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!

[rich weyand]

The stock Qjet you put on it is leaking fuel from the machining plugs on the bottom at high vacuum (idle), which is making it idle richer than hell. That's why the oil is turning black. This is a common problem once a Qjet gets old. You can get a quality rebuilt Qjet from Sean at SMI Carburetors.

The other thing that can give you rough idle when it runs fine otherwise is having the vac advance hooked to ported vacuum. Move it to unported (manifold) vacuum, so it advances properly at idle. Ported vacuum is emissions nonsense. It will also make the engine idle hot and is hard on exhaust valves.

[cadillac_al]

That jumping needle is probably a valve problem, either sticking or burnt or adjusted too tight. You forgot to tell us if it has had a tune up recently. It could just be plugs and wires.

[TylerW]

Ok guys, there are some interesting developments with this:

Today I dived back in and pulled all eight plugs. All of them looked fine, light tan, no evidence of oil burning or fuel fouling. That rules out carb problems with that Qjet everyone likes to condemn. It was just rebuilt by me including sealing the well plugs and works great.

I then went back through and did a compression test and this time got 120-125lbs across the board. Whatever is or was going on with #5 didn't show up this time.

Next up, I put it back together and made a thorough video for everyone to watch. I have a vacuum gauge on it during this and also give video of the exhaust so that you can see how it runs at idle, revved in park and loaded against the converter. There's a lot of good evidence here.

A few things you will notice are-

1. No smoke. Corresponds with no blowby and no oil consumption.
2. The engine is mechanically quiet.
3. If you rebuild and adjust a Quadrajet correctly they will start cold in 2 revolutions like this one did even with a weak cylinder. It starts that way everytime. It has it's original hot air choke and the thing works FLAWLESS.

I have a pretty good idea what's going on now...I think one of you nailed it already but I'd like to get this out to help others. I'll post what I do to fix it.Thanks again.

https://www.youtube.com/watch?v=cnvS...ature=youtu.be

[gmachinz]

I vote for either a sticking or crudded up exhaust valve. If it doesn't close properly and let's air past, it will give you a low leakdown number. Try dropping a little vinegar in the carb with half throttle and keep doing it until the engine smooths out without it. You'll see a lot of black exhaust as it cleans the carbon deposits out. Vinegar dramatically raises the combustion temps temporarily and dis-lodges carbon build up and blows it out.

[70STOVEBOLT]

So what was it? I have a similar problem with my freshly rebuilt 402. I used an infrared thermometer to check the temp at each exhaust port on the manifolds and one is about 75-100 degrees colder than the rest. Starts right up every time and runs good but does have a slightly rough idle and a vacuum gauge shows low vacuum (around 10 ") at idle with a jumpy needle. It does have a somewhat lumpy cam (.501/.527 and 314/324 degrees duration). I have not adjusted the valves since its been running but it's quiet and seems to run ok except the cool #2 cylinder.