Rough running engine

[kimlinh]

My recently rebuilt engine runs rough. It starts and idles fine (when its running on the idle jet). But when I drive it the engine has a more or less steady miss. The more power I use the more intense the miss is. At first I though it might be electrical. I replaced the condenser. No change. I don't think it is a leaky manifold, All the spark plugs look the same. They are not wet and they don't look like they are running lean. I've also tried 2 different rebuilt carburetors. ( rebuilt by the same guy, me, so maybe that's the problem) Does anybody have any ideas?
Thanks in advance

[Marshall V. Daut]

Possibly a bent valve that missed the builder's attention is hanging up in its guide. Or there wasn't enough clearance in a guide for the valve to slip through easily. Run a compression check on each cylinder, both when the engine is cold and warmed up. That will reveal if a valve is sticking. This is always a good starting point in order to eliminate an internal engine problem before troubleshooting the electrical and fuel systems. The best ignition and best carb in the world won't overcome internal problems.
Marshall

[bobbader]

Merely replacing the condenser without it changing how the car runs does not guarantee it's not electrical. You rebuilt 2 carbs and using either of them results in no change. Did you do anything to the distributor? Rebuild? New points? Distributor cap long body? Clean the points and reset gap?

How did the engine run before the rebuild? Is it possible the miss was there before the rebuild? That would indicate the problem is outside the engine.

I think the first thing I'd try is point gap on each lobe of the distributor cam. If that's OK, check the gap between the blade on the rotor and each terminal in the distributor cap body. Keep going from there.

[rotorwrench]

That's true. A worn distributor bushing will play hell with the breaker points.

[1crosscut]

Realizing that you don't think it is a leaky manifold I would still go ahead and do a check to be sure.
While at idle spray starting fluid all the way around the manifold gaskets. If there is a change of engine speed there is a leak.
If not then you know for sure there isn't a leak.

Does your car have a vacuum wiper? If so remove the line and temporarily plug the port.

You've switched carburetors. Now try switching the distributor with a known good one.

[Randall]

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Here's a video showing a worn dist. bushing. https://youtu.be/sAImvK8dnl4?si=TPY8xxAc1lHmdcil

[Mulletwagon]

Worth investigating a sticky valve. Recommend raising the rpm and dripping Marvel Mystery Oil down the carb intake. Had the exact same presentation which was fixed by 4 oz. of MMO in the tank with each fill up. Be sure to let us know the fix. Good Luck.

[BButturff]

Could this be something as simple as a partially plugged fuel line? The engine gets enough fuel at idle but doesn't when the demand for fuel is increased?

[nkaminar]

One thing to do is to get one of the clear distributor caps. That way you can watch the sparks to each cylinder as the engine is running. Best done in a dark garage (with the door open) or outside at night.

[Bob Bidonde]

Do a compression test!

[Randall]

One of the Fayetteville NC members was having an miss at different times and brought it to me, the miss would come and go ,did compression test and checked fine,I drove it and kept pulling plug wire till I found the cyl it was missing on, would not miss at idle , pulled the head and the valve seat was jumping out and then would go back in, so I used my Kwik Way portable seat cutter and installed oversize seat in car repair, sometimes you never know what you're going to find.

[Big hammer]

Fresh rebuilt engine , lots of nice new paint , check for poor grounds