Yep, I had a similar one. turns out al lthat was wrong was a dodgy connection in one of the spar plug leads, and I probably should have tqwigged that if th eprevious guestimator had altered every throttle mechanism & fuel mix adjuster the spark advance would have been tweaked as well.....
It sang for about 30 seconds before destroying itself. I decided to rebuiold & therefore know what I was dealing with from then on. But yeah, been there, done that.... I hope the rest of the season runs a little smoother. (on the plus side, you could probably break it for spares & get some of tyour dosh back.)
As an aside, no idea where you are based, but I guess you've read about the Forth trip on Saturday?