on an 1979 Buick Park Ave, can’t change from 1st gear to drive. Preliminary diagnosis is a low vacuum at the transmission regulator valve(?). Car has 207,xxx miles. Is this a carburetor problem that may be clogged with carbon deposits, or what else can be causing the problem. That’s for any definitive insight.
A carburetor problem is a possibility, but most of these transmissions get manifold vacuum from a manifold tee. I’d suspect a collapsed vacuum line or a line plugged with carbon/sludge. It shouldn’t be hard to trace the line from the vacuum modulator on the transmission to the junction point on the carb or manifold.
There is a short length of rubber vacuum hose coming off of the modulator at the transmission, it connects to a metal tube which runs from underneath the vehicle, up past the exhaust manifold and up to the intake manifold where you will find another short length of vacuum line connecting the metal tube to the vacuum port on the intake. Make sure both rubber lines are good and the metal tube is free from blockage. Also observe, when removing the vacuum line from the modulator, that there is no fluid dripping from the modulator vacuum port or from the hose. If there is fluid coming from the modulator or vacuum line, you will need to replace the modulator.
Transman is the expert. First thing though is to put a vacume guage on the engine itself to make sure it is generating good vacume. Then check the lines. If not good at the engine you would have a worn engine and other troubles like hard brakes I believe.
If previous suggestions have not found the cause check the ignition timing. But if you tested the manifold vacuum and found it low timing would be at the top of the trouble shooting list. And also a restricted exhaust.