CrossedFlags77

Background: 1977 Corvette, assuming its a TH350 transmission. I swapped the intake manifold yesterday for a Performer intake. Transmission shifted fine before changing intake.

Today: At first the transmission wouldnt shift in to 3rd at all, only when I would manually hold it in 2nd and then put it into 3rd and it would only go into 3rd when I had reached high rpms. I have done the basics, checking fluid, checking vacuum lines, reset the downshift mechanism. The vacuum line to the manifold is fine. I replaced the rubber vacuum line that connects the hard line to the modulator down at the transmission because it was cracked. That helped a little bit but I it still won't shift automatically into 3rd until I reach about 3000-3500rpm while cruising. When it does shift into 3rd it is a very hard shift. I can manually shift through the automatic with no problem. It shifts where I want it to when I do it manually. I assume it would be a vacuum leak but there is only one line to the transmission that is a vacuum line and I have checked it. Any suggestions would be appreciated.

Happy 4th!

wnmech

For sure it has something to do with your vacumn modulator because on that transmission the shifting is controlled thru it. Make sure you are getting enough vacumn from the port on your new manifold to allow the transmission to shift. Since it worked normally before you changed intakes I would suspect that is the problem. If that all checks ok then spend around $10-$15 dollars and buy a new modulator. Does the engine still idle alright? If not you may have a intake leak somewhere. Good luck

694speed350

CrossedFlags77

The engine idles fine, and feels like it has more power than before. I will check the vacuum to the modulator tomorrow. How much vacuum is adequate?

wnmech

I am not exactly sure how many inches of vacumn it takes to operate the modulator but it will vary because the differant inches of vacumn will vary by engine speed allowing shifting ranges. You did tee off the vacumn modulator line off of the manifold and not the carburator? Good luck and let us know what you find.

kevinator80

I had the same exact problem with an 80. I found a vacuum line disconnected from the check valve on the drivers side firewall area. Sorry I can't remember exactly which line. I reconnected it and everything shifted fine so I did not bother tracing the line. I had the exact same symptoms so look for a vacuum leak. Good luck.

wnmech

Adding to what kevinator80 just mentioned you could cap off all other vacumn sources from the manifold to where you are only using vacumn for the transmission and see if it shifts. Good luck

CrossedFlags77

Turns out I had a small leak somewhere between the manifold and the modulator. I replaced the hard line with a regular rubber vacuum line for testing and it shifts perfectly. I knew it had to be something simple. Thanks for the help.