I am new at tuning my 390, so here is my problem. It idles rough but runs great through the rpm range except right off idle, I have swapped carbs with no change also full tune up, my old plugs were white and my new plugs are white also, so I am thinking vacume leak. The problem is I don't know how much vacume my 390 should be making, at idle it only has 12" and at 2,000 it has 20". The motor is all stock.

 

have you tried adjust your idle mixture screws ?
if you have a autolite 2brl they are under the front of the carb .
gently turn them all the way in then back out 1 1/2 turns . this is a good base point to start from .
hook your vacuum gauge up and adjust the screw in or out to get max vacuum .
make sure both screw are adjusted the same .

 

Well....I have bigger problems, #1 cylinder only has 30# of compression

 

that would be a big problem . did you do a wet test ?

squirt some oil in cylinder , if compression jumps up problem is rings , if not you have a valve problem or ?

 

I'll try that..I did a test after running the engine and when it was cold.[same results]

 

Everything went from bad to worse, even though my truck has been running rough for a couple years now, I thought I would try to fix what I thought was a vacume leak, then I find a cylinder down on compression. So I decided to just run it like it is for the winter while i tried to build a new engine. This is when my bad luck hit, I took it for a trip into town for fuel and it developed a backfire through the carb and alot of blue smoke from the right side exhaust [#1 cylinder is the culprit]. I hate to waste $1,500 on a stock rebuild right now, when I really want to build a nice stroker [but can't afford], I have another truck 2wd that runs good with a very faint knock in the bottom end, so my thought was to get the crank turned and run that in my highboy until I can afford to build the engine that I want, any suggestions here would be great