Thanks gents, for your comments. I have fine tuned the carburetor on a chassis dyno and it idles about 1,000 RPM (my genuine Smiths tach doesn't read that low);
rough idle when it is cold, but smooths out a bit when it warms up. I would have trouble keeping it running if I slowed the idle down. I'm using the plugs Shelby gave me, Champion RN7YC, and they appear to be OK. As far as sharp edges in the combustion chambers, etc., I have no idea, but would be very surprised and disappointed to find anything like that. I've been told by some experienced folks that this is a common problem with compression ratios over 10:1, so I think I am just going to live with it. Shutting the beast down with the clutch is almost automatic now anyway. But I am still waiting for the first E85 fuel to be available in my neighborhood and all this run-on stuff will be a thing of the past. Rich