I have a 1970 F250 Camper Special with a 390 and an aftermarket 4bbl carb (not sure the brand). It used to run really well. Then I made the mistake of letting it sit for a couple months. After running it again, it started to idle really rough unless I gave it a lot of gas. I was able to get it running decently again by using carb cleaner, replacing the fuel filter, replacing the PCV valve, adjusting the idle up a bit, and running fuel system cleaner through it.

Now, though, it has started to have a new problem. It starts ok (not great). It will stay running as long as I give it a LOT of gas. Then, as soon as I stop giving it gas, it dies immediately with no sputtering or shaking. It just dies quickly. I do hear a bit of a sound when it dies though. It sounds like somebody blows out all their breath really quickly and loudly. Like a "puh" sound.

Any ideas?

 

Sounds as though the idle circuit may be clogged. Counting the number of turns, screw in the idle mix jets. "Usually" around 2 turns. Remove them completely and blow out the passages with compressed air/ carb cleaner. Reinstall all the way and back out the same number of turns on each.

 

Edelbrock (AFB) carb?
Check the primary venturi boosters for debris. Make sure the little tubes on the bottom side of the boosters aren't clogged.
(Part #32 on page 21)
http://www.edelbrock.com/automotive/...ers-manual.pdf

 

Also check all rubber lines for vacuum leaks and cracks throughout your whole vacuum system using quick start spray or carb cleaner.

Plus check dizzy vacuum advance for leaking & sucking air.

It may be time to do a over all tune up, to include rebuilding the carb.
That would mean doing Compression test also.
Plus check the fuel filter for any water/crap from the tank and the rubber tank lines sucking air or leaking.

If you don't know when the last time any of this has be done then it's time to do it.
full tune up!
Orich

 

2X all the above but I'd also check the timing chain. Sounds like it skipped a tooth or two. Next time you get it running throw a timing light on it and a dwell meter. If the timing mark bounces around the chain is loose. The dwell should only change 2-3 degrees when you Rev it. More than that is a worn ot Dizzy or it's points breaker plate.