Expert: Brad Sears - 12/7/2009

Question
I am working on a 1965 thunderbird for a friend that has a 390 with the original autolite 4 barrel and auto transmission. I removed the carburetor and discovered the the vacuum diaphragm for the secondaries was torn and the secondaries had not opened for some time and were not moving. After a complete cleaning and reassembly with new gaskets and diaphragm and setting all adjustments to factory specs the carb is back on the car and the secondaries still will not open. I removed the rod that connects the diaphragm to the butterflies and the diaphragm seems to work great when the throttle is opened and I can manually open the secondaries but it seems like manifold vacuum is holding them closed. It seems like the diaphragm wants to do its job but the secondaries need a little extra to open. Any help would be appreciated.

Answer
Time for the carburetor to come back off. Make sure that the secondary throttle shaft is not binding in the slightest amount. The vacuum that works on the diaphragm is Venturi vacuum and amounts to way less than manifold vacuum. Then check the diaphragm cover and the carburetor body to make sure that is not warped and leaking. Also check where the throttle body attaches to the main body for warping. Also be sure that the new diaphragm does not have a pin hole in it. Keep me posted.
Brad