Hi Michael,
Whilst we have spoken about this problem, I felt an answer here would help anybody else that may have had an interest in this problem.

My problem turned out to be old fuel that ran fine in four strokes but caused problems in two of these OS .61 RF ABC-P's, I found the problem by running a brand new engine still in the box and it exhibited the same symptoms and fuel was the only common denominator. New fuel cured all problems....

I have been turning APC 13 x 9 props to slow things down a bit and improve down hill braking and the pipe length fro the Glo plug to the first baffle is 484mm (19.01")

Hope that helps

Mike Chipchase