'02 KX125 engine dies at high rpms

[Shadow]

Well, the bike worked perfectly for 3/4 of a lap and then the engine seized up. Seems a sleeved cylinder needs much more running in than I thought.

Ok, what are my options now? Is there any chance of salvaging the sleeve? I imagine the piston at the very least is toasted, but would the cylinder still work with a new piston?

Second thing, how can I remove the piston from the cylinder with minimal damage to the cylinder sleeve? It's melted somewhere enough that I can stand with all of my 180lbs of bodyweight on the kickstart and it won't budge.

[citabjockey]

Ug, so sorry to hear about this failure! Does seem like you have a tough project on your hands.

Generally the cylinder is tougher than the piston so it may not be in that bad a shape. That said, if its really frozen the jug as you say the sleeve may be toast -- I assume no oversize pistons are available? How does the spark plug look? Kinda sounds like it was too lean. Did you WFO it on the new rebuild or did you take it a bit easier?

You will know lots more when you get the engine opened up. Keep us posted.

Well, the bike worked perfectly for 3/4 of a lap and then the engine seized up. Seems a sleeved cylinder needs much more running in than I thought.

Ok, what are my options now? Is there any chance of salvaging the sleeve? I imagine the piston at the very least is toasted, but would the cylinder still work with a new piston?

Second thing, how can I remove the piston from the cylinder with minimal damage to the cylinder sleeve? It's melted somewhere enough that I can stand with all of my 180lbs of bodyweight on the kickstart and it won't budge.

[Shadow]

Oversize piston could be an option, as when I had to sleeve the cylinder during the winter, I opted for the smallest piston for starters precisely because of the possible future need of an oversize piston.

Warmed the bike up, took a couple of easy laps on the small bike track to get the bike warm/test it on moderate throttle. Headed to the track, tested whether it would die on high rpms like before on the first straight, it didn't, so I started to ride the track properly.
I was indeed going wfo through a whooped out sand straight when the bike seized up.

I opened the top end as soon as I got back from the track. There's some heat damage visible on the sleeve and piston, but they're both remarkably intact considering. Still, the piston's stuck onto the cylinder walls and I have no experience on how to get it moving again so I had to leave it there.

Checked some advice on getting a seized piston moving and it doesn't seem that promising to be honest. One site said to pour oil and increase the pressure by injecting grease into the cylinder through the spark plug hole. Trouble is, the piston's stuck so low in its stroke that the ports are showing, which makes any attempts at increasing pressure moot.

Spark plug was the normal tan color.

[SachsGS]

Pull the head off and soak the piston/cylinder in penetrating oil overnight.Pull the magneto cover off and take a piece of wooden dowel and with a hammer tap the top of the piston,every so often trying to move the flywheel clockwise and counterclockwise with a wrench.Be gentle and the piston will start to move.

With a steel sleeve you should have at least 3 to 4 oversize piston sizes.Once you get the piston free I would carefully scrape off the aluminum deposits and give the cylinder a light hone.Unless there are deep scratches you should be able to put another stock size piston in the jug without boring it.

Check your timing and make certain that coolant is circulating.Raise your needle and go up at least a couple of sizes on the mainjet.As the motor breaks in you can optimize your carb jetting but let it run rich for now untill you get everything sorted out.

[Shadow]

Thanks for the advice. I just got back from pouring oil on the piston so it can soak overnight. I'll introduce the piston to a hammer tomorrow.

http://img534.imageshack.us/img534/3759/p6040010.jpg
http://img836.imageshack.us/img836/1005/p6040012v.jpg
http://img37.imageshack.us/img37/8494/p6040013u.jpg
Here's the damage from my extensive dumbassery. How does it look to you guys?

[citabjockey]

Was the ring end gap in spec? What piston/cylinder clearance did you run? tan sounds a touch too light for the plug. A nice milk chocolate brown might be safer. What oil ratio did you run?

[Shadow]

Oh heck. That's the first time I even hear of a ring end gap and piston/cylinder clearance. I just got the standard sleeve meant for the bike's cylinder, had it sleeved and machined to match with the piston in a workshop.

Oil ratio was 1:32 as per the manual, air screw was 1 and 7/8 turns out when the default is 2 turns. I've had to adjust the carb's needle with one spot leaner due to the bike running too rich previously.
There was no usual symptoms of too rich or lean mixture while riding.

Oh and just to add, the bike's been sitting unused those few months after I had the cylinder sleeved and installed due to time constraints and the then spring weather which makes tracks unrideable here for a while. So basically the cylinder was still brand new when I started riding again.
My guess is just insufficient amount of running the bike in, so it seized up thanks to that when I started to ride properly with it.

[SachsGS]

Your piston/cylinder looks very "dry" to me,check for an air leak somewhere that may be leaning the mixture out.

[TotalNZ]

Steel sleeves are a terrible option for a motor that previously had electro plating, in my experience they never work properly and i believe that steel sleeve is gonna cause you continuous problems.
Does look like a lean seize though or like has been mentioned incorrect ring end gap.

[Shadow]

Can't get much worse than how it's been so far. The bike's cursed I say!


What's the problem with steel sleeves and why won't they work properly?

[Coop]

How light is the "normal tan color" on your plug? I agree with the others, looks like a heat/lean seize to me.

What are your carb specs? Pilot jet, main jet, clip position? You have an air filter in correct? How are the reeds?

Check the carb boots for cracks also and ensure the clamps are tight. If the bike was running I could tell how to check for an air leak but that's not really an option right now.

[SachsGS]

Steel sleeves retain more heat and don't transfer heat as well.As TotalNZ has stated,a cylinder engineered for electroplating may not tolerate a steel sleeve.

Husqvarna once did tests on breaking in motorcycle engines and found that with the proper procedure a motor would last up to 4 times as long.
 
Your KX isn't cursed.Your Kaw is a highly tuned racing engine and it takes time to learn the idiosyncracies of a machine.Can you imagine if this was a 4T and you had rods flying through the cases and dropped valves blowing your topend?

[Shadow]

At least some fate is on my side! The piston needed only a light tap and it started moving.

Tried searching for a leak somewhere between the carb and cylinder but found none. The reed valves are in fairly good condition still. Clamps were tight. Been making sure of that everytime I've installed the carb.
Of course have an air filter, properly oiled up and greased the bottom of it. Had to get a filter skin on top of it too because the track I ride at is almost completely deep and fine sand which gets everywhere.

Carb settings:

Main jet #360           Default is #350
Pilot jet #35             Default is #45
Needle jet was #633 8-4         Repair manual I've got doesn't list its default size.
Jet needle was 6BEK1-69         Default is 6BEK1-69-3        Couldn't see the last "-3" part on the needle when I checked it. Could it still be the default one?
Clip is on the highest groove. Had to adjust it there in the past because of problems with the bike running too rich. Default would be 3rd groove from the top.
All the jet changes are after the previous owner. Only change to the carb I've made has been the jet needle's clip position.

Made a mistake last time. The air screw should've been 1 and a half turns out. So I was running it too lean when it seized up.

Here's the spark plug.
http://img823.imageshack.us/img823/4844/p6050008.jpg

Damage to the piston:
http://img864.imageshack.us/img864/1856/p6050009.jpg
http://img687.imageshack.us/img687/3161/p6050010.jpg
http://img705.imageshack.us/img705/293/p6050011.jpg

Reed valve. Apart from the corner being damaged, they were fine. There's no warp on them at all.
http://img508.imageshack.us/img508/1623/p6050017.jpg

Damn the power valve was in need of a cleaning. Filled with black tar like shit and carbon build up when I opened it.
http://img831.imageshack.us/img831/4840/p6050021.jpg
What's the best way to remove that carbon from the exhaust valve? Just scrape it off with a knife?


SachsGS: Sheez! I don't want to imagine having a 4-stroke. Too many moving parts in its engine.

[citabjockey]

Someone correct me if I am wrong but that spark plug looks pretty cooked. The pebbly surface on the center electrode looks like the engine was pinging. The color is really not that bad so maybe its timing as opposed to mixture? I would suggest next time around try:

Verify the timing. You have a service manual?  Don't start it until you are convinced the timing is perfect. Check the specs that came with your sleeve and piston. The shop should ask you want clearance you want. The closer the clearance the better heat transfer will be from the piston to the sleeve (within reason). Piston manufacturer should have the spec. They probably also spec the ring end gap. Verify that when the jug comes back from the machine shop. The ring came with the piston I assume?

Go richer on the needle -- you said it didn't run real well but if you can get it to go to break in the engine thats ok. Go up 10 on the main too. Go ahead and put the air screw to stock but that really only has an effect at very low throttle settings. Also, run 24:1 oil during break-in. With the richer fuel mixture this should be ok.

First, check for leaks. Start it and spray silicon spray around the carb, the intake boot, and the reed valve and the cylinder base gasket and head gasket. If the engine tone changes or it dies then you have a leak wherever you were spraying. If you find a leak you have to address it before anything else. BTW - do you have new main crank seals in the engine?

Assuming you are good to go for leaks, ride it for 10 min with varied but low (1/2 or less) throttle settings keeping the revs pretty low. Let it cool and check the (brand new) spark plug. Make any adjustments you think it needs for milk chocolate plug with a SMOOTH center electrode. Run it again for another 10 min with varied throttle settings. Get up to maybe 2/3 throttle. Let it climb to just below being on the pipe. Let it cool again and check the plug again. If you want to really be sure, pull the top end and check the piston or just pull the pipe and shine a flashlight up the exhaust port -- making sure smooth metal is staying smooth.

Next time out ride it the same way for 1/2 hour or so. Let it climb into the power band a touch a couple of times. Check the plug again right after one of these higher power shots. Make any mixture adjustments you think it may need. A touch of blubbering at partial throttle at low rpms is really ok as long as it snaps back to life when you want it to.
let it cool and check everything once more.

NOW ride it for 1/2 hour and let it climb fully onto the pipe a couple of times, Never for more than a second or two. Check the plug right after this and make any main jet changes you think it needs.

If its all good ride it for another couple of hours -- again, keep the full power durations really short but go ahead and have fun with it. Check everything one more time. If you haven't pulled the top end before this do it now and check for problems.

After that, assuming the top end is still looking good, it should be ready for the real world! You can consider going back to 32:1 for oil at this point as well. You may notice the mixture being a tad richer at this point if you do. If the engine runs ok then leave it rich.

Good luck!

[Shadow]

Here's another theory on the engine's fate provided by a bike shop's mechanic.

After he took a look at the piston, said that running lean was not a cause of the seizure. Based on where the damage marks are on the piston, it supposedly looks like the bike had been cold when it was given gas. So the piston had warmed up, enlarged, started to chafe on the other side and then get stuck finally on the other side where most of the damage is.

Isn't the piston made of some aluminum alloy?

Could the seize have happened just because the piston warmed up more quickly than the steel sleeve?