having carb related issues with 250, single barrel rochester, I was having issues with truck running very poorly, little to no vacuum at the top of the carb.

over the weekend i took another carb that i knew to be good and put on the engine. when it first started up ran very smooth, had lots of vacuum at the top of the carb, enough to pull your hand down tight against the rim. I let it run for just a few minutes and noticed a small fuel leak around my line.

i shut the engine off and fixed the leak, connected the throttle linkage and when i started the engine back up it was not running quite as smooth and the vacuum was once again gone from the top of the carb. could somebody please tell me what in the world is going on with this thing?

Well usually I measure vacuum with a gage on the intake, at idle you shouldn't feel much vacuum at top of carb.

That said, the symptoms sound like dirt in the fuel lines/tank, BTDT - known good carb, put that on, runs good, then craps up like the old one. Usually dirt getting in there. I like those clear plastic FRAM filters in between fuel pump and carb so you can see dirt in an old tired fuel system.

ok well i may be ok on the vacuum at the top of the carb then. i just assumed it should have enough to suck your hand to the top of the carb.

i did check the vacuum with a gauge at the intake, and at idle it is around 12 but the needle is really bouncing back and forth between about 10 and 15 very quickly

i know that sounds a lot like a burnt valve issue but when i did a cylinder leak down test to check for that all cylinders were within about 5% of each other. so i am wondering where in the world could i be loosing that kind of vacuum. from what i have read i should have right around 20 lb at the manifold.

i have checked the intake/exhaust gasket by spraying carb cleaner all around the intake and exhaust while it was running and there was no noticeable leaks. i could have missed something in behind the exhaust pipe however.

How is it cruising out on the road?
Sometimes the idle circuits will clog up but the mains are fine.
So you could have 12" vacuum at idle due to the dirt, but cruise out on the road at 18" which is about normal.

That would say dirt is getting in and messing up the idle circuits first. Usually a good clean & air blow (gasoline, then thinner or acetone, then air blow) will solve this and the carb will run good again - until more dirt gets in there.

Of course, a vacuum leak could have the same symptoms- rough idle, fine cruising. I have even had these where it will idle good COLD, but when it gets hot, the exhaust manifold expands and pushes the intake away, creating a leak and rough idle. Pesky problem. The little Fords used an integral intake to avoid this problem, which works great until you want to add a 4bbl or multiple carbs.