Some good info there pokey, thanks.
Bike is a Yamaha WR500 (owner since 2004), set up for road use (supermoto/back road blasting) so have been asking it to do things it's not designed for.
Bike was (professionally) fully rebuilt 12 mths ago, including lapping all crankcase/cylinder/head join surfaces. Have had ongoing issues with detonation, tuning and high operating temps. This gearbox issue is just one more hard-to-explain event. Pretty sure the bike is cursed.
![Smiley :)](https://forum.ozvmx.com/Smileys/default/smiley.gif)
Re: bottom end charge sucked not pushed. Didn't know that. Thanks!
Re: g'box overfilled or breather blocked. Neither of those.
Re: hydrolocking. Do have a fair bit of oil around the crank but pretty sure it's not filling with fuel mixture and is not ultra smoky after start-up.
For the record, the bike is definitely not neglected in terms of maintenance and upkeep, quite the opposite. Having said that, I'm not a mechanic, but (probably because I've had so many dramas with this bike) I do now try and learn as much as I can about how these old 2 strokes work.
Also, have just had crankcases checked for microcracks and both are fine. Will get it back together and try again.