RE: ys engine surging at full throttle
Success, the engine now works perfectly. When I originally had a problem with the engine surging I spoke to Phil at Probuild and he told me the cause was either a leaking rocker box gasket; a leaking airbox gasket or a crack in the air box casting. I had noticed previously that on turning over the crankshaft, bubbles of air could be seen escaping around one of the cap screws securing the airbox. I assumed that a new gasket would cure this as there were no sign of cracks on the airbox casting. Having replaced the gaskets the engine continued to surge so I took the airbox to my foundry and crack detected it with a die penetrant. The penetrant came straight through the corner of the casting even though visually there was no sign of a crack. Having replaced the airbox the engine ran much better than before but I was still not happy with it so I decided to do the modification suggested by Cyberwolf ( piston Porting). Success at last! I also had a rear bearing cage failure which stirred up a bit of the crankcase. Fortunately it did little damage to the engine but on examining the bearing race, corrosion could be clearly seen. The moral of the story 1) Don’t buy a second hand engine.
2) If you do buy second hand replace the rear bearing.
3) Replace the paper gaskets they are easy to make with 0.5mm gasket paper.
4) Try piston porting ( 4 X 1mm holes drilled through the top (crown) of the piston exiting at the back of the piston ring groove) if you have the facilities.
5) Go for the easier option as suggested by DAVID BATHE, replace the piston ring. Many Thanks to everyone who has helped me.
Tony