OK, last night the lack of fore-aft freedom of the crank caused me to inspect the prop drive washer. I wanted to make sure that locating pin was still in place and that it's groove in the washer wasn't too deep.

When I pulled the prop and spinner I noticed that the back of the spinner was worn from the backfiring. So I pulled out a new spinner.

Then I pulled the drive washer and noticed something. There was a black residue on the back of the washer. It looked like the aluminum was either scuffed (which it couldn't be unless the front ball bearing was seizing) or had something like plastic baked to it. The stuff wouldn't wipe off with denatured, so I rubbed it off with emery cloth. The pin was where it was supposed to be. (of course, it fell out and I had to find it)

The washer went back on and maybe, just maybe the crank turned over a bit free'er. That and the newly indexed airbleed screw caused me to want to give it a run to see if there was any improvement.

The engine usually won't start unless the throttle is more than half open. It'll now start dependably if that's done. I finger choke it to fill the delivery line, and apply the starter. It usually runs after the fuel moves the last little bit and gets to the carb. It cranked good and I started around the airplane to pull the igniter. It was running like it does. Steady with a stumble. I was reaching for the igniter when it turned into a machine gun. It always had been prone to pop a backfire now and then, but not when running above 1/3 throttle, and only one at a time at low motor. This time it cut loose. I got no idea why it kept running but it just about beat the airplane to death. The impacts appear to have damaged the leading edge of the stab where the hold down was holding. The hold down has thick foam around 1" dowels but the LE looks less than pretty now.

I got to the TX and cut the throttle and it quit. It was blazing hot and had destroyed the backplate of the new spinner.

It's time I found out from Tower if they'll take it back. It's either going to Tower or better yet, to OS so they can do some R&D on it. I'd love to find out why it's so "user unfriendly". Unfriendly it is. It's wasted I'm guessing about a gallon of fuel, turned two brand new OS FS plugs into "used but probably good" (I seldom reuse plugs. Life is short and plugs are usually cheaper than what my flight time is worth to me.), chewed up one spinner backplate (and wore another one to marginally useful), and got me questioning my engine debugging talent. I wish it'd done this right away.

Looking the engine over now, about the only odd thing I can see is some kind of black gunk that appears to be forced out at the base of the pushrod shrouds. Those covers fit so well, I can't believe the stuff could be from inside, but there is no reason I can think of that anything like that would just happed to deposit itself in that fairly inaccessable place. It looks like it could be the same stuff that was sticking to the back of the drive washer.

Sorry to have taken so much of your time. I would have deleted this whole thread if the forum permitted since I'm no longer interested in trying to solve an "engine doesn't run smoothly" problem. It's more than that. It's going back to somebody. Hope I can get my money back.