And the answer is...

Just spoke with John Haytree who has stripped and rebuilt the problem 1.25.

He said the engine was in lovely condition, everything to tolerance and all looking correctly run-in ... except for the piston, which had two VERY high spots on the skirt making the piston a poor fit in the cylinder. Apparently, in a Saito, the piston should be a very free fit in the cylinder, the ring is what seals it; this is why the skirt often looks worn because there is a degree of piston slap in normal operation.

He believes that the friction from the two high spots was causing a localised heat build up / increased friction cycle leading to the fall off in rpm. The only thing he can think of is the piston may have been dropped or whacked in the factory, distorting it slightly and resulting in the high spots.

But all is well now and the engine is winging its way back to me. I'll post some figures once it's back on the bench. Maybe I'm not a muppet after all.

HS

:-)