350 running bad----need help!

[Farmstore34]

Hey all,
I need some help on a problem with my 72 K20. It has an older crate 350 with an edelbrock intake, carb, and mild cam. It has always run and idled fine. All of a sudden it started running poorly and will hardly idle. It runs fine under acceleration and above 1800rpm but below that it acts like its running on 5 cylinders and idles erratically (seems like its running very lean). I changed the fuel filter, then cleaned out the carb but no luck. I then swapped in another Edelbrock carb that was working fine and that made no difference. I suspect a vacuum leak, but have found nothing around the carb, no bad hoses.
I have a feeling it may have been over revved on the day this started, so I'm not sure where to start. I dont hear any knocking or ticking, and other than at idle it runs fine. Any ideas?
Thanks!
Colby.

[WorkinLonghorn]

What are you running for a distrib?

[rick0312]

fuel pump maybe?

[Farmstore34]

I have an MSD distributor, coil, 6AL box and plug wires. The 6al box was just rebuilt by MSD a month ago. I have great fuel flow, although I have'nt tested it while running (just by cranking).

[Sammy]

How long ago were the plugs & wires replaced? Are the wires touching the manifold? If the wires & plugs are in good shape I would be doing a compression check on the cylinders. A burnt valve/cracked head can cause a rough idle.

[cobb_ben]

Is your dizzy loose? Cracked, fouled plugs? Dirt in the carb? Is it an old holley possiable blown power valve...ect. good luck!

[Farmstore34]

progress so far.....................
I checked the timing tonight, it's still set right and the distributor is not loose. I did discover that if I pull the big vacuum hose that goes to the pcv valve from the carb, the idle does not change. Seems to me there is a vacuum leak somewhere? I put my thumb over that vacuum port and there's good suction, but it runs the same open or blocked. I'm gonna do a compression test tomorrow, and then maybe pull the intake off and check the gaskets. This is very frustrating!
Colby.

[berencam]

I wouldn't pull your intake just yet. Check your distributor cap and rotor for one. Also check to make sure you aren't running rich or lean. Your pcv shouldn't be making that much of a difference but if it worries you change it, its like 5 bucks. Also check your vacuum advance on your dizzy it might have broken and now has caused the advance to be stuck wherever it was broken.