[sidecarbod]

Pipe bang and detonation are two different things, it's sounds like you are talking about detonation but just to be sure pipe bang happens on a closed throttle when the engine is on 'over run, , i.e the rear wheel is turning the engine over at above idle speed, you will hear the odd 'bang' in the pipe and at the same time the bike will surge forward s little bit.

 

[sidecarbod]

It does sound like detonation and this can be down to a few things, for starters you could try a higher octane rating fuel, this might help but it may well be a lean air fuel ratio. The reason it happens more when the engine is hot is that the fuel in the combustion chamber is more 'excited' when its hotter.

Your head has been skimmed 0.3mm, that might also be the issue, 0.3mm is probably not enough to get the squish band working properly, at a guess your squish gap is probably still around 1.6mm which is too big, on top of that you will have pushed up the compression ratio, obviously the higher the CR the more likely the fuel is to detonate. Really you want the squish gap to be around 1-1.2mm and then you want the combustion chamber opening up so that the final CR is somewhere around 12.5:1. You could probably push the CR up to 13-13.5:1 as long as the squish band is working but in my opinion its not worth chasing a super high CR, the gains are not worth the hassle if you go too far. A working squish and a CR of 12.5:1 will give you a great results.