Got a 2 speed Jetaway in my 68 Cutlass Supreme w 350. When accelerating pretty heavy continously the car starts to hesitate and jerk at 40-50mph. Almost as if it is trying to shift but can't catch or it isn't getting fuel. Makes a vroom ....vroom .....vroom ...until I let go of the gas and accelerate soft. Once I let go and repress it goes over 40mph fine. I thought it was carb related but after I had my intake and carb swapped it goes the same thing. Does this sound like it is trans related or maybe fuel pump? Want to dump it and get a th350 eventually

 

Is your modulator leaking transfluid? Is the vacuum line secure to it and the manifold. Does it have good vacuum? Is the fluid a nice red color? How does it shift when you drive normal?

 

I believe the governor was leaking but the seal was replaced and now it does not leak. Just had a new intake and carb put on so everything was reconnected properly. The trans fluid is a bright red. I rarely take it over 70mph. Ive felt it shift a few times and its a really hard knock/kick once it does. It drives fine and normal most the time. But if I take a turn a bit faster sometimes the trans slips. Same if I try to floor it up a steep hill. Or when I floor it and gas heavy from 0-40 once it gets to 40 it jerks/hesitates and doesnt goes above 40 until I let go of the gas and repress it.

 

Fuel pump, or clogged sock in the tank!
I was going to say fuel filter, until I read you changed the carb.
Trans isn't in the best of shape, either - I'd start looking for a 200r4 bolt-in.

 

That is what I am starting to lean towards. Sorry but what is the difference between the TH350 and 200r4? There is a local shop that sells rebuilt TH350 short tail for $399 with shift kit.

 

The biggest difference is 1 more gear!!!! The 200r4 is an overdrive trans, the t350 is a 3spd. The $399 is probably for chevrolet, not an oldsmobile trans. Anything over st300 would be an improvement. The 200r4 is really the way to go. It has a great 1st gear and the o-drive will help with economy.

Getting back to your problem, generally most shifting problems associated with your trans is with the modulator. They are not expensive to replace and very easy. Also look at the vacuum lines again, if they are britle, replace them very cheap and very easy. Ensure you have manifold vacuum at the modulator.

Also, make sure your kickdown switch is adjusted properly and that the solenoid in the trans is not sticking. It takes 12v to actuate.

 

Never been in an old car upgraded to the 200r4, but really like the th350 in my cutlass and they are pretty cheap. I would compare installed cost for both and balance against your budget and perfomance expectations.

 

One thing to consider on a Jetaway to 200 swap is the shift detent gate in the steering column. You need two more positions. The detent plate can probably be modified, but remember 68 is a one-year-only column and may not mod as easy as a 69-later. I DK of anyone offhand who's done this with a 68 column shift car.

 

I'm going to bring this one back out because I haven't figured this out yet. After taking changing the transmission modulator into consideration I loaned a MityVac from AutoZone and test vacuum lines and the modulator. Both the steel line from the intake and the modulator held vacuum. Does this rule out the modulator is bad? I am most likely going to replace it anyway as the rubber hose on it is a little brittle. The hesitation/surging is guaranteed when getting to 40-50mph if you accelerate continuously pretty heavy. It will not go above those speeds until I release the gas and press again soft

 

I don't think it's the trans - now I'm thinking more the carb!
Almost sounds like a partially clogged jet!
Try this: warm the car up, take a short ride, and come back.
Remove the air cleaner, and while running, run the motor to about 4000 rpm and put your hand over the carb 'til it just about dies - remove your hand 'til it cranks up again, and cover it again - do this 4-5 times and go for another ride - should help, if it's crap in the carb!

 

The carb is brand new along with the intake. It did this before I swapped the carb. Def not the carb.

 

Yeah - I read that - but if you've crap in the tank - - - what about filters??

 

Does the transmission make any strange noises when you are driving?

If not, then its possibly a mal-adjusted throwout bearing. You need approximately 3/4-1 inch of freeplay on the clutch pedal. If it is not the throwout bearing, then I would suspect the pilot bushing is worn.

 

Yeahbuddy, have you gotten your issue resolved??

 

You may want to hook a clean fuel source to the supply side of fuel pump and re-test, this does not eliminate fuel pump!

BTW whein the last time you did a complete tune up?

Regards, Pat

 

What are we working on? make/model? As general rule on clutches, if the clutch freeplay is adjusted properly, then I would suspect the input shaft bearing is getting noisy! I had this happen to me before on a clutch job, put it back together without checking shaft play, had to R&R Input shaft bearing.
If you got it apart checking for what causing the noise you might as well replace pilot bushing/bearing and throw out and inspect the clutch and flywheel!

Regards, Pat