When I stripped the motor to flush all the bits of piston I discovered that there wasn't much in the way of sealant left at the bottom of the cases so it was probably sucking air as well the timing being set a bit a bit too advanced
I rebuilt the engine with a new piston kit & rebored the cylinder which it needed anyway.
I made sure to use a cotton thread in the sealant( no silk as stated in the manual) as insurance against the same thing happening again
Sudman came around 1 evening & we timed the motor to fire at 3.5mm at maximum advance to be safe, really need to do some proper testing on the timing at some stage to see where it can be moved to
So far it's run like a train since the rebuild & really jumps out of corners, starts easier & idles better as well
If you need any info I'm more than happy to help out
