I have a 1978 Ford E150 4.9L / C6 / 1 bbl Carter carb, and it's stumbling quite a bit at idle.

I checked it for vacuum leaks and found none. It sometimes diesels a bit when I shut off the engine, but never for very long. It doesn't run that bad overall, it's responsive under acceleration, yada yada.

However, when I took the air cleaner off to look at it, I left the one vacuum line that goes from near the base of the carb to the air cleaner open, and voila - the engine smoothed right out.

So it's running rich, is my guess, because when I "create" a vacuum leak, the engine smooths right out.

Also, I can tighten the idle mixture screw all the way to the bottom, and there is little change in the engine idle - it actually seems to get just a hair better with a bottomed-out idle mixture screw.

So, do I need to re-jet the carb? Do I need a different idle mixture screw? The previous owner said he had the carb rebuilt at some time. I can rebuild a carb no problem, but I haven't ever re-jetted a carb - the only problems I've had with a carb before were with running lean conditions - i.e. a vacuum leak or adjust the idle mixture a bit.

Any help you can give will be greatly appreciated

Thanks,

Mike

P.S. I bought the van from the original owner, he still had the 1978 title. 80 year old guy, took it camping in the summer, 121k miles when bought, zero rust top or bottom or door tracks He appeared to take great care of it.