Classic/Antique Car Repair: 1951 Chevy w/216 wont idle and hesitates at shift, windshield wiper motor, hairline crack

Question
Dick-

I have a 1951 Chevy Styleline Deluxe with a 216ci that is stock that I purchased last year.  Last summer, I noticed that I needed to run with a partial choke to really have it idle decent at a stop.  It also seemed a little doggier that it should, so I just had the Rochester C carb totally cleaned and re-built.  I have it all re-assembled, and I am still un-able to get the darn thing to idle without some sort of choke on.  And at that point, it is idling pretty fast.  I took it out for a drive last night, and when I was on the gas and cruising, it seemed to run pretty good, however, when I would jump on it, or shift, there was a delay.  After the delay, it would really kick in go.  Do I have some sort of timing problem? or is there something else that could be causing me troubles.  When I drove it last night, I had the air/fuel mix screw turned 3 1/2 turns, and the rpm adjustment as high as I could get it.  The cruising season is upon us, so any help would be appreciated.

Thanks!
Aaron

Answer
I think you have a vacuum leak, probably at the base of the carburetor, but possibly at one of the fittings that go into the intake manifold, or at the other end of whatever line that fitting is connected to, like the windshield wiper motor or the fuel pump if it has a vacuum booster.  It could also be a hairline crack in the carburetor base itself, or worn throttle bushings/shaft.

To diagnose this, set the idle mixture screw about 1 1/2 turns out from the inner seat, then adjust the idle speed (or hand throttle if you have one) to keep the engine running - even if it is running really rough. Then, take a propane torch, turn on the gas valve BUT DON'T LIGHT IT!, and move the tip of the nozzle around any suspicious areas - for instance the base of the carburetor, the sealing surfaces of the intake manifold to the block, the wiper plumbing etc.   When you find a vacuum leak, the engine will speed up and run better because it will be sucking in the propane gas instead of pure air.

The hestitation on initial acclerator operation may be a result of the same problem, or your acclerator pump may not be working or adjusted right.  I have had very poor results with professionally rebuilt carburetors - I have learned to just buy the kit and do it myself - there are so many pitfalls in rebuilding a carburetor that only a dedicated car owner with good eyesight, the ability to read and follow directions and a clean well lit place to work can do a good job.

I think all your problems can be explained by the carburetor rebuild and the vacuum leak - so don't start fiddling with any other parts until you clear that all up - you might just muddy the waters if you start changing other things.

Dick