1957 Automatic Transmission Stutter

[Howard of Florahome]

Transmission "stutters" (shakes, slips, hesitates) when starting at low speed.

 

[55blacktie]

Automatic or manual?

 

[Howard of Florahome]

Automatic or manual?

Sorry. Automatic.

 

[55blacktie]

Have you tried manually shifting into low and starting from there? If so, any difference?

 

[willcarterdc]

I also have occasional hesitation/stuttering from the engine under hard acceleration from a complete stop. I’ve always referred to my issue generically as being from the “engine”, so perhaps you’re referencing something different when you specifically say it’s from the “transmission.” What I’m describing on mine doesn’t involve the transmission slipping, so we may well be talking about different things. If so, please disregard the rest of this.

In any event, while mine still has such problems occasionally, it has gotten much better as a result of four things:

1. I always start out driving in low to the extent I can while the car’s still warming up; and even after it’s fully warmed up, I start out in low from a complete stop if I’m stopped on a significant incline or in a situation where stalling out would be dangerous (busy intersection).

2. My mechanic fixed a fuel leak on the carburetor and did a tune-up.

3. I started using ethanol free gas.

4. I vented my gas cap by drilling a very small hole in it (taking it off of and away from the car to do so, obviously). Note: I bought several (4) gas caps that were billed as being vented. All four of them resulted in noticeable vacuum and/or pressure in the tank when removing them after driving even for a short time, and regardless of the ambient outdoor temperature. It is possible that all four were defective; it is also possible that I have some unknown underlying problem that is being compensated for by having drilled the cap.

As to the latter two: correlation isn’t causation and all that, so I cannot say with 100% certainty how much of the improvement came from switching to ethanol free and venting the gas cap. I can say that on a scale of 1-10 where 1 is “undriveable” and 10 is “drives like a brand new modern car: perfect with no hesitation at all ever,” steps 1 & 2 above took me from about a 3/4 to a 5/6 and steps 3 and 4 then took me from a 5/6 to about an 8. Again, though, I’m sure there were multiple confounding variables (weather, driving the car more/less, etc.), but even accounting for those, I’m convinced that switching to ethanol free and drilling the gas cap made a significant difference. Gonna get the carb completely rebuilt, which will hopefully get it up to a 10 on this issue.

(And if I’m off base here, apologies! Feel free to ignore or delete.)

 

[Howard of Florahome]

Thanks for the feedback and response.
I have not tried manually shifting but that is a good suggestion and I will give it a try.
I have been using ethanol free gas since the rebuild so hopefully that is good.
I am using Dextron III (I know it is Dextron, unsure of the numbers) trans fluid. Before we rebuilt transmission I was usingType F.
I drove the car again today and it did not “stutter”when it was cold.
As history, I’ve owned the car since 1991. We finished a 4yr body off “rotisserie” restoration last year (working on it as time allowed) and replaced/rebuilt everything that appeared worn or had given trouble in the past Including brakes, shocks, springs, electrical, wiring harness, engine, transmission, rust repairs, paint, etc. Just trying to get the bugs worked out of it now. Thanks for all your suggestions and help!

 

[55blacktie]

The Thunderbird Restoration and Details Manual recommends Dexron, so I would look elsewhere for the problem. Type F will give firmer shifts but shorter lifespan, unless you change the fluid more frequently.

 

[Ward 57]

I also have occasional hesitation/stuttering from the engine under hard acceleration from a complete stop. I’ve always referred to my issue generically as being from the “engine”, so perhaps you’re referencing something different when you specifically say it’s from the “transmission.” What I’m describing on mine doesn’t involve the transmission slipping, so we may well be talking about different things. If so, please disregard the rest of this.

In any event, while mine still has such problems occasionally, it has gotten much better as a result of four things:

1. I always start out driving in low to the extent I can while the car’s still warming up; and even after it’s fully warmed up, I start out in low from a complete stop if I’m stopped on a significant incline or in a situation where stalling out would be dangerous (busy intersection).

2. My mechanic fixed a fuel leak on the carburetor and did a tune-up.

3. I started using ethanol free gas.

4. I vented my gas cap by drilling a very small hole in it (taking it off of and away from the car to do so, obviously). Note: I bought several (4) gas caps that were billed as being vented. All four of them resulted in noticeable vacuum and/or pressure in the tank when removing them after driving even for a short time, and regardless of the ambient outdoor temperature. It is possible that all four were defective; it is also possible that I have some unknown underlying problem that is being compensated for by having drilled the cap.

As to the latter two: correlation isn’t causation and all that, so I cannot say with 100% certainty how much of the improvement came from switching to ethanol free and venting the gas cap. I can say that on a scale of 1-10 where 1 is “undriveable” and 10 is “drives like a brand new modern car: perfect with no hesitation at all ever,” steps 1 & 2 above took me from about a 3/4 to a 5/6 and steps 3 and 4 then took me from a 5/6 to about an 8. Again, though, I’m sure there were multiple confounding variables (weather, driving the car more/less, etc.), but even accounting for those, I’m convinced that switching to ethanol free and drilling the gas cap made a significant difference. Gonna get the carb completely rebuilt, which will hopefully get it up to a 10 on this issue.

(And if I’m off base here, apologies! Feel free to ignore or delete.)

Interesting you had several caps that weren't venting. I thought all the suppliers had addressed that issue. It is very important. I've never had an issue using ethanol infused gas but have a station close by that has pure gasoline.
I'd look to the power valve on the carb. Either not working at all or not properly adjusted. When you flip the throttle ( when not running )and look into the carb you should see a good shot of gas into the throttle body. It's designed to stop stumbling on take off.

 

[zeroroadkill]

Had the same stumbling at full throttle, turned out the replacement fuel pumps (3) were garbage.. I installed an electric fuel pump (kept the original for the vacuum wipers) and solved the problem. The electric fuel pump mounts on the side of the chassis and uses the two bolt holes where the clutch pedal mechanism goes. No more vapor lock. Also cut the gasket around the inside of the gas cap about 1/8” to allow venting.

 

[doug7740]

Here is a Tech Tip I wrote four years ago about the fuel cap venting on a 1955/1956 Thunderbird-

Vapor lock problem on a 1956

A update on my vapor/??? problem. I've done some more testing and here's what I'm finding: I had a bit of a chat with Chris Ames (the water pump spacer guy) about vapor lock problems and he told me a couple of things that helped him. One was insulating the line from the fuel tank to the...

forums.fordthunderbirdforum.com

doug7740
1955 Thunderbird Blue

 

[Howard of Florahome]

Had the same stumbling at full throttle, turned out the replacement fuel pumps (3) were garbage.. I installed an electric fuel pump (kept the original for the vacuum wipers) and solved the problem. The electric fuel pump mounts on the side of the chassis and uses the two bolt holes where the clutch pedal mechanism goes. No more vapor lock. Also cut the gasket around the inside of the gas cap about 1/8” to allow venting.

Thanks for the feedback. I hadn’t considered vapor locking may be my problem because I have been running an elec fuel pump for some time. I’ll make a vent hole in my gas cap to see if that helps. Also my transmission seems to be slipping (stuttering) and there is a viberation that can be felt in the car. Yesterday I noticed a hesitation to engage when I placed it in reverse.

 

[willcarterdc]

Before drilling the gas cap, it might be worth first trying a replacement vented gas cap for a while to see if that makes a difference; and then, if that doesn’t make any difference, cutting away some of the gasket on that fuel cap as recommended by others and trying that for a while to see if it makes a difference. Neither of those ultimately ended up working for me, but I did try them first, following the “additive/a little bit at a time” approach.

I mention this only because I feel sure that drilling the gas cap is not ideal even if ultimately necessary and even though it’s what I ultimately did. I don’t know exactly why I feel it’s not ideal (I am far from an expert), but I do have some intuitions: e.g., gas fumes in garage, roll-over danger, spill/leakage danger on sharp left turns if the tank is very full, etc. These might all be minimal or hypothetical risks, but they do exist. So, I consider having drilled my gas cap to be a temporary stopgap solution rather than a permanent or first-choice solution.