OK- - - -all fixed and headed home! There was a hole approximately 1/8" in diameter that was directly in line with the throttle blade when it was fully closed. That would have applied straight intake manifold vacuum to the advance at idle. Also, there would have been an air leak past the throttle blade that might have raised the idle sped too high, even with the butterfly fully closed. Bad juju!
I tapped the existing hole for a 10-24 flat head machine screw, which I inserted from the outside after grinding the head down a bit to allow it to pass through the hole drilled for the 1/8" pipe thread. I installed the screw with a drop of blue Loctite after cutting it off so just a couple of threads would protrude into the carb throat, then smoothed off the projecting stub with a Dremel tool and a very fine grit grinding stone. The screw slot was left vertical, and I drilled a 1/16" hole near the top of the slot. Now the hole will be slightly above the throttle blade at idle, and will allow the vacuum advance to work as designed- - - - -no advance at idle, then it comes in at off-idle and light throttle cruise. On acceleration or a hard pull with a wider throttle opening, that advance goes away due to low vacuum to prevent detonation when the engine is pulling hard.
Jerry
