Many thanks to all of you helpers from all over the world. I opened up the FS 90 and saw that I had it about 10deg too advanced. I have now reassembled it correctly and it runs great.
Many thanks
Check out my other problem with an OS 91 VRDF blown hole in the piston, which I subsequently repaired by hand. That awaits testing but I need somewhere desolate as re-bedding in an ABC engine will be very noisy!
teddykan