ok 350, good operating condtion. 16" vac at idle. idle is 850 rpm (stick in neutral) timing is 11 btdc. sometimes when i shut it off, it acts like it is shutting down then bump bump bump.... sometimes still runs, somtimes stops after that. there is no short to ignition (stock hei) idle mix is good (just a pinch rich) to cool chambers. throttle blades are not open too much and no vac leaks. i am stumped. please brainstorm with me.

With that much vacuum, I'd try idling it down to 750 and see if that helps. Stock tachs aren't the most accurate things in the world and you may actually be idling higher than you think.

try changing to colder plugs and better fuel. the higher the octane, the better it resists pre-detination.

i run 93 octane gas, so unless the winter mix of fuel is messing with it.... is that a possiblity?

do you 'blow the cobs out' regulary? if your running on the rich side, you may have carbon build up causing a hot spot or a glow plug effect. too hot of a plug will do the same. check your plugs for carbon build up and go from there.

First, make sure it's not voltage bleeding into the ignition somehow- yank the plug at the distributor as a diagnostic test to shut down. Don't rely on the ignition switch.

If that makes no difference then I have a question..... vacuum advance connected to a full manifold vacuum source? If so, that's your problem.

just an upadate, it was a vac leak at the master cylinder. fixed now, and problem is gone. (the ign is on ported vac for the advance)