GM-GMC: Choke, vacuum tester, vacuum line

Question
I have a 1978 Chevy Van with only 26,000 miles. The choke butterfly never opens.  It appears to be connected to a vacuum controlled pulloff.  Any thoughts on what the problem might be?
Thanks,
Marshall  

Answer
Either the butterfly is stuck closed because of carbon, or the vacuum pull-off module (or its source of vacuum) is bad.

Get a piece of vacuum tubing and connect it to the pull-off on the side of the carburetor (remove the vacuum hose you see connecting to it).  Use a vacuum tester, or suck on the other end of the hose.  If there is no resistance and the butterfly does not open, the pull-off is bad.

If there is resistance to the vacuum you apply, try using some carb cleaner to free the butterfly.

If the butterfly opens (or you free it with cleaner), reconnect the original vacuum line and start the engine.  The butterfly should open at least halfway.  If not, remove the vacuum line at the pull-off (with the engine still running) and check for vacuum.  No vacuum indicates a bad or clogged vacuum line or a defective thermo-switch (if so equipped).  Follow the vacuum hose back to its source on the carb or intake manifold.  Something along the path is preventing the vacuum from reaching the pull-off.