1939 Plymouth Coupe Engine Hesitation

[1952B3b23]

So i've been plugging away on the Plymouth and was finally able to take her for the first test drive on Saturday. It brakes nicely and shifts smoothly so i was happy with that. I just drove around the block a few times but my engine isn't running quite right. The engine is the original flathead Six 201 cubic inch with a single barrel carter carb (D6A1 to be exact). Here's the issue:

When i blip the throttle from idle its as if the engine gets no fuel and starts to bog down and want to die. Its almost like the accelerator pump is only kicking in after the pedal is depressed to a certain point. If i gradually depress the throttle the engine revs up smoothly. Its the quick blips of the gas pedal that do her in.

The carb was rebuilt by a friend of mine who's been a mechanic for 30+ years so i'm pretty confident that its together correctly.

As a side note, the engine had the same problem with the "blipping" of the throttle prior to rebuilding the carb. At that time i figured it was the accelerator pump that was acting up so a rebuild was in order.

Im wondering if maybe i need to raise my float level? Its set at the factory spec right now.

Any help will be greatly appreciated.

Thanks,

-Chris

 

[Old Iron]

The accelerator pump may not be engaging soon enough.
Look down in the venturi of the carb and move the throttle if there's not a good stream of fuel squirting into the carb, rebuilt or not, the accelerator pump is the problem.

 

[1952B3b23]

The accelerator pump may not be engaging soon enough.
Look down in the venturi of the carb and move the throttle if there's not a good stream of fuel squirting into the carb, rebuilt or not, the accelerator pump is the problem.

Thank you Old Iron. I will look into this and report back.

If the accelerator pump is indeed the problem does that mean i need a replacement carb? They have a couple on ebay now that would fit the engine.

Im also going to check and see that the vacuum advance is working. My guess is probably not but who knows.

Thanks,

-Chris

 

[Bamamav]

Yeah, sounds like a pump problem to me, too. Mine did that when I first got it started, put a new pump in and it cured it. Yours may not be adjusted correctly, or the squirter might be stopped up.

 

[1952B3b23]

Yeah, sounds like a pump problem to me, too. Mine did that when I first got it started, put a new pump in and it cured it. Yours may not be adjusted correctly, or the squirter might be stopped up.

Hmmm i wonder if the replacement pump may have been faulty. Or maybe my mechanic friend didn't put it in right. I'll have to check into this, hopefully tonight ill make it out to the garage.

Thanks for the reply,

-Chris

 

[Old Iron]

If he installed a leather one it has already dried out from the high quality fuel we have.
If its a vinyl one check to see if it has a spring under the perimeter of it.
There's a check ball in the pump circuit if it's not in there it won't work either.
Heck check to see if the rod is hooked up.

 

[donsrods]

Everyone's diagnosis seems correct, the most common cause of this is an accelerator pump not doing it's job. The reason you are able to slowly get the rpms up is that the motor will respond if you gradually ask it to speed up, but if you stomp the gas and open the butterflies, it needs that shot to overcome all the air rushing in.

As mentioned, look down the throat with the motor off and move the throttle arm. You should see a good shot of gas getting sprayed into that area. A vacuum leak can also cause something like you describe, or a bad vacuum advance. Put a timing light on it and speed up the motor to see if the advance is working.

If there is a strong shot of gas go straight to the distributor as your next suspect.

Don

 

[1952B3b23]

Thanks a lot for the great responses Don and OI! I will try these things and report back.

I'm 95% certain that the replacement accelerator pump is leather. If its already dried out do i need to try and find another made of different material?

Thanks again,

-Chris

 

[martin53]

I hope you figure it out I'm having the same problem with my except my engine is a slant 6. Good luck.

 

[Old Iron]

Thanks a lot for the great responses Don and OI! I will try these things and report back.

I'm 95% certain that the replacement accelerator pump is leather. If its already dried out do i need to try and find another made of different material?

Thanks again,

-Chris

Yes, leather will not stand up under fuels we run nowadays.
The vinyl style used in modern rebuild kits is the one to use.
I would have top see it before I could tell ya which one might fit.

 

[1952B3b23]

Thanks OI. I'll have to look at the rebuild kit and see what type of accelerator pump came in it.

I did take a look down the throat of the carb and hit the throttle. I'm getting a good squirt of fuel so it seems as if the accelerator pump is working. Next I'm going to check and see if the vacuum advance is working. I ordered a new one off eBay just in case, should be here next week sometime. If I don't need to replace it then I guess I have a spare.

Thanks for the reply,

-Chris

 

[1952B3b23]

I hope you figure it out I'm having the same problem with my except my engine is a slant 6. Good luck.

Thanks. Once I figure it out I'll definitely post up the solution. Hopefully that'll help you and other people in the future.

-Chris

 

[Yankee Transplant]

Timing ok ?

 

[1952B3b23]

So I swapped out the vacuum advance and that fixed my problem. Thanks everyone for the help.

-Chris

 

[martin53]

Heck yeah. Glad to here that. Mine ended up being a freakin spark plugg

 

[1952B3b23]

Heck yeah. Glad to here that. Mine ended up being a freakin spark plugg

That's good news. At least it was a simple fix.

-Chris

 

[1952B3b23]

Well Guys im having engine troubles again. After fixing the vacuum advance issues the car was running alright. One thing it did always do was have some blow by, you could see and smell oil vapor coming from the oil breather and crankcase vent pipe. I didnt think to much of it since the engine is 75 yrs old.

The more i drove the car the worse it got though. It then had much more oil vapor out of the oil breather. To the point where it would really stink up the cabin of the car (especially at stop lights). The last time i drove it which ended up being about a 20 minute ride. When i stopped not only was there oil vapor coming out the oil breather but there was liquid oil on the breather cap, i hadn't noticed this before.

When i changed the engine oil prior to driving the car on the road i cleaned the breather cap. Im thinking that i'll need to do the piston rings this winter. My plan as of now is to pull the engine and tranny out of the car. That way i can not only fix the engine mechanically but paint it and the tranny. Then it'll allow me to paint the engine bay as well.

My question is, should i be checking any other stuff before pulling the engine? Nothing is disconnected yet so i can still fire it up and do diagnostics tests if need be. This is the first time I've had to do this so i appreciate the advice.

Thanks,

-Chris

 

[JBailey]

What kind of oil did you use when you changed it?

 

[1952B3b23]

What kind of oil did you use when you changed it?

I used straight 30w oil.

 

[smallfoot]

A simple compression test will tell you lots. If the breathers are smoking chances are valve guides are worn. Are you smoking through the exhaust too? Probably ring issues if that's happening too.