C4 auto transmission shifting issue

[DeepC73]

Hello again everyone,
My 1973 w/c4 auto transmission has started not wanting to shift. I had it rebuilt about 1500 miles ago. It has ran perfectly up until yesterday. It has always shifted at 25 and 45 mph unless I’m really on it. After it warms up now and I have been driving it for a few miles it shifts hard at around 35 and 55mph. It’s really causing the engine to rev pretty high rpm before shifting. Any ideas. I checked all of the vacum lines, they seem good. What else could cause it. The only recent modification is a new intake manifold gasket and I went back to the original air cleaner assembly.

 

[75MIKE]

Kick down lever isn't bound up and is pulled forward with a little spring tension?

 

[Dfroman]

I would check the rubber hose for cracks or holes.

 

[DeepC73]

I checked the hose, no cracks. I checked the kick down lever; the wire for the neutral start switch was against it. I moved it, but no change. Is there away to adjust the shift points (rpm) lower? It is still shifting around 32 mph, much harder than normal. I adjusted my timing a few days ago, could that have changed anything?

 

[jckkys]

The vacuum modulator is adjustable with a screw driver in the hose nipple. I think counter clock wise lowers the part throttle shift point. A shorter pin under the modulator does the same thing. It's been about 20 yrs. since I set mine up, so I would look up the specifics. The kick down lever sets the full throttle shift point.

 

[Rustytruck]

make sure the kickdown rod is being pulled back towards the carb add a spring if necessary. stock there is the spring that pulls the rod up and another spring at the lever where the rod attaches at the carb. cant let the rod float around on its own.

 

[DeepC73]

Rustytruck,
The rod is indeed floating. It only has one spring on it, on the carb. Where does the other spring belong?

I adjusted the screw in the modulator a bit. It is much better, but I’m still puzzled by the sudden change.

 

[Rustytruck]

At the very end of the rod up at the carb the factory had a spring from the rod pivot on the carb linkage pulling towards the radiator and hooked below the factory throttle position solenoid. just enough spring to add positive pull on the rod.

 

[DeepC73]

Rustytruck, that spring is there. However, if I understand you correctly, there is another one?

After I adjusted the modulator today, all was fine. My new wheels came in and I had to install them. I went from the factory 15” wheels, to a 17”. Now the transmission is doing the exact same thing again. Is it because of the larger diameter wheels or is it just coincidence?

 

[Rustytruck]

the kick down lever at the tranny has its own spring control. just doesnt hold position with the weight of the down shift rod.

 

[jckkys]

I repeat the kick down lever effects full throttle shifts, or as you called it "realy on it". Any time you're not realy on it the shift point is established by the vacuum modulator and the governor. Just look at the throttle lever on the carb as the throttle is fully opened. It won't even touch the kick down lever till the throttle is about 90 percent open. The kick down valve in the C4 valve body has a coil spring to return the lever that works by itself when there's no binding in the rod. The spring on the carb is a back up for that spring. If you depress the kick down rod, does it come back to the original position when you release it?

 

[DeepC73]

Update: I have tried EVERYTHING! It appears it is an internal issue with my transmission. The guy who rebuilt it last April thinks it could be the Valve body. I will be dropping it off on Tuesday. Thank you to everyone who tried to help me out. This forum, and the people on it, is AWESOME!