Hi All,

77 F100, 302, 2150 2bbl, 3spd manual, manual brakes, bone stock with most(?) emissions intact.

I've struggled with the carb on this thing since I got it a few months back. Sometimes it idles smoothly, sometimes like crap. Currently it's surging at an idle. Here's the scenario:

-Engine at operating temperature

-timing set at 8º, vac advance disconnected

-air cleaner on

-idle mixture screws out 4 turns (will hardly idle at all with anything less)

With vacuum gauge attached to full manifold vacuum, it idles very roughly bouncing between 12" and 15". then every 24 seconds like CLOCKWORK, it will smooth out and increase to a steady 20"-21" for about 3 seconds then back to 12 or 15. I've lap timed it with a stopwatch for over 5 minutes and it consistently surges every 24 seconds. If I pop the top off the air cleaner and cover the airhorn with a rag, it instantly jumps up to 21" or better and runs smoothly until I remove the rag.

Apart from the clockwork surging, it sounds like a classic vacuum leak but I can't find one anywhere. I've sprayed carb cleaner all around the carb/EGR, etc (in between surges) and gotten virtually no change in rpm/vacuum.

What in the ever-loving-hell is going on with this thing?? Any suggestions would be much appreciated.

Thanks

 

Have you tried the old propane trick. I agree. Sounds like vacuum. Get a wand that will allow you to discharge propane at a point source. Slowly go around everywhere until you find the spot that makes the surging stop. That's your leak. Other than that, I've got nothing!

 

Agree on a vacuum leak.....4 turns out on the mixture screws is a lot....EGR functioning normally? How about a vacuum check around the throttle plates on the outside.....gaskets in good condition?

Sometimes those little vacuum lines and plugs get brittle and will crack. A thorough going over sometimes yields good results.

 

If you have egr double check it's not stuck open

 

If you have an EGR plate under the carb they are know for burning out internally. Also I think you need for initial timing.