Help...! What did I do now...?
Thread Starter
Obsessed
Joined: Apr 2019
Posts: 627
Likes: 394
From: Valencia, Spain
2025 C3 ('68-'73) of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
Help...! What did I do now...?
Went to my buddies shop today to make some adjustments on the carburetor to get the secondaries to open up at WOT... Got some good advice on how to adjust the slack in the throttle cable and make the adjustments on the Quadrajet... That went well, made the adjustments and gained a bunch more power. We had a look at the timing made a slight adjustment there but was already pretty good anyway.
Now, put everything back together and driving home I notice the turbo 400 is not shifting well... It won’t shift to the next gear until 3500 or 4000 RPMs and at times feels like it’s slipping. Starts out fine, feels great, but doesn’t want to shift until it’s revving way up there. If I back off to bring the revs down and the give it gas again feels like I’m in neutral almost...
This must have something to do with what I was working on but what? Maybe vacuum advance or some vacuum line got disconnected??? Really worried now, hope it’s a simple fix. I have a ralley next weekend... Anyone know what could be causing this weirdness ? Was running great before... Sometimes I don’t know when to leave well enough alone..🤷♂️
Now, put everything back together and driving home I notice the turbo 400 is not shifting well... It won’t shift to the next gear until 3500 or 4000 RPMs and at times feels like it’s slipping. Starts out fine, feels great, but doesn’t want to shift until it’s revving way up there. If I back off to bring the revs down and the give it gas again feels like I’m in neutral almost...
This must have something to do with what I was working on but what? Maybe vacuum advance or some vacuum line got disconnected??? Really worried now, hope it’s a simple fix. I have a ralley next weekend... Anyone know what could be causing this weirdness ? Was running great before... Sometimes I don’t know when to leave well enough alone..🤷♂️
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I am thinking you lost the connection for the vacuum modulator.......that would cause the symptoms you are stating and you were playing around where vacuum lines are up top.
Verify all connections......
Jebby
Verify all connections......
Jebby
Burning Brakes
Joined: Dec 2008
Posts: 971
Likes: 45
From: Middlesex NJ
Sounds like the vacuum line going to the vacuum modulator on the trans...
Maybe you cracked one or ripped one.
I recall that it will shift very late if the vacuum is low (almost like under full throttle).
Retrace your steps you did yesterday and look for the vacuum line going down to the trans. There is usually a lil rubber section down at the modulator.
Not sure what year you have or what mods have been done. The line going down to the trans in most cases was steel with rubber at each end from what I can recall. Havent had an auto old car in quite some time so not sure what you have there especially when it could have been modified over the years.
Maybe you cracked one or ripped one.
I recall that it will shift very late if the vacuum is low (almost like under full throttle).
Retrace your steps you did yesterday and look for the vacuum line going down to the trans. There is usually a lil rubber section down at the modulator.
Not sure what year you have or what mods have been done. The line going down to the trans in most cases was steel with rubber at each end from what I can recall. Havent had an auto old car in quite some time so not sure what you have there especially when it could have been modified over the years.
Thread Starter
Obsessed
Joined: Apr 2019
Posts: 627
Likes: 394
From: Valencia, Spain
2025 C3 ('68-'73) of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
I have a 1971 350/ 270hp, no mods all original all matching numbers... I suspect it’s a vacuum problem as well... I’ll have to wait until Monday to bet it back to my buddies shop. 
Unfortunately I don’t have a garage or place to work on it at home.
Unfortunately I don’t have a garage or place to work on it at home.
Last edited by Daveinspain; Feb 22, 2020 at 12:22 PM.
Melting Slicks
Joined: Oct 2012
Posts: 2,156
Likes: 273
From: Dunedin NZ.
I have had the symptoms you describe when the vac line came off the vacuum modulator at the transmission end. If you put the passenger side wheels up on a kerb you will have enough room to peek underneath and see if that is the problem. I would be looking at the top end first as mentioned by others.
Thread Starter
Obsessed
Joined: Apr 2019
Posts: 627
Likes: 394
From: Valencia, Spain
2025 C3 ('68-'73) of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
Fixed..,!!!
Just wanted to let you guys know I was able to get the Vette up on a lift today and sure enough I found the vacuum tube going to the modulation valve on the transmission disconnected... Stuck it back on and everything is back to normal... 😊
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