thanks so much for the informative replies mates! I suspect that this could be the problem and will sort it out in the next couple of days. Luckily I have a spare engine here so parts wont be a problem (no decent bike shops in the Philippines and ebay takes so long with tax to pay when items arrive!)...

An unrelated thing was my mate showed up on one of my old xtx's with a bad knock in the engine last month. wasnt backfiring but was noisy. turned out this was the counterweight key. I had the same problem on the old xt and tt 600 models. I've seen damage from a key shattering at 100kmph, it smashed a hole in the top of the engine big enough put a hand in!