OK, she's now idling nicely at 800-900 with some room to spare on the idle screws. I can take it down to a somewhat rough 500 rpms if I so choose.

The airflow is balanced with a carb-balancer. When I lift the #1 test pin, the engine increases idle and then drops down low. I get the same behavior when I lift the #2 test pin.

What was the problem? Not entirely sure, but I adjusted several things in the process.

#1 I pulled the carbs off, loosened the idle set screws and loosened the throttle plate screws. I then let the butterflies snap smoothly and fully shut and re-tightened the screws.

#2 I shortened both slightly and increased throttle return spring tension and reattached the them when I reinstalled the carbs.

#3 I adjusted the center linkage. I loosened up the two set nuts and let the throttle plates snap fully closed with return spring tension. I then played with the middle linkage until the pins caught the throttle forks at precisely the same time AND the residual tension from the main throttle linkage pulled the butterflies fully closed.

#4 I reattached the choke cables and started the car with full choke. After it had warmed up, I released the choke and balanced the airflow and throttle to around 500 RPMs. I then adjusted the mixture 3 flats and a time until the idle was smooth and level...

#5 I balanced the mixture using the idle response to the lift pins.

#6 I adjusted the idle up to 800/900 RPMs and then readjusted the timing back to 10 degrees BTC. I re-balanced the airflow and idle speed afterwards.

This left me with an engine that idled fairly nice and promptly shut down w/o dieseling. I did not have time to take it for a test drive, but I'll probably give that a go tomorrow.

My overall verdict on the probable culprit? I think it was a combination of slight mis-adjustment of the throttle plate in #2 and the middle linkage being slightly out of synch.

My lifters have been adjusted several times recently, but with all the carb swapping and timing adjustments, I probably need to re-check my valve lash and adjust as needed. Followed (of course) by re-adjustment of timing, mixture, and throttle.

I think I may be over this hurdle and can get back to the dozen other issues/tasks on the car.


Thanks to all who gave suggestions and responded to my requests for help!
I'll post back after I give her a test drive or two.