Transmission Upshift and downshift after carburator change

[David P. Barter]

Hello Folks,

So I have replaced the factory Q-jet with an Edelbrock carb with Electric Choke and everything is plumbed back properly, wired properly and I cannot find any thing not connected like it was before the change. However I did remove the electric module that controls the downshift when the Q-jet came off as there is no place to make it work with the current new carb. I am not sure if this is related, my gut tells me it is, and what I am finding now is that the there is a long delay when upshifting to 4 gear. The car seems to stay in third a long time. Once it shifts it is fine and downshifting is not really the issue. I have also flushed the tranny by changing the filter and fluid in the pan, cleaned the pan well, replaced the gasket and refilled. Fluid level is on the money and it drives really well all but that shifting into 4th issue.

Any thoughts? Thanks folks!

["Cadillac Kid" Greg Surfas 15364]

David,
What year Cadillac do you have.  I don't recall any with a 4 speed transmission and a Quadrajet Carb.
Greg

[David P. Barter]

Sorry Greg, i meant 3rd gear. it is a 1969. Deville Convertible with the standard 472.

[David P. Barter]

Thanks for the info George. It shifts fine down and up until it goes 2nd to third. and it eventually shifts if the rpms and speed are correct but it takes a long time. So I am wondering if it is a vacuum issue. that is all I can think of. But thanks!

["Cadillac Kid" Greg Surfas 15364]

David,
You seem to be on the right track suspecting a vacuum issue.  Check to see that the vacuum line from the transmission modulator to the carburetor connection is not separated or broken.
Greg

[Dave Shepherd]

That switch that is now not installed allows the trans to down shift to second for full power passing. ( kickdown switch)

[TJ Hopland]

The electric switch on the throttle linkage is the 'kickdown'.  If you are going less than about 60mph in 3rd and that wire gets a +12 signal it will downshift.   As long as it is engaged it will tend to stay in 2nd.  The switch only engages when the pedal is basically on the floor.  Most of the GM vehicles that used the THM400 transmissions had the switch on the firewall inside the cabin and the gas pedal activated it.  It was just Cadillac and some of the other cars that had it up on the carb so it would not be a great surprise that the aftermarket does not offer an option for that switch.    If I get my 73 out later today and remember I will try to take a pic of the switch that I rigged up on my EFI system. 

The only external control for 'normal' shifting is the vacuum modulator.   No or low vacuum signal assumes you are trying to accelerate hard so it will firm up and delay shifts.  A strong vac signal assumes you are under light acceleration or cruising.  The transmission signal is a manifold vac port which is usually the physically lower ports on the carbs.  The higher up ports tend to be 'ported' or 'timed' ports which will have different signals than a manifold signal and are usually not present at lower or closed throttle settings.  On all the Q-jets that I can remember working on the transmission port is center back side either above or below the port that goes to the brake booster.  Its usually a short piece of rubber line then steel line down to near the modulator where it goes back to rubber.

Personally I have never been happy with carb swaps on engines after about 66 when they started with emissions stuff.  Once they started adding emissions stuff the carb / engine combo became much more of a matched system package and if you mess with one part of the package without understanding and likely modifying the whole system you usually don't end up with as good a package as if you just got the original system properly repaired.

[David P. Barter]

Thanks TJ - it turns out that the line was the wrong size for application so the vacuum was not acting properly, therefore the vacuum modulator was not engaging properly etc. I have since replaced all that line from the steel tube to the manifold vacuum port on the front of the carb and it has solved the upshifting issue. I have also ordered the B&M Detent Switch kit and will hook that up to solve the downshift issue. Thanks for all the feedback.