Hard flat-spot on acceleration above 3000 rpm and running hot 39 Plymouth

[whippersnapper48]

Greetings Mopar gurus!

 

  So I've been working on a friends 39 Plymouth convertible for about a month now, and I just cant get this thing running properly.  The car runs and idles great, but when you are accelerating in 2nd gear to about 20 mph she bucks hard and just wont go any faster. same in third at about 35-40 mph.  Someone worked on this car before me, and shot gunned a ton of parts. The whole distributer was rebuilt including the vacuum advance, the carburetor was professionally rebuilt, new fuel pump, new spark wires, new flexible fuel line, and a new coil.  I timed the car 2 weeks ago, and she needed a little advance according to the vacuum gauge.  I took the car for a spin today, and its still bucking hard like its running out of gas.  There is no backfiring at all, no white, blue, or black smoke, and the carb is getting gas.  Today the car over heated on me, and I had to take it back and let it cool down.

 

Im at a loss on this one, but Im not defeated.

 

Thanks for the help in advance,

 

Jon

[TodFitch]

For the rebuilt distributor, were the vacuum and mechanical advance curves checked. You can do that with a hand vacuum pump/guage and a variable advance timing light.

 

Is the exhaust system clear with no blockage?

 

You may also want to check the valve timing. There is a pipe threaded plug on the top of #6 cylinder that can be removed and an indicator used to find actual piston TDC.

[whippersnapper48]

Thanks for the input TodFitch.  I plan on doing a timing gun check using just that method you described.  I really think this is gas and not timing though.  I asked if I could pull the distributer and run it on my Allen distributer machine, but he feels that since it has been rebuilt, it is not needed.

[Merle Coggins]

Double check the points. In particular the spring. If the spring doesn’t get locked in correctly the tension will be low and will lead to points bounce at higher RPM. I know this from experience. Idle was smooth and timing right on, but when driving it would start bucking and loosing power badly as I sped up. I was able to confirm with a timing light. When I’d rev up the engine the timing would get wildly erratic and retarded. I checked and found the points spring not locked in correctly. The copper strap was the only thing acting as a spring. Once I corrected that it ran perfectly. 

Edited May 11 by Merle Coggins

[Ivan_B]

Are you able to "accelerate" it without issues in neutral? Is the float level correct and accelerator pump appears to be working? The fact that someone rebuild stuff, etc., does not mean a thing. You can still find all kinds of "things" in there, just like I do, after the PO had someone working on the car for him... Humans make mistakes, that's a fact.

For example, the other day I noticed that my choke was about 35-40-degrees closed with the knob is all the way in. Turned out, I did not connect the cable properly when I re-routed it, while fixing the radio, last year... So I was riding the choke all this time... Oh well 🙄

Edited May 12 by Ivan_B

[DonaldSmith]

Ditto to Merle Coggins' post.  Dummy me had the points in wrong. so that the spring was not operative.  Running well at idle and slow revs, but flattening out at mid revs.  Of course, I had been working on carb issues so the carb got the blame.    

[LazyK]

Disregard

Bad suggestion 

Edited May 12 by LazyK

[D35 Torpedo]

I agree with ignition being the culprit. 

[whippersnapper48]

11 hours ago, Ivan_B said:

Are you able to "accelerate" it without issues in neutral? Is the float level correct and accelerator pump appears to be working? The fact that someone rebuild stuff, etc., does not mean a thing. You can still find all kinds of "things" in there, just like I do, after the PO had someone working on the car for him... Humans make mistakes, that's a fact.

For example, the other day I noticed that my choke was about 35-40-degrees closed with the knob is all the way in. Turned out, I did not connect the cable properly when I re-routed it, while fixing the radio, last year... So I was riding the choke all this time... Oh well 🙄

Thanks for your input, and no, it does the same thing with no load.  He said that he was driving the car last fall, and all of the sudden it started doing this. It hasn't run properly since.  He insists its timing, but in my experience cars don't just jump time.  If they do in that rare occasion I have found them to run terribly across the board.  

[Sniper]

59 minutes ago, whippersnapper48 said:

He insists its timing, but in my experience cars don't just jump time. 

 

Changes in dwell can affect timing, however the culprit is the dwell not the timing, the timing change is a symptom.

 

Just put a dwell meter on it and see how it acts above and below the RPM level where you're seeing the issue.  That will help sort it out

[Jerry Roberts]

Also , the fuel might be partly  restricted , perhaps at the tank . Some of these cars have a filter inside of the tank . Try running on a gas can . 

[Ivan_B]

I've just re-read the original post. If the car won't accelerate past a certain point, also check that the throttle is opening all the way up, when the pedal is pressed. I've had this problem once. The car would not go past a certain rpm