41 Plymouth coupe wants smoothness

[Povertycove]

I recently purchased a nice 41 Plymouth coupe. After having the fuel tank cleaned, repaired and sealed, it runs very nicely, and has a nice turn of speed -- a very easy car to drive except for one thing -- it tends to "lope" in all three gears, at its worse almost a bucking at times. The carb has been professionally rebuilt. Timing is good. It comes down to a nice low idle easily. The two theories I've had are 1. a higher rear end gear (I don't think this is the case, though, because I can pull easily in high gear from a very slow speed)or 2. a faulty vacuum advance. I've had a number of these old l-heads, and this one is livelier than most, suggesting, possibly, that somewhere along the line someone put a hotter cam in there, if that's possible. Anyway, I'd just love to hear some more thoughts on the subject. I don't have to do anything about it -- it trots along the interstate at 60 very easily. Its just around town driving that I'd like to tame._

[Guest Richie09]

Hi, I don't know if this will help what you are describing but if vacuum advance is in question you might want to look at the distributor. The plates with ball bearings that allow the rotation of the spark advance could be gummed up. I found the spark advance mechanism in desparate need of cleaning as the grease was almost solid and dried up in my '40. I have not completed getting this engine running as it is a "ground-off" serial number and though it is a 23" head, I have located an engine with correct serial prefix for that year and it has not been installed yet. Richie

[Rusty_OToole]

Maybe you finally got a good one. They were a fairly peppy car when new, but most of them are worn to the point where they are down on power and don't have the old oomph anymore. They can fool you because they have such a well behaved motor. It can be in an advanced state of wear and still start and run normally except down on power.

I know a guy who has practically the same car. He set out to improve it first by having the engine rebuilt, then by adding overdrive or changing the gear ratio.

As soon as he drove it with the rebuilt engine he forgot about any changes to the gearing or transmission. It ran fine and had all the power and speed he wanted. And this was a car that ran well before, just had low compression and one or 2 burnt valves.

I would start with a compression test and then put a vacuum gauge on it. Check valve clearance, spark timing, points, carb adjustment, basic tuneup then the vacuum gauge. There must be something wrong, I hope it is something minor like a bad plug wire.

[Povertycove]

Compression is good, and even across the board. All the basics that you mention have been done. I just took it out today for a little toot, and the car has just amazing pep -- it just has this jerkiness. I'm thinking that it may be a sticky valve, so I'm running a fair amount of Marvel in it. Odd though, that it starts, runs and idles so well; has terrific pep; but it has this jerkiness to it in mid-range of power. It almost feels, at times, like a beginner wwas learning how to use the clutch. Ha! I'm wondering if the carb could be fluttering somehow. Thanks for your ideas on this -- its just a great car, rust-free and nearly perfect.I just wan't to get this one issue resolved. Miust be something simple.

[ply33]

In my older B&B carb there is a "power circuit" which is actuated via manifold vacuum that controls the transition between the idle circuit and the main jet circuit. I believe that your 41 has a similar circuit but implemented a bit differently. Anyway, on mine the piston and/or bore the piston runs in that controls this circuit had worn and the result was really bad transitions between running under power and idling. You might want to check out that part of your carburetor.

[Rusty_OToole]

Is this bucking like the engine cuts out? Could be a loose wire in the ignition circuit, or a wire worn bare and shorting out. When it shorts out the engine cuts out for a second. Then it jumps and the wire moves around enough to make contact again.

Or maybe some dirt in the carb that floats around and gets in the jets once in a while?

It is hard to diagnose something like this long distance. I'm sure if you go over the motor with a fine tooth comb you will find it.

Did you try the vacuum gauge? They can help pinpoint a problem.

[hchris]

Good compression and smooth idle does not necessarily mean smooth running is going to occur at higher power settings, the mode of operation for fuel and ignition systems is quite different at higher speeds.

The problem is how to analyze which system is playing up; the fact that the fuel tank has been disturbed would cause me to check that there are no fuel blockages all the way between the tank and the carb. If no faults are found there I would be looking inside the carb to see if there is debris floating around inside.

As far as ignition goes it might be worth your while to stick your head under the hood with the engine running in the dark; rev the engine up and if there are any ignition breakdowns they are quite visible as blue flashes along the wires or around the plugs.

[Povertycove]

I'm interested in the power circuit thought, and I'll check that out, as well as sticking my head down in the dark to see if there is any crossfiring. The fuel lines are clean, and the carb has been refurbished by a competent guy. This is an awfully good car, and as the old mechanic used to say to me back in the hills of North Carolina, "just drive it." It needs some miles. Its a funny problem though -- it isn't cutting out, and I'd feel comfortable setting out for a 500 mile trip. Its just annoying around town. Thanks for your help, fellows.

[Rusty_OToole]

If you do the after dark plug wire test it can help to spray with water from a spray bottle. This duplicates conditions on a wet day with rain water blowing in thru the rad. The most likely time to see blue sparks or cross fire.

Incidentally plug wires can cross fire if they are touching and parallel to each other. You need to keep some air space between or cross them at an angle.

[Rusty_OToole]

Is there a good ground between the body and engine or frame and engine? The rubber mounts can isolate the engine under certain conditions. As it rocks or moves it makes and breaks contact. The factory always put a small ground wire between the body and motor, and a big one between motor and battery.

[Rusty_OToole]

If it seems to cut out and back in sharply like someone threw a switch it may be electrical. If it blubbers and stumbles then slowly clears up more likely carburetor. But this is by no means carved in stone, just the most likely.