89 KX250 revs really high once it's warmed up

[crispy]

Hey all I've had ongoing issues with my kx250 that i'm trying to sell at the moment. It's an 89 kx250 G1 model, had to get the stator coil rewound which I didn't have a problem with this job. The cdi unit is faulty and had no spark so I test fitted a later model cdi unit off a 91-92 kx125 and had spark . (it's the only cdi unit that works and i'm not prepared to purchase any other parts for this) When you kick the bike over from cold it takes a bit to get going which I'm not really bothered about, but once starting the revs slowly but steadily increase. I choke it which helps to kill the revs slightly but once I take the choke off after 1 minute the revs just increase then goes to almost wide open throttle. The kill switch doesn't work when this happens however when it's idling cold or warmish it works fine. Could this be the cdi unit?? I haven't got the original cdi unit unfortunately for the bike so I can't tell for sure this is the cause of the issue, but it's very coincidental. I thought it was float level and running low on fuel and have adjusted I and will try tomorrow

[Katman]

I would be suspicious of the left hand crankshaft seal allowing air to be sucked past it.

[drift77nz]

I would be suspicious of the left hand crankshaft seal allowing air to be sucked past it.

I agree, sounds like it. Or a base gasket or a intake manifold gasket.

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[crispy]

Thanks for the replies guys, so reason I want to know this all is i'm selling the bike as is where is, and don't want to spend any money on parts anymore, rather my own time spent in labour


I had another look at it yesterday and the throttle cable had been adjusted incorrectly, there was no free play in the cable. I found this when I had the carb off and cleaning/inspecting it. Looking at the carburettor slide it was sitting 1/4 open when throttle was fully released. Backed off the cable adjusters and it now idles, but it hunts up and down, Smells like it has a lean "hunt" issue as it idles up and down. Kind of like a car with big cams if that makes sense. You can just about smell that it's lean at idle. It did this originally when I had the bike going months ago. It doesn't rev like before (Thursday/Friday night) just sitting, but i'm still thinking it has an air leak in the stator side seal (left side crank seal). or the intake manifold (has light cracking on the outside) Spraying carb clean or similar stuff doesn't make a difference at idle, so unless these light cracks all the way around are big enough that an aerosol spray just doesn't make a difference I won't know until the manifold is removed.

[drift77nz]

Thanks for the replies guys, so reason I want to know this all is i'm selling the bike as is where is, and don't want to spend any money on parts anymore, rather my own time spent in labour


I had another look at it yesterday and the throttle cable had been adjusted incorrectly, there was no free play in the cable. I found this when I had the carb off and cleaning/inspecting it. Looking at the carburettor slide it was sitting 1/4 open when throttle was fully released. Backed off the cable adjusters and it now idles, but it hunts up and down, Smells like it has a lean "hunt" issue as it idles up and down. Kind of like a car with big cams if that makes sense. You can just about smell that it's lean at idle. It did this originally when I had the bike going months ago. It doesn't rev like before (Thursday/Friday night) just sitting, but i'm still thinking it has an air leak in the stator side seal (left side crank seal). or the intake manifold (has light cracking on the outside) Spraying carb clean or similar stuff doesn't make a difference at idle, so unless these light cracks all the way around are big enough that an aerosol spray just doesn't make a difference I won't know until the manifold is removed.

Haha, yeah the throttle being at 1/4 throttle wouldn't have helped:yes:
Another place it can leak is at the crankcase halves. Don't overlook the rh crank seal also, although it will generally show up as blue smoke out the exhaust from the gearbox oil being sucked into the cylinder. Using carb cleaner to find it is best at idle as this is when you have the greatest vacuum and it will show the gremlin easier.

[crispy]

Took the bike for a ride today and performed alright (used shit fuel, not by choice but from lack of time to get organised), bike rode alright but had the high idle occasionally along with running very average. Got home, drained fuel put fresh fuel, drained carb etc. Once it ran normally again the idle is fluctuating, I thought this is maybe a leak in the intake manifold but spraying carb clean around here didn't make a lot of difference. Maybe the switch to 95 improved it slightly. I'll try spraying around the cases when it's running to see if there are any other leaks.
My main concern here is using a cdi out of a 92 kx125 most probably isn't ideal. It's my only option to get the bike running but what damage can this do to the engine?? I removed the carb and intake checked the reeds and look good. one reed valve has .2-.3mm gap between the cage, would this also cause this issue. Also the carburettor slide has a few grooves worn in the surface so would this cause issues also?? The slide I thought is sticking in the carb, is there any way you can free this up or lubricant you can put on it?? I was thinking auto trans fluid just to help things slide easier, I can't be too confident this wouldn't cause any issues with running of it

[drift77nz]

Took the bike for a ride today and performed alright (used shit fuel, not by choice but from lack of time to get organised), bike rode alright but had the high idle occasionally along with running very average. Got home, drained fuel put fresh fuel, drained carb etc. Once it ran normally again the idle is fluctuating, I thought this is maybe a leak in the intake manifold but spraying carb clean around here didn't make a lot of difference. Maybe the switch to 95 improved it slightly. I'll try spraying around the cases when it's running to see if there are any other leaks.
My main concern here is using a cdi out of a 92 kx125 most probably isn't ideal. It's my only option to get the bike running but what damage can this do to the engine?? I removed the carb and intake checked the reeds and look good. one reed valve has .2-.3mm gap between the cage, would this also cause this issue. Also the carburettor slide has a few grooves worn in the surface so would this cause issues also?? The slide I thought is sticking in the carb, is there any way you can free this up or lubricant you can put on it?? I was thinking auto trans fluid just to help things slide easier, I can't be too confident this wouldn't cause any issues with running of it

Not sure how the CDI is going to effect it (different model and cc may have different ignition curves etc.).
Reeds not seating can cause problems as the pressure wave of the air/fuel mixture in the crank cavity being push up into the cylinder on the downward stroke of the piston is reduced.
As for the slide in the carb, DO NOT LUBE IT. lube around it will not stay there and could cause it to stick even worse. Polish the slide and piston with fine wet and dry sand paper until it slides freely. Make sure to clean it well in hot soapy water.

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[crispy]

I guess 125 has a different ignition curve to the 250 but didn't really feel much different to the last time the bike ran before I had trouble with the stator. I've cleaned the carb again, drained the bowl and polished the carb slide, reassembled and ran fresh 95 with premix and once that fuel had gone through the bike rode a hell of a lot better. Someone is keen on the bike and took it for a ride yesterday when it ran poorly. Would be good if he took it out now to compare it. Starts a lot better an will definitely give him a surprise compared to last time. Idled better also just had a slight hunt in the idle. Maybe if he hasn't totally made up his mind I could talk him into another ride this weekend if I haven't heard back. Might try a can full of 98 with premix too when it gets low in the tank

[Grasskarter500]

I used to own a '89 Kx250, same model as yours, And did have some of the problems that you described. I know that it probably won't suit you, but I ended getting as much new parts for it as i could, getting parts for that particular bike is quite a mission. It uses the same motor as the KDX250, just with different gearing/porting.

For your carb issue, I find that using the finest sand paper you can get and lightly sand the grooves out. Does help with the sticking, but not a good long time solution. Throttle cables (old ones) can cause some sticky acceleration issues, so get that replaced.

For what i ran in it, I used castrol VMX80 for Gearbox, and 95 premium with Maxima Castor 927 mixed at 33:1, running a BR8EG. Mind you, the motor was quite old, and was running poorly from donor bike, but it took a while, and was running like it was new.

If you need to know anything else about the bike, let me know, and i might be able to help. I don't
know much about bikes in general, but self taught myself this stuff when doing up the old girl.

[crispy]

I used to own a '89 Kx250, same model as yours, And did have some of the problems that you described. I know that it probably won't suit you, but I ended getting as much new parts for it as i could, getting parts for that particular bike is quite a mission. It uses the same motor as the KDX250, just with different gearing/porting.

For your carb issue, I find that using the finest sand paper you can get and lightly sand the grooves out. Does help with the sticking, but not a good long time solution. Throttle cables (old ones) can cause some sticky acceleration issues, so get that replaced.

For what i ran in it, I used castrol VMX80 for Gearbox, and 95 premium with Maxima Castor 927 mixed at 33:1, running a BR8EG. Mind you, the motor was quite old, and was running poorly from donor bike, but it took a while, and was running like it was new.

If you need to know anything else about the bike, let me know, and i might be able to help. I don't
know much about bikes in general, but self taught myself this stuff when doing up the old girl.

I've cleaned the slide up twice now and it's the best I can get it, buying new bits is getting a bit long in the tooth with wifey but I want this as problem free as I can before I sell it. I actually have a Br8eg plug in the garage as wanted to see if it makes a difference to starting it cold and hot. (This engine is most likely a bit worn also) did a compression test yesterday (110psi cold), yet to do one when it's hot. I too am self taught in working on bikes, got myself a repair manual not long after I got the bike. So much useful info you can use on other 2 stroke engines/bikes.

[ktm84mxc]

Had a prob with my 84 KTM it was giving me so much grief hard to start, back firing , wont rev . Turned out to be a loose coil wire a quick pinch and elly tape end of dramas, my advice go through every thing and recheck.

[crispy]

Thanks, i'll double check all the connections are tight even on the spark plug and clean earths

[crispy]

Bike seems to have been the best it's been, started it up last week after cleaning the carb slide. releasing the throttle it actually went straight back to idle. Someone has bought the bike now and he seems happy with the way it ran, so all good

[Murray]

Bike seems to have been the best it's been, started it up last week after cleaning the carb slide. releasing the throttle it actually went straight back to idle. Someone has bought the bike now and he seems happy with the way it ran, so all good

good on you for getting it right before passing it well done