Basil, I have a TT .91 FS on the bench right now. I started breaking it in last year, and everything went great up to about 8000 rpm. Over that speed I would hear a crackling noise, and when I would try to restart it for the next 5 minute run, it would be hard to start, or not start at all. I would take it back, check the valve settings, and find the lock-nuts on the valves were loose and the clearances way too loose. I would re-set the valves, tighten the nuts and go back to the field for another session. I went through three cycles of this difficulty, and then ran a search on this engine here on RCU. I came up with a report from one of the moderators that this was a common problem with the TT .91 FS. His recommendation was to take off the valve cover before starting any break-in, loosen any screws or nuts under the valve cover, set the clearances, then re-set everything with loc-tite. Other than that, it's a great engine. I am just waiting to get a couple other projects flying before I give my TT an alcohol bath, since I don't know if loctite will stiffen up well when applied to an oily surface.