Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: Shadow on March 01, 2011, 06:29:08 AM
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Went for a ride first time after a top end rebuild. Bike starts up and idles fine but every time I get into high rpms and then release the throttle, the engine dies instantly. No bogging, no cutting or skipping or anything like that. It just dies without any warning.
I'm seriously at a loss as to what might cause it.
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If you continue to hold it at a high rpm does it keep running fine or start to slow down?If you do the same thing at about 3/4 throttle will it do the same thing?Does it immediately start back up again or does it take a few kicks?
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Have you run it in.
Sounds a bit like a seize :-X
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check the spark plug wire
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Sorry for a late reply. Didn't have a chance to take a look at the bike until today.
Somehow the bike had been cured miraculously while it was sitting at the garage. Started it up to check what ford said, at first it bogged a bit like it was too rich but then it opened up, and started revving pretty much perfectly and didn't die. This was while the bike was in neutral though. With spike tires on it and the whole area near the garage being asphalt, I can't really test in gear there.
TMKIWI: The issue actually came up during the run in. Never knew that kind of behavior is a symptom of an engine seize. I always thought it's more like bike dying completely and won't even start again.
McClung: Checked, no issues there.
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Update: Managed to test ride the bike properly only now, and the problem still persists.
From 3rd gear and up, when I let off the throttle on high-to-top rpms the bike shuts down. I can keep the rpms up no problem but every time there's a drop in the rpms the bike dies.
Can't hear any odd sounds from the engine, bike starts well, picks up rpms well, low to mid-high rpm range works normally and the problem is not present when the bike is in neutral, only when the engine is under load. Air screw did not seem to have any effect.
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Check the power valve connecting rod. it may have been installed improperly.
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Do you mean the governor shaft lever + the idle gear lever and main shaft gear alignment?
Pardon me, but I have no previous experience with KIPS or power valves and the repair manual I've got doesn't list a part named "power valve connecting rod".
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yeah that's what I meant, just using laymens' terms
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Alright, just making sure, 'cause I don't want to strip the whole KIPS apart if I don't have to.
Checked that and it MAY have been misaligned. Not too sure since I saw the mark on the main shaft gear only after I took it off, but at least now it is aligned without a doubt. Will head to the track tomorrow to test the bike again.
Makes troubleshooting a bit tiresome when the bike is stored some 10km from my house and I can't test ride there, so I need to drive to a nearby track every time I want to ride the thing a bit. Fortunately that's only a 10 min drive.
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10 minutes?? wow, the closest place for me to ride is 45
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No luck. Still dies in high rpms.
Remember when I said that it doesn't happen in neutral? Well screw that. Today it died in neutral, not even sure if I let off the gas when it died. I'm completely confused at this point.
Starts back up on first kick after it's died. No issues with compression. Only other problem I have is that the clutch doesn't disengage properly but that shouldn't have anything to do with this.
Could the problem be with the main jet in the carb and it being clogged or something?
Haha, yeah, I've been fortunate enough to have always lived near a track. And the riding cost for a season is 30 euros. :D
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Give the carb a good cleaning and focus on the idle and pilot circuits. ;D
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So the failure mode is when you rev it, you can hold the revs with a bit of throttle but when you chop the throttle the engine slows and then dies -- even when you are re-opening the throttle? When it does this, take out the spark plug. Is it wet?
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Yeah, reopening the throttle does not help.
Update: Alright, the carb is now squeaky clean. What little I can test the bike at the storage depot, the bike picked up rpms frighteningly well and showed no signs of dying. I could keep the throttle wide open without problems. That's in neutral though, so how it works at a track still remains to be seen.
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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.
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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.
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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.
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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. ;D
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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://img534.imageshack.us/img534/3759/p6040010.jpg)
http://img836.imageshack.us/img836/1005/p6040012v.jpg (http://img836.imageshack.us/img836/1005/p6040012v.jpg)
http://img37.imageshack.us/img37/8494/p6040013u.jpg (http://img37.imageshack.us/img37/8494/p6040013u.jpg)
Here's the damage from my extensive dumbassery. How does it look to you guys?
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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?
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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.
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Your piston/cylinder looks very "dry" to me,check for an air leak somewhere that may be leaning the mixture out.
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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.
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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?
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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.
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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? :-X
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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 (http://img823.imageshack.us/img823/4844/p6050008.jpg)
Damage to the piston:
http://img864.imageshack.us/img864/1856/p6050009.jpg (http://img864.imageshack.us/img864/1856/p6050009.jpg)
http://img687.imageshack.us/img687/3161/p6050010.jpg (http://img687.imageshack.us/img687/3161/p6050010.jpg)
http://img705.imageshack.us/img705/293/p6050011.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 (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 (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.
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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!
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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?
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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?
Yes definately, this is classic case of the probs a steel sleeve can cause.
As was mentioned earlier, the steel sleeve builds and dissipates heat differently than what the manufacturers intended. As a result the rates the piston and liner expand are out of sync and can cause scuffing and in some instances a seizure.
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Yes, that is quite correct about cold seizure. You do have to have it fully warm before asking for lota power - esp when on a new bore and piston. Did you also have him look at the plug? If he is ok with it then you may be fine. My break in procedure still applies.
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No, I didn't show him the plug. Forgot to mention to you guys, that it's an old plug, and the pebbly surface was on it already when I put it in the bike. The current cylinder/piston usage did not cause that.
Got a new piston ordered, once it's here, will drop the cylinder and piston to a workshop to be fixed and I will make sure it will get within the piston/cylinder clearance specs.
Also picking up an '02 kx250 tomorrow. Will use that for a kx500/250 conversion project once I get a bit more money and find a suitable donor bike for the engine.
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(http://img812.imageshack.us/img812/2509/07062011309.jpg)
:)
Got a workshop recommended to me, getting the piston tomorrow, and then the cylinder is off to be fixed.
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Man, I do not get a break.
Got the bike back together, went to the track to break it in. Engine works fine, no issues with it now, but afterwards I noticed the swingarm pivot nut had disappeared. Good thing that the rear brake lever stops the pivot shaft from getting out on these kawis. Could've been an interesting turn of events otherwise.