truck would stay running i'd try spraying starter fluid around the base of the carb or near vacuum lines to double confirm no leaks, but i can't keep her running. Fuel filter is full of fuel. Both sight windows for the front and rear float bowls have fuel. I'm kinda baffled. Thoughts?

[HO455]

What is the choke doing?

There is a test that can be done but it has a dangerous element to it. Put some good leather gloves on and when truck is running hold your hands over the carburetor. Your trying to slowly choke the air off to the engine. If you can block air off and get the engine to run better then the engine is running lean or getting too much air. Could be a vacuum leak or a bad gasket in the carburetor or other internal carburetor issue.

If the problem gets worse then the engine is getting too much fuel. Could be stuck/sunken float, bad gasket in the carburetor, too much fuel pressure or several other things.

If this only makes the engine mildly worse then I would dive into the ignition system. As they say 90% of all carburetor problems are electrical.

DO THIS AT YOUR OWN RISK!

Carburetor backfires can cause burns.

Good luck and keep us posted.

[Mendo70C20]

Originally Posted by HO455

What is the choke doing?

There is a test that can be done but it has a dangerous element to it. Put some good leather gloves on and when truck is running hold your hands over the carburetor. Your trying to slowly choke the air off to the engine. If you can block air off and get the engine to run better then the engine is running lean or getting too much air. Could be a vacuum leak or a bad gasket in the carburetor or other internal carburetor issue.

If the problem gets worse then the engine is getting too much fuel. Could be stuck/sunken float, bad gasket in the carburetor, too much fuel pressure or several other things.

If this only makes the engine mildly worse then I would dive into the ignition system. As they say 90% of all carburetor problems are electrical.

DO THIS AT YOUR OWN RISK!

Carburetor backfires can cause burns.

Good luck and keep us posted.

It's just odd that it was running great then now this.

[Rust_never_sleeps]

Cliff's "tip-in" test is a less flame-worthy version of the above procedure
I'm not sure from your description whether that's where the problem lies, but it might help to rule some things out.
Mine(which isn't a Holley) runs as you describe on a cold day because I currently have no choke, BUT it will restart

You can use our “tip in” procedure to help verify the primary part-throttle calibration. With the engine completely warmed up, set the speed via the fast-idle or idle-speed screw to 2,000 to 2,200 rpm. Gently “tip in” the choke flap or partially cover the choke housing if the choke is not used. Don’t close the flap enough to choke out the engine, but just enough to richen up the mixtures a bit. You should see an engine speed increase of about 50 to 100 rpm. This tells you that your primary settings are just on the edge of lean. If there is no engine-speed increase or RPM fall off some, pull a small-manifold vacuum hose from the carburetor. If the engine speed increases due to the additional amount of incoming air, the mixtures are a bit rich. If you are using an APT carburetor, set the metering rods to provide a very slight RPM increase during the “tip-in” procedure. Verify the settings by pulling off a small manifold vacuum hose; the engine RPM should stay about the same or the engine should slow very slightly. This procedure provides a good starting point. You can now place the carburetor in service and do some real-world testing. Future metering changes may be needed, as some vehicles prefer slightly rich or slightly lean settings for best overall efficiency.

Ruggles, Cliff. How to Rebuild & Modify Rochester Quadrajet Carburetors (S-a Design) (p. 100). CarTech Inc.. Kindle Edition.

[Rust_never_sleeps]

Maybe get a mirror over the carb and see if your accelerator pump is doing its thing

[Palf70Step]

First thing I normally do with something like you describe is check/change the fuel filter. If it is a see thru one, see if any trash is in your filter that may indicate something got thru and clogged something.. I also spray carb cleaner down breather to see if it clears up everything. IF it starts and still sounds bad I look and spray carb cleaner to check for vacuum leaks. If that didn't clear me up I start going deeper into carb and/or ignition.

[1967GMCC20]

Choke settings or Vacuum Leaks, most common. Did UT occur, when Temps drop.

[LockDoc]

-
If you have never replaced the sock filter on the fuel pickup tube, in the tank, That would be one of my first suspected problems. They do disintegrate and plug up the tube or filter.

https://americanclassic.com/1967-87-...3-8-sku-23-598

They just slip on the end of the fuel pickup tube in the tank. Any parts vendor should have them.

If you think it might be flooding hold the accelerator pedal all theway to the floor when you are trying to re-start it.

[RichardJ]

If it's a Holley and running rich, did you check the power valve. Power valve could be blown, loose or bad gasket.
A power valve problem can happen without warning.