250 Techno Revving Flat Out

[rallymadsam]

Went out on my 96 250 Techno yesterday, rode for about an hour, nothing strenuous, just around the fields in the snow, mostly 2nd/3rd gear very low revs 'pootling' around.

I had noticed the fan hadn't come on, but it was pretty bloody cold, and hadn't been using the throttle much at all, so wasn't overly concerned.

Was all running fine, then I stopped for about 2 minutes, sat there with the bike upright and fuel left on, kicked her up and she revved out of control. The kill switch had no effect, so I pulled the plug lead, turned off the fuel and she gradually slowed down and stopped, but not before she'd boiled and spat all the coolant out. It wasn't running absoultely flat out, but close to it, for approx 30 seconds I'd say, could have been a touch longer (hard to tell when you're panicking like feck). The fan didn't run when it was boiling.

There wasn't a big bang, and she hasn't seized, but it's got me worried.

I had no choke or throttle on at the time, and the throttle seems ok from testing it after, the twistgrip's returning and you can hear the carb slide shutting.

So what could have caused it? And what should I be checking?

Our initial thoughts were that she's got very hot due to the fan not running, and the plug was burning fuel without needing the spark (hence the kill switch not working & running on when the HT lead was removed). But what was she burning? As to my knowledge the throttle was shut, and fuel turned off. The kill switch normally works fine by the way.

Help.

Sam

Edited February 6, 2012 by rallymadsam

[neilnh]

Carb icing maybe ??

[benwall]

check the throttle routing ocasionly it can get stuck on something e.g. going into the carb, engine mouinting brackets, other cables and mainly zip ties. It can free it self most of the time so after it has happend it can be hard to find sometimes.

As your kill switch was not working i heard the reason for that is that at very high revs it becomes imposible for it to work.

Hope this helps good luck.

also if you have bar ends this can grip the grip to the bars.

Edited February 6, 2012 by benwall

[t-shock 250]

I'm also thinking carb icing, but hard to tell over the internet.

[rallymadsam]

So you think that water vapour (caused by temperature difference) could have frozen in the carb causing the throttle to jam open?

[digger144]

almost certainly moisture in carb or cable freezing,best way to stop revving engine is to stuff glove or something similar up exhaust tail pipe that stops them

[boni_tou_1]

im going for carb icing also. winther dry fuel or a cap of meths spirts in a gallon of your normal 2 stroke mix should help

[rallymadsam]

Quick update:

Topped the coolant up tonight, checked the plug condition, reset the idle screw somewhere near (wound it out in attempt to stop her yesterday).

And, she fired up 1st kick, sounds ok, no knocks or rattles or anything, ran her for a minute or 2, all seemed ok. Kill switch worked fine.

So hopefully just 'carb icing' which is a new phenomena to me.

I'm going to clean the carb and check the throttle anyway just to rule any issues with them out.

Also tested the fan by crossing the terminals and it runs fine, so next thing to check is the thermostat. Under £20 for a new one if it is knackered though, so not too bad.

So, all of the above leads me to 2 questions:

1: What should the idle screw be set at?

2: At what temperature should the fan kick in?

Thanks for the help and advice so far.

Sam

Edited February 7, 2012 by rallymadsam