You are both right !
Took the engine off & peered into the hole where that shaft was located to see.... the conrod staring back at me, lol.
I guess air coming in from the above 'void' was making it run massively lean (& hot very quickly!) ? I think it all makes sense now
I nearly threw that pull start away too, glad i bagged it up now !
Well i've put it back on, minus the coil & string, gave it another go & it both started & ran as sweet as
(Dave, i hear where you're coming from, lol. Next time i'll invest wisely...)
Cheers chaps.
(ps, i think had i done the same with my other engines i wouldn't have had the same problem as removing the one way bearing & pull start mechanism wouldn't leave a gap as the bearing just slides directly off the crank)