KX250 locking at wide open throttle...

[Nickbob43]

I'm also posting this on a couple dirtbike forums but I figure I'll post it here to see if anyone has any ideas. I'm going to give all the variables that I possibly can but I am kind of stumped by this.

2002 Kawasaki KX250 pretty much bone stock besides jet kit and pipe.

This problem has been happening off and on since mid summer last year. The throttle will lock wide open. I thought at first it was a kinked or bad throttle cable so I purchased a new one and made sure the cable was routed perfectly. I know for a fact its not the throttle cable now. It's not the return mechanism for the slide on the carb either or the throttle tube on the handle bar. When the bike is off if I crack the throttle wide open and let go it returns exactly like it should.

Every once in while when I'm riding the thing, it will lock wide open. It's pretty damn scary especially with how fast the power rushes on. So the only thing to do is pull in the clutch and when I do that the motor rips full bore max RPM until I hit the kill switch. Well yesterday when a buddy (Jack) and I were riding it was doing it off and on and the last time it did it I had my hand on the kill switch and the ****er would still not die. It’s almost like it’s dieseling. Even though there is no spark the piston is still firing at FULL MAX RPM. After that last time this happened when I go to kick it the thing feels like it has no compression

I thought it might have been old gas but I can’t figure out how that would cause it to lock wide open. I was running U4 mixed with some regular pump gas yesterday with a pre mix of about 28:1. I mixed the oil a little fat yesterday because the U4 makes the bike run more lean. I just had the gas sitting around and figured I should burn it up sooner than later. If it is the gas which I’m doubting how would that make the bike lock in the wide open position?

The motor has never been gone through. It’s stock from 2002. I did a complete leak down test and compression test before ice riding season in November and all the numbers came back OK. After 2 kicks the compression was at about 165 psi. But now, like I said above it feels like it has no compression. The bike was locked in the WFO position and would not die and I think that did the motor in. I’m going to take it apart tonight and see whats going on.

I’m wondering if the exhaust power valve might have something to do with it. I inspected the reeds which are fairly new when I changed the jets the last time and they look fine, no cracks or breaks.

Any ideas?

 

[IndyMX]

I was running U4 mixed with some regular pump gas yesterday with a pre mix of about 28:1. I mixed the oil a little fat yesterday because the U4 makes the bike run more lean.

Any ideas?

If you mix "a little fat", Using more oil, you are leaning your mix, not richening your mix.

So, you made a lean condition more lean by using less gas & more oil.

Lean / Rich have nothing to do with the oil in the mix.

Plus.. and this has been beaten to death on here, don't use your oil / gas ratio to jet.

Pick a nice safe ratio, say 32:1 and jet properly. If you happen to be lean, put a fatter jet in to richen the mix.

 

[Nickbob43]

But why is the thing locking in the WFO position?

 

[IndyMX]

That's a pretty good question.

It has to be something in the slide, cable or throttle tube...

But, one way you may be able to kill it when that happens is a combo of the killswitch and locking up the rear brake while in gear.

When it happens, after you kill the engine, does it release?

 

[Nickbob43]

That's a pretty good question.

It has to be something in the slide, cable or throttle tube...

But, one way you may be able to kill it when that happens is a combo of the killswitch and locking up the rear brake while in gear.

When it happens, after you kill the engine, does it release?

I inspected the thottle return mechanism in the carb very closely. Thats the first thing I went after. I even replaced the throttle cable for good measure and peice of mind and made sure the thing is routed perfectly. There are no kinks or funny bends. When I twist the throttle tube to wide open and let go when the bike is off it returns perfectly. I don't think thats the problem. Could the power exhaust valve cause this? Could a lean condition cause this? Could something wrong with the motor cause the thing to go wide open without any warning? Because thats whats happening.

 

[IndyMX]

I wonder if it's a weak return spring on top of your slide???

 

[Nickbob43]

It feels great man. It snaps back to the closed position just like it should every time. When it goes wide open, I makes sure the throttle is closed in my hand and usually just hit the kill switch when it happens. I had the entire carb, throttle cable and throttle tube off of the machine on the work bench and inspected its operation ever so closely. It does what it should, thats why I'm chaising something else.

 

[IndyMX]

Hmmm...

Perhaps what's going on, when you are at WOT the engine is enough vacuum on the slide that it won't drop.

See if a new return spring helps.

I doubt that testing with the engine off is going to be a very good method.

 

[Nickbob43]

Hmmm...

Perhaps what's going on, when you are at WOT the engine is enough vacuum on the slide that it won't drop.

See if a new return spring helps.

I doubt that testing with the engine off is going to be a very good method.

It shoots up to wide open when I'm just on maintenance thottle riding around. If I'm on the gas just a little bit it happens. It doesn occur when I'm at Full Throttle.

 

[Moto Man26]

If your bike's throttle locks at full throttle, I would think that it's something like your throttle cable, the carburetor slide or something else related to the carb... But your suggestion about power valve system may not be that plausible. Because your power valve only engages at certain rpm's... But it doesn't hurt to check... Also, I would suggest tearing apart the motor and freshen up the top end, and replace any other part that is in need... Because when your engine revved really high, you probably shot the top end. And that's why you have no more compression... Hope this helps:)

 

[bmcd308]

Would an air leak somewhere past the carb do this? Maybe he is getting a lean runaway from some sort of intermittent air leak that is on the engine side of the carb.

 

[jcarautos1]

I only have one suggestion. Have you checked if your mixture needle is too short or the tip broken off? If it is the throttle would stick cause the needle did not go back into the jet.

 

[BigRedAF]

Sounds like the seat to handlebar connection is loose.

 

[fatboyslim]

Clean and adjust the powervalve assembly, check for air leaks in your rubber boots and also mix your fuel at the proper ratio with new gas. See what happens after that.

 

[whenfoxforks-ruled]

Isn't u4 a 4 stroke fuel? What does your plug look like? And to get a point straight, the engine climbs by itself to full throttle, not the carb hanging up? if so, then you have a champion air leak some where. The plug should tell.

 

[BigRedAF]

Clean and adjust the powervalve assembly, check for air leaks in your rubber boots and also mix your fuel at the proper ratio with new gas. See what happens after that.

Slim, good info but it has nothing to do with RPM climbing to WFO on it's own?

 

[DLHamblin]

I'm also posting this on a couple dirtbike forums but I figure I'll post it here to see if anyone has any ideas. I'm going to give all the variables that I possibly can but I am kind of stumped by this.

2002 Kawasaki KX250 pretty much bone stock besides jet kit and pipe.

This problem has been happening off and on since mid summer last year. The throttle will lock wide open. I thought at first it was a kinked or bad throttle cable so I purchased a new one and made sure the cable was routed perfectly. I know for a fact its not the throttle cable now. It's not the return mechanism for the slide on the carb either or the throttle tube on the handle bar. When the bike is off if I crack the throttle wide open and let go it returns exactly like it should.

Every once in while when I'm riding the thing, it will lock wide open. It's pretty damn scary especially with how fast the power rushes on. So the only thing to do is pull in the clutch and when I do that the motor rips full bore max RPM until I hit the kill switch. Well yesterday when a buddy (Jack) and I were riding it was doing it off and on and the last time it did it I had my hand on the kill switch and the ****er would still not die. It’s almost like it’s dieseling. Even though there is no spark the piston is still firing at FULL MAX RPM. After that last time this happened when I go to kick it the thing feels like it has no compression

I thought it might have been old gas but I can’t figure out how that would cause it to lock wide open. I was running U4 mixed with some regular pump gas yesterday with a pre mix of about 28:1. I mixed the oil a little fat yesterday because the U4 makes the bike run more lean. I just had the gas sitting around and figured I should burn it up sooner than later. If it is the gas which I’m doubting how would that make the bike lock in the wide open position?

The motor has never been gone through. It’s stock from 2002. I did a complete leak down test and compression test before ice riding season in November and all the numbers came back OK. After 2 kicks the compression was at about 165 psi. But now, like I said above it feels like it has no compression. The bike was locked in the WFO position and would not die and I think that did the motor in. I’m going to take it apart tonight and see whats going on.

I’m wondering if the exhaust power valve might have something to do with it. I inspected the reeds which are fairly new when I changed the jets the last time and they look fine, no cracks or breaks.

Any ideas?

My friends 2005 did the same thing last summer. He had two good get-offs because of it.

We did as you and had checked everything but found nothing obvious. A few people mentioned this wasn't to un-common on the KX-250 if the slide/bore in the carb gets worn (that the slide can **** a bit sideways and hang).

Anyway; to make a long story short he replaced the carb and so far its been fine. He found a new in the box 2005 motor from a dealer on Ebay (he seems to find all the good stuff...) including carb.

This winter he swapped out the rest of the motor.

His symptoms were exactly like yours, we cleaned, swapped cables etc and it still did it (always at the wrong time). Wasn't till he replaced the carb that it seemed to be fixed. His seemed to do it after a good bump or going through some whoops etc.

 

[rmc_olderthandirt]

In order for the engine to produce any significant amount of power it needs the appropriate mix of air and fuel.

There are a number of things that could allow the engine to rev really high at idle, such as a leak in the air intake, an accumulation of raw gas in the case, etc. There is no way that the bike could produce a significant amount of power from this so if you are feeling the power at the scary level I can assure you it isn't from one of these type of problems.

The only way that the engine could develope that level of power is if the throttle valve was indeed open. It can't be from anything else, so rule out the powever valve, reeds, air box, etc.

For some reason, the throttle slide is NOT returning to the idle position.

I suspect that now the top end has been blown due to over-reving. Like IndyMX said, leaving it in gear and jamming the rear brake as hard as you can would probably have killed it and spared your top end.

This is kind of a wild thought, but check the bolts that hold the engine to the frame. It is possible that if the engine was shifting a bit under torque that it could pull on the throttle cable.

Otherwise, I think that you should tear the carburetor apart and clean the throttle slide really well. Something is jamming it.

Rod

 

[Shaw520]

As stated above, sounds like the needle (broken/bent) or the slide is hanging up. Both of these components can act differently when engine is running.

 

[Nickbob43]

I really appreciate all the input guys. Thank you. I'm going to tear into to it tonight and keep everyone posted what the problem is.

 

[yathump]

I'd check the throttle tube for abnormal wear, or just replace it. I had a 90 CR250 that did this and i replaced the cable and looked everything over and it checked fine.

Turns out the throttle tube had a wear spot in it that when on the bike and if you would put pressure on the outside of the tube it would stick.

 

[atkkid250]

i would agree with many of the people on here, if wat u say is true and it isn't ur cable, throttle tube etc, and u say it has weak compression, i would def. replace the top end, probably a little more then a little worn haha. good luck, a broken bike sucks