Running Rough

[Wood and Steel]

Hi everyone. I have a '51 Plymouth that sat for a couple of years while I was working on the body, and now that I have it back on the road again, it's running pretty rough. I'm new to older engines in general, and while I have a fundamental understanding of how they work (thanks to YouTube and other resource), I'm having a hard time diagnosing the problem.

 

Symptoms:

Running rough, almost sounds like a bit of a gallop, getting worse as it gets up to temperature.

The Timing mark bounces when using a timing light.

Vacuum gauge doesn't quite hold steady. Hopefully you can see it in the video.

Seems to be more smooth when under medium load, such as going up a slight incline. 

 

What I've tried so far:

Changed dis. cap, and coil. (I was getting bit of a shock when adjusting advance before)

Checked timing with light. Mark doesn't hold stable, as mentioned above.

Checked vacuum. Also doesn't hold stable, but does stay between ~17-20, dropping when it cuts out a bit.

Checked Dwell and rpm. Both check within spec.

Adjusted air fuel screw using vacuum gauge. (disconnected vacuum advance, gauge hooked to manifold). Vacuum drops and stalls the engine if I turn to far clockwise, but I see nor hear any change regardless of how much I turn counter clockwise.

 

Other info:

The plugs, wirers, and points are a couple of years old, but all of the other wiring is pretty crispy.

The spark plug wells are pretty rusty from sitting. I'm planning on swapping heads in the spring.

Can still run 55mph just fine, with an occasional stutter. 

6v

The engine is out of a '56

The video and audio attached were poorly captured, but I didn't check them before I put the car away. It might be a little easier to hear the issue in the audio file, but there is some unfortune crushing of leaves, and a speaker pop when I got the phone too close. I think you can still hear the cadence though. I can get better video and audio if that would help. I might even be able to show the timing mark if I drag a 12v battery over to the car.

 

 

 

 

 

 

02.07.2023_1432PM.aac

[keithb7]

No adjustment with air screw is an indicator that it may be time for an ultrasonic carb clean. Fluttery vacuum indicates valve seats could possibly use some hand lapping. Check your points anchor screws. Tight? Points may be moving around a little bit, seen by the jumping mark with the timing light. Return spring arm on points adequate? Wear block and points cam in good shape. Closely inspect wire from coil to distributor, junction, and internal wires in distributor. All solid? No stray wire strands occasionally grounding slightly? Any possible porcelain cracks in the spark plugs?  Fire up you car in the dark. No lights. Look for possible sparks to ground. Check wire from ignition key switch to coil. Set points open. Set a multimeter to reference ground. Check continuity of all wires in ignition system that they are not "beeping" the multi-meter indicating they are grounding somewhere. 

 

 

Edited February 9, 2023 by keithb7

[allbizz49]

If it sat for a couple years, carb is probably crusty. Sounds like you part swapped everything and still have a problem......Pull the carb off and clean it. Even a good soaking and blowing out all of the passages will work. Soak it, sonic clean, whatever gets the job done. These things are so simple, even a caveman can tune one. Good luck.

[OUTFXD]

vacuum advance might have a pinhole leak. would get worse as the engine builds more vacuum.

 

Just my two bits.

[vintage6t]

Possibly a stuck/sticky valve. Put some Marvel Mystery Oil in the crank case and see if it smooths out after running it a bit.

[Sniper]

There shouldn't be any vacuum advance at idle. 

 

Fluttery idle vacuum usually indicates a valve related issue

 

https://www.onallcylinders.com/2016/03/31/vac-visual-quick-guide-vacuum-gauge-readings/

[Bryan G]

Sticky valves was my first thought as well. I'd start with some Marvels in the gas and some more in the oil. If that's what it is, it should free up with more running. 

[Sniper]

Brake fluid works real good on freeing up stuck valves.  I did a write up on that some where here.

 

 

[LeRoy]

2 hours ago, Sniper said:

Brake fluid works real good on freeing up stuck valves.  I did a write up on that some where here.

 

 

I had not heard the werd recalcitrant before reading your post. I'm curious how you came to know it?

[Sniper]

12 minutes ago, LeRoy said:

I had not heard the werd recalcitrant before reading your post. I'm curious how you came to know it?

 

I spent many a deployment in the Navy reading.  At one point I read everything on the destroyer that wasn't a Navy publication, including the dictionary.  Then I started reading the Navy publications. 

[kencombs]

1 hour ago, LeRoy said:

I had not heard the werd recalcitrant before reading your post. I'm curious how you came to know it?

I had to read Sniper's answer before I realized that you didn't mispell wierd!    Middle finger, wrong hand, common for a touch typist.   'word recalcitrant' makes much more sense than weird.

[LeRoy]

8 hours ago, kencombs said:

I had to read Sniper's answer before I realized that you didn't mispell wierd!    Middle finger, wrong hand, common for a touch typist.   'word recalcitrant' makes much more sense than weird.

Sorry, I am a horrible speller so I now purposely misspeel werds as a jab at muhself.

I'm sure I find it a lot funnier than others do.

[Wood and Steel]

The mystery oil seems to have really helped. I'm planning on doing a carb rebuild and valve job when I get the chance, but it's running well enough for cruising around town now. There's definitely a short in the ignition system somewhere. I get a little shock every time I adjust the distributor. I've replaced the cap, wires, and coil but that hasn't fixed it. Further investigation required.

[Sniper]

16 minutes ago, Wood and Steel said:

I get a little shock every time I adjust the distributor.

 

Not uncommon.  Wear a dry leather glove next time.