Help with rough running engine

[RedRider]

1970 C-10 w/ a 350 small block that has been sitting for 10+ years. Fires up just fine but idle and acceleration are very rough, like it’s not firing on all eight.

When I brought it home last fall:
Changed oil and filter
Squirted Mystery oil into cylinders and over valvetrain
New plugs/wires
Dumped old fuel, new inline filter.

At that point it started and ran briefly, just wanted to make sure it wasn’t seized or dead on arrival.

Fast forward to now, I’ve come full circle ready to revisit the engine. This issue was first noticed when I added a new intake (Edelbrock) and carb (Demon). It sounded like a dead cylinder so I checked compression (135-140 psi all around). I installed a new HEI distributor too (set at 8 degrees) but that didn’t solve the problem. There’s some white smoke out the tailpipe but I suspect that’s the Marvels oil burning out of the cylinders.

At this point I’m not sure where to go next. It certainly seems like an ignition issue but the plugs, wires and distributor are all new. Maybe the carb, but it’s new too. I’d appreciate any troubleshooting suggestions or maybe a second set of eyes to see if I’m missing something.

[toolboxchev]

They did not come from the factory with model year 2018 Marvel Mystery Oil. Burn that crap out of there. Clean the plugs, check your vacuum advance make sure its working and plugged into the correct port. Focus on ignition and timing first, then carb.

Check out hotrodders forum links to Crankshaft Coalition. A ton of info there.

[RedRider]

Originally Posted by toolboxchev

Check out hotrodders forum links to Crankshaft Coalition. A ton of info there.

Pretty slick website, bookmarked. Imagine if every automotive forum member contributed a bit of their expertise...that would be quite a database of information.

[toolboxchev]

Originally Posted by RedRider

Pretty slick website, bookmarked. Imagine if every automotive forum member contributed a bit of their expertise...that would be quite a database of information.

I followed some of their links around and ended up calling Edelbrock tech line after cleaning and dialing the HEI on my truck. Dude was awesome, ask what cam, carb, vacuum, timing, ect.. I had and then sent me a couple of tuning springs.

Running like a champ now, even smoothed out considerably, with more felt power.

[Kudzupatch]

Originally Posted by RedRider

..... This issue was first noticed when I added a new intake (Edelbrock) and carb (Demon).

HUGE hint right there. If I read this right and it was running good before, then it was not. Start with what you changed, odds are great you did something that is causing the issue.

[Dead Parrot]

New intake + one cylinder missing likely = intake gasket vacuum leak. Carb gasket leak usually causes several lean cylinders or the entire motor depending on manifold type.

Might pull and check the plugs. One might have been fouled by too much oil. Also could have got a dead out of the box plug.

[68gmsee]

Seems to me it's never really run very good -right?

A rough running engine can be a bunch of thing -even a combination. You've done some of the preliminaries so next is to do the simplest, easiest things first.

Make sure you don't have any vacuum leaks by plugging all even the vacuum advance. When unsure if the gas in the tank is contaminated I disconnect the fuel hose from the tank to the fuel pump and then place a fuel hose into a one gallon can under the truck and run it from there.

From there it's a matter of elimination; ignition components and/or internal engine components like worn out cam lobes. Just do the simplest, cheapest first.

[YoungPup1977]

[RedRider]

Originally Posted by Kudzupatch

HUGE hint right there. If I read this right and it was running good before, then it was not. Start with what you changed, odds are great you did something that is causing the issue.

My thoughts exactly, like did I miss a tooth on the distributor? Timing is good though, but that’s not to say something else changed.

Originally Posted by Dead Parrot

New intake + one cylinder missing likely = intake gasket vacuum leak. Carb gasket leak usually causes several lean cylinders or the entire motor depending on manifold type. Might pull and check the plugs. One might have been fouled by too much oil. Also could have got a dead out of the box plug.

Pulled the plugs and they look alright, smell a little like gas though. I am running a questionable 1” carb spacer (got it at a swap meet, didn’t sit exactly level but thought the gasket would compensate to seal it). But if it was a vacuum leak from the carb gasket, wouldn’t that cause a pretty high idle?

Originally Posted by 68gmsee

Seems to me it's never really run very good -right?
A rough running engine can be a bunch of thing -even a combination. You've done some of the preliminaries so next is to do the simplest, easiest things first.
Make sure you don't have any vacuum leaks by plugging all even the vacuum advance. When unsure if the gas in the tank is contaminated I disconnect the fuel hose from the tank to the fuel pump and then place a fuel hose into a one gallon can under the truck and run it from there. From there it's a matter of elimination; ignition components and/or internal engine components like worn out cam lobes. Just do the simplest, cheapest first.

Yes, direct “fresh” fuel may be next, good call. The stuff coming in looks questionable looking through the clear fuel filter. I could see old gas causing a rough idle, but on acceleration too?

Thanks for the tips everybody, I’ll keep you posted of my progress.

[68gmsee]

Originally Posted by RedRider

...
Yes, direct “fresh” fuel may be next, good call. The stuff coming in looks questionable looking through the clear fuel filter. I could see old gas causing a rough idle, but on acceleration too?

Thanks for the tips everybody, I’ll keep you posted of my progress.

The reason I mentioned it is because I got burned some years back. Engine had been running great but hadn't been driven in a while. Like a dummy I assumed it was something in the engine. Later found out the gas was contaminated.

[RedRider]

Welp, the solution to this problem looked something like this:



Long story short, the electric choke wasn't working, closing off the air intake. I opened it up and the engine fired to life, on all eight! It turns out that lack or air sounds strikingly similar to lack of spark; a fine lesson learned.

But the true lesson here boys and girls is when you're troubleshooting air, fuel and spark...don't discredit the air part.

Thanks again for the help everyone. I hope info this helps another poor soul in the future.