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Old 07-28-2020, 06:28 PM
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Question Turbo 350 Question

Hi I have a turbo 350 street/strip package behind a 383 with low vacuum in my 41 coupe. I have a very soft shift through the gears, no real sense of shift but you can see the rpm's drop on the tach. I have the vacuum line to the modulator on the constant vacuum side of the carb , not the ported side. Is there any type of adjustment to firm up the shift. Any help or ideas would be greatly appreciated.


Thanks

Joe

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Old 07-28-2020, 06:46 PM
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What is the stall speed rating on the torque converter?
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Old 07-28-2020, 08:07 PM
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Crosley , thanks for the reply.I checked the paperwork that came with the trans but it didn't say, I thought they said it was either 2200 or 3200 but I don't remember. the torque converter is light blue if that helps. also the rear gear is 2:73
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Old 07-29-2020, 08:28 AM
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Did you try it with full manifold vacuum?

"The vacuum modulator, together with the governor, controls the shift points on the TH350 transmission. All stock TH350s were manufactured the same; their differences in shift characteristics for specific engines and vehicles were set with the governor and modulator.

The governor lives behind a cap secured by a bail on the drivers side of the transmission in front of the tail cone. It is a little widget that spins with the output shaft. There are flyweights and springs on this spinning widget, and when the flyweights overcome the springs and hydraulic pressure, the flyweights push open a rod valve that lives in the center of the governor shaft. Adjusting the governor weights and springs can be done to adjust the wide-open throttle shift points of the transmission.

The vacuum modulator adjusts the hydraulic pressure to the governor dependent on the engine vacuum. At wide-open throttle (low vacuum), the full hydraulic pressure is provided to the governor and the wide-open throttle shift points are selected. When there is engine vacuum on the modulator, the hydraulic pressure to the governor is reduced, and the governor finds it easier to overcome the hydraulic pressure to open the shift valve, so earlier shifts are selected. Adjustment or replacement of the vacuum modulator can be used to adjust the part-throttle shift points.

The kick-down cable on the TH350 bypasses the modulator and provide full pressure to the governor, enabling the wide-open throttle shift points. The result is that, when the pedal is depressed past the kick-down point, the modulator is bypassed, and the full hydraulic pressure provided to the governor, which will cause the transmission to downshift if it is below the RPM for the wide-open throttle shift point.

Does that help?"
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th350 Transmission Vacuum line to intake??? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

I remember back in 1975, the TH350 in my new '75 Monte Carlo had barely perceptible smooth shifts... it lasted 200K miles but was only behind an emissions 2 bbl. 350"... and my schoolteacher wife usually drove it...
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Old 07-29-2020, 08:34 PM
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Update

Hi I had time to check some things out today. I pulled the vacuum line off of the carb and put my mighty vac tool on it to see if the modulator would hold a vacuum , it didn't . took the line off and bench tested the line, wouldn't hold vacuum . I made a line out of break line and two short hoses (one each end ) holds vacuum now took for a test drive and a good shift between 1st and 2nd but going into drive still soft. With the help I got here I am moving in the right direction , now where do I go from here ? Thanks again for all the help.
Joe
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Old 07-30-2020, 05:23 AM
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Quote:
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took the line off and bench tested the line, wouldn't hold vacuum Joe
Yeah, the modulator hose on my Ford truck also got a mysterious hole in it...
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Old 07-30-2020, 07:44 AM
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Quote:
Originally Posted by tools View Post
Hi I had time to check some things out today. I pulled the vacuum line off of the carb and put my mighty vac tool on it to see if the modulator would hold a vacuum , it didn't . took the line off and bench tested the line, wouldn't hold vacuum . I made a line out of break line and two short hoses (one each end ) holds vacuum now took for a test drive and a good shift between 1st and 2nd but going into drive still soft. With the help I got here I am moving in the right direction , now where do I go from here ? Thanks again for all the help.
Joe
No vacuum results in very high slighty hard shifts.
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Old 07-30-2020, 07:50 AM
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Hi So with the shifting firmer from 1st to 2nd how do I get a firmer shift into drive ?
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Old 07-30-2020, 05:12 PM
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Some TH350 modulators are adjustable using a screw that is inside the vacuum line fitting. However, I'm not sure how to tell whether your modulator can be adjusted. I don't think the OEM modulators have the adjustment screw.

Also, I'm not sure this even relates to your shift problem, but check the adjustment on the TH350 kickdown cable. It determines when the car will downshift from 3rd to 2nd when you accelerate. The cable is clipped into a bracket right behind the carburetor. If you look at the back of the black plastic fitting around the cable (the side next to firewall) you should be see there is a tab in the middle that can be slid up. You slide this tab up, push the carb linkage to full throttle, and then slide the tab back down to lock the setting.

Bruce
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Old 07-30-2020, 05:23 PM
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That converter is going to soak up your shift points. Especially with a highway gear like you have. How does it do on Full Throttle shift? It will be soft everywhere else... But that's not a bad thing.
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Old 07-30-2020, 08:18 PM
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Hi I did remove the hose on the vacuum modulator and there is a slot on the piece that looks like a jet inside , so I am going to assume that it is adjustable . To firm up the shift should I screw it in or out . Thanks Joe
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Old 07-30-2020, 09:30 PM
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Quote:
Originally Posted by tools View Post
Hi I did remove the hose on the vacuum modulator and there is a slot on the piece that looks like a jet inside , so I am going to assume that it is adjustable . To firm up the shift should I screw it in or out . Thanks Joe
Firmer shift is inward(clockwise looking into the nipple), putting more pressure on the internal spring in the modulator.
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