I had this truck running really good and smooth but this spring it has taken a turn for the worse and I have no idea why. I tuned the carb and it was all good but a week later it wouldn't start without keeping my foot on the pedal. Yesterday I went out to see if adjusting the carb would help with that and it fired right up with no problems.... at first.

Shortly after, maybe 2 minutes or so, the engine started to get weaker and weaker and on the verge of stalling out. It would rev up and then drop down, rev up and then drop down, over and over again. I turned up the idle to try and help that, it would help for a moment, and then start stalling out again. When I took off the air filter we could also hear a very loud hissing sound but all the lines are connected in the correct places.

I tried to post the audio (mp3) but I'm not sure if I was successful with that. I had to convert from mp4. I could easily email the actual video to anyone willing to listen. I can't upload mp4 on the forum.

Any one have any ideas what could be making this happen?

 

Check the Auxiliary Acceleration Pump (AAP) on the carb. If you don't have a factory service manual, you can download from the link below.

https://web.archive.org/web/20110102...ttora.com/FSM/

 

Is there a particular way to check if that is the problem? Or just take it apart and see if the diaphragm is warn out? I see the diaphragm on ebay costs around $120.

 

There are diagnostic checks in the factory service manual. See the link I posted earlier. Buy a carb rebuild to get a diaphragm. Rockauto has rebuild kits for a reasonable price.

 

Also make sure your entire fuel system is clean with no dirt or debris. This symptom is often caused by a blocked or partially blocked idle circuit. Junk or trash in the fuel system can easily clog up the tiny idle passages.
Since you have a 1980 truck, consider that it's now 45 years old. The tank could have some trash in it and/or the pump filter sock could be detached, missing or falling apart (if you have the in-tank electric fuel pump).
Drain, drop & clean out the tank & at the very least get a new filter sock on the electric fuel pump. If you have the mechanical fuel pump on the head it just uses the fuel filter (see below).
The rubber fuel lines may be older and not designed to handle today's Ethanol-laced gas. The Ethanol makes older rubber fuel lines shed particles inside and those can cause idle passage blockages.
How old or dirty is the fuel filter? It should be in the engine compartment on the passenger side and is cheap & easy to replace.

It might sound like a lot of work (I consider it moderate) but checking out these things and making them right is more than worth it in the long run.

 

Are your running the stock Asin carb? Is your truck a CA (smog vacuum line nightmare)? I have an '80 that is a CA truck and I'm having the same issues. It is very important all the lines are not leaking and they are connected to the proper ports. I think my issue is with the dash pot for the choke. I'm trying to track it all down. If your truck is stock is should have a vacuum hose diagram on the hood...I know mine does. Still, it is very complex system with a manifold and one way check valves all over the place. It could be something as simple as a cracked line or one that came off a port. Good luck to both of us.