Hi,

I rode the bike for most the day a few weeks ago and After using it for a few hours it became very hot, It would bog a bit when the throttle was opened and seemed to have less power, the engine smelt hot and the exhaust silencer was too hot to touch.  After lunch I refuelled it and it seemed better and didn't get as hot but still seemed too hot. The cooling fan cut in as expected but I've since found there was only water in it and not specific coolant.

I hadn't used it for over 4 months before that so the fuel was probably a bit stale.

I have checked the water pump impellor and all is well, I have the carb off and will clean this, I've taken the barrel off to check for partial seizeing and to check conrod play as I suspected it was running lean. All is ok, bore/ piston good and the small/ big ends are fine. The only thing is the top of the piston is very sooty and so is the inside of the head. I'm new to trials but have mx bikes and I'm used to a browny colour with no soot on the piston crown. I now realise ive been running the oil too strong at 40:1 but the soot build up and hot running can't be down to this. Could it be as simple as stale fuel? how does the vacuum fuel pump work, could it be that?

Any ideas would be gratefully received.

Rob

 The St300 akways had a problem cheaper to move on.

A retarded ignition can make the exhaust pipe too hot/hotter than usual.

Sounds like it’s running lean.

 

Thanks for the replies, yes i thought maybe its running lean but why so much carbon build up? How can the ignition timing go out with a CDI and how can i advance it?

Newer ST's don't have pipe drain hole like first models. 

Oil can acumulate inside pipe if you ride for long periods slow. If after that, you ride it hard (uphill open trothle for long periods) it can ignite "gum oil" inside pipe and get very hot with a lot smoke. I think this hapened with me when bike melted rear fender.