Ive just picked up a mk3 and after riding it once ive already got a running issue so dont know the bike very well at all. It will run for 5 mins then it starts to rev highly with the throttle off then cuts out. The first time it happened I got it started after about 5 mins and it just red lined with the throttle and choke off for 20 seconds then cut out. Any ideas anybody?

when it rev's high....how high?
when it does this high rev bit can you stop it with either the kill switch or ignition switch?.........

Stuck throttle or running lean.

It sounded like it was redlining and then it just cut out.When it redlined I didn't try to turn it off, I twisted the throttle on and off to see if it made a difference but it didn't. After about 10 seconds of it doing this it cut out.It would then takeabout 5 mins to start again.I could then ride it for a few minutes then it just cut out again. I did wander whether it was running lean but that wouldnt cause it to redline would it?I havnt checked the throttle cable, whats the best way to do it?

Cheers

depending on how lean it will check the FAQ section for std carb settings

Any one be twiddling in the carb already?
Frayed cable? possible
Air leakage? probable.
Badly under jetted? possible.
Lots of time sorting?probable.
Mike

Ill try the carb setting first and see what happens.I only picked the bike up yesterday so Im not sure what twiddling's been done.Thanks for the tip offs though.And thanks for the reality check Mike that it's a 16 year old bike and I'm gonna have to get my hands dirty haha!

All the specs for the model are available in FAQs.
The inlet jet on the carb is also a suspect, as is the fuel tap filters.
They let enough fuel thruogh to allow running for a short while, then the fuel supply is used quicker than it's replenished.
The mixture then goes extremely weak, leading to, well you know the rest.
Take the 17mm drain plug of the bottom of the carb, OUTSIDE, and then turn on the fuel supply, look for a steady supply running out of the outlet.
If you loosen the jubilee clips and gently rotate the carb, you fire a bit of carb cleaner into the main jet and pilot circuit.
Check the drain plug for grunge.
Mike

Hey Mike, could that also be the reason for the problem I'm having? The other night when I checked the bike out before buying it, she fired up easily. Yesterday morning I topped up fuel & oil etc then she was a cow to get going. I checked previous posts and advice etc and eventually got her started, leaving her to idle with choke open (thought maybe it just needed to warm up a bit). After a few minutes idling though it just cut out and refused to start again. I tried everything again - no luck, and eventually got so p-d off I left it until this morning. Same story - eventually got it going and revved the crap out of it but wouldn't idle without choke. When I thought it was ok and hit the kill switch, the cow didn't want to start up again?? The guy I bought it from is a bike mechanic so I'm assuming all carb settings are ok (btw, can't seem to find the standard settings under FAQ, can you give me the link?), and the plug looks brand new. I thought maybe I'd flooded it but I don't think so (no fuel smell etc), I actually think it's getting too little fuel. Maybe because it's so damn hot here at the moment (night low 25 deg C, midday highs 35!!!), maybe the mix is too lean for the heat - what do you think? I'm no expert on 2-strokes so don't want to go fiddling and stuffing it up. Maybe I should just send it to a local shop and let them give it a once over to check everything and service etc? I just want to ride my 'new' bike for crying out loud!! But I don't want to get stuck in the middle of nowhere trying to kick the bike back to life if it does the same. Advice please buddy??

http://www.crm250.com/specs.asp

carb settings and most other things are here

SAM The problem is not too little fuel it is too much. The hotter the temperature the richer the mixture gets.

There is a knack to starting any 2 stroke as has been said already. When you start it DO NOT twist the throttle until it has fired. Also keep the choke on for as LITTLE time as possible. DO keep the engine going by blipping the throttle when it won't idle after starting. By leaving the choke on you are gently flooding it.

Check the jetting.

Thanks for the quick replies guys. I'll give it a go as you suggest - just with a bit of choke to start up then turn it off and keep blipping it for a bit to see if she idles after a bit or take it out for a quick run straight away. Keep you posted....

You know- my CRM has some funny habits that sound a bit farmiliar to this. Took me a few days to work out what it was, but it was fairly simple!! She starts to rev her head off, then cuts out after 3 or 4 minutes....the reason- she has run out of fuel!!! The float chamber fills up, then runs low and leaves a pressure vacuum- so she revs her head off, but when there is no more petrol in the chamber after 1 or 2 mins, she just stops!
If you simply switch onto reserve, she starts again. I reset the float heights, cleaned the petrol filter and cleaned the carb jets out. Try checking that fuels is flowing into the float chamber and also check the float tang height- these could all affect the pressure of fuel running into the jets. Alternatively, it could be you have a cruddy build up in your petrol filter in your tank blocking the petrol flow. Drain the tank, remove filter and tap ad clean out.

Maybe this will help...

njhardy, you are a friggin' LEGEND!!!! Who would've thought it could be so simple? I've been fiddling with the carb settings as per the recommended specs etc and it was a bit better but still cutting out. Was just on my way to get carb cleaner and get down to stripping it all etc, but luckily checked in and read your comments. Changed to reserve, kicked it twice and WA-HEY - she lives!!!! Idles nicely, doesn't cut out - all good!!! So instead of spending the afternoon stripping the carb, I've been playing in the bush and a new housing development under construction round the corner (nice piles of sand, roughly graded roads etc) - awesome! Really surprised at how responsive the bike is and she goes like a rocket (actually a bit scary at full throttle??).
I'll still get everything cleaned and checked out properly but I'm pretty sure there's not much to fix up on the motor.
Thanks again for all the help guys - especially you njhardy! I was really starting to get pissed off......

Not just a pretty face!! -Glad to be of help. Took me a few hours of head scratching to solve this one myself, but woke up the next morning and had the answer!! Unfortunately not everything on the CRM is that simple to fix!?