This is a first for me. My GTS300 sounded pretty dire for some time - very 'clattery', but ran just fine otherwise. I decided to check the valves first, and that made it slightly worse. Then I had to leave it alone for some months (other things took priority) and we had a spare scooter anyway.
Eventually, with some time on my hands I decided to take the engine out and check it all out. The first thing to do is to take the exhaust off - blow me if there wasn't some coolant inside! Not a huge amount, but a puzzle. Head gasket? That was my first thought. So with the engine on the bench I examined the old head gasket - it looked fine with no tears or signs of any problem. Having put a new oil control ring on (the old one looked ropey and fell apart very easily) I put new gaskets on and and checked everything else out that I could think of.
Fitted the engine back in, filled with oil and coolant, and she started on the second turn, sounding very nice after the oil pressure came up (about three seconds). However after a few more seconds, steam started rising from around the downpipe-exhaust joint, and there was some coming out of the back of the exhaust. Never mind, thinks I, there's always a bit of burn-off after a re-build.
BUT it just got worse, and on revving the engine, huge clouds of steam came out of the exhaust.
So, have I buggered up putting the new head gasket on (if so, why isn't exhaust getting into the coolant), or is there some other way coolant can get into the exhaust? An internal crack in the head?