Normally I wouldn't seek advice on such a seemingly simple problem, but I have been battling a hesitation on acceleration. The vehicle is a stock 1967 Mustang, California 289 W/Autolite 2100. Usually when this occurs, I replace the accelerator pump diaphragm and everything is back to normal. Not this time, so I tried a few simple things like adjusting the timing and checking the vacuum advance diaphragm for leaks. Still no fix.
After that I Checked the ported vacuum switch, Took the carburetor off and thoroughly cleaned and rebuilt it including replacing, the float, accelerator pump and changing to a different booster cluster. Still no good. I replaced the PCV valve and isolated the entire vacuum system. I have a Pertronix 0ne and Two, so for the hell of it I switched them out. With the exception of the hesitation, The car starts idles, and runs normally. The manifold vacuum at idle (2500ft elevation)is a steady 17in. The only thing I can think of is maybe the exhaust system has a partial block somewhere in it. I grasping at straws now and looking for suggestions.