Stock 1979 350 with a rough idle

[TylerW]

Hey guys:

A few months back I picked up a '79 SWB as resto project. In the meantime I have been working at getting it roadworthy so I can use it. It was off the road for a long time and when I got it, it ran poorly. There was an off the shelf Holley carb and air-gap intake which I removed and replaced with the stock stuff from a donor truck.

The poor running persisted until I discovered the timing was retarded due to the timing cover/timing tab being replaced at some point with the wrong pieces. The engine in question is a reman 350 that is pretty clean inside. I have no history on the engine. I think the truck was restored 15 years or so ago and was let go downhill after that.

Now that that is cleared up, it runs mostly better but isn't right. When the engine is cold, it drops a cylinder. It runs that way until the engine comes up to operating temp. Afterward, it runs on all eight until I let off the gas, then you can hear it drop back out randomly.

When I swapped the intake around, I did some tests to try to determine the condition of the engine, including compression, chain slack and cam lift. It passed all those except compression. One cylinder only built 90 psi cold. A little oil brought it up to 130, about where the others were already.

Here's where I need some other brains on this-

I assumed it has a ring or a cylinder problem somewhere, but...it's not giving the symptoms I would expect. It has no blowby, hasn't used or leaked any oil, doesn't smoke. It runs quiet and aside from the weak idle, it is torquey and is a nice driver for as beat up as it is. It's not backfiring, so I'm a bit stumped. The only thing I did notice is that the oil turned black right away after being changed. It isn't overheating or kicking coolant out.

I had a vacuum gauge on it recently when I fine-tuned the idle and it pulled 15 inches but with a jumping needle.

I'd appreciate any feedback on this, thanks!