So I was out riding my 1991 KDX250 yesterday. I was on the highway, when I decided to see what my top speed was. I made it to 130km/hr when the engine seized, and the back tire locked up. So I coasted down to about 50km/hr and popped the clutch. Bike fired back up. I was able to ride about three km, before I came to a stop sign. I managed to keep the engine running while I was stopped, but as I pulled away she finally died again. I got it home (on the back of a trailer) and checked the compression, only 55psi. I'm gonna do the top end myself, what KX250 pistons and gasket kits fit in my KDX250? Is there a preferred brand of piston? What are some things I should be looking at when I have the top end off? Any other hints or advice would be appreciated? Thanks.
KDX250 Blown Top End
- Thread starter bajus22
- Start date
In your case, the piston has most likely melted around the exhaust port area due to excessive heat. 2 strokes don't like to be held at one rpm for any period of time, whether it's at 4000rpm or 12,000rpm. This means that the exhaust side of the cylinder doesn't get any chance to cool down. When you back off on the throttle, the unburned air/fuel mixture cools the piston and exhaust ports/cylinder very quickly as it passes over and through them.
When you have the top end off..
-check the condition of the powervalve (make sure it all functions correctly when moved by hand)
-check the conrod big end bearings (make sure they have no up and down movement)
-make sure that the cylinder plating isn't worn (if you can catch your fingernail on the plating it needs to be replated or sleeved)-If your seizure was due to the piston failing, you'll most likely need to do one of these things
-Make sure no debris is in the crank case
-Check the condition of the reeds (sitting flush with cage, not chipped or cracked/warped etc)
-Make sure the intake side of things is 100% clean before re-assembly (filter, airbox, intake boot etc)
As for KX Gaskets, I think you can use the thinner head and base gaskets from a 89 model (Not 100% sure) You'll be able to find more info on that if you do a search.
Most people I know have opted for a forged piston, either wiseco or wossner. I've always used wiseco pistons, just because they're forged and have quite a good reputation. Forged pistons are less likely to shatter than cast pistons, and according to popular belief, last longer. Although are slightly more expensive. Vertex pistons are the major aftermarket brand for cast pistons, and also have a good reputation.
In the end it's up to you. Chances are, with a fresh top end it'll feel like a whole different bike.
Good luck.
When you have the top end off..
-check the condition of the powervalve (make sure it all functions correctly when moved by hand)
-check the conrod big end bearings (make sure they have no up and down movement)
-make sure that the cylinder plating isn't worn (if you can catch your fingernail on the plating it needs to be replated or sleeved)-If your seizure was due to the piston failing, you'll most likely need to do one of these things
-Make sure no debris is in the crank case
-Check the condition of the reeds (sitting flush with cage, not chipped or cracked/warped etc)
-Make sure the intake side of things is 100% clean before re-assembly (filter, airbox, intake boot etc)
As for KX Gaskets, I think you can use the thinner head and base gaskets from a 89 model (Not 100% sure) You'll be able to find more info on that if you do a search.
Most people I know have opted for a forged piston, either wiseco or wossner. I've always used wiseco pistons, just because they're forged and have quite a good reputation. Forged pistons are less likely to shatter than cast pistons, and according to popular belief, last longer. Although are slightly more expensive. Vertex pistons are the major aftermarket brand for cast pistons, and also have a good reputation.
In the end it's up to you. Chances are, with a fresh top end it'll feel like a whole different bike.
Good luck.
- Thread starter
- #3
Thank you for your reply. You were dead on with your assumption, the piston did melt on the exhaust side. I've taken off the cylinder, and it wasn't as bad as I thought. I removed the melted aluminum (with some acid) and the liner is intact. I ordered a Pro X piston, 67.355mm. Is it alright to hone these cilinders (gently) with a 720 grit stone? It was recomended I do that by my local shop. I also ordered a gasket kit for a 1988 KX250, so I'll hopefully get the thinner gasket. I'm also going to put in a new needle bearing. I cleaned out the bottom end and checked the bearings. I have zero up/down movement, but I do have some side-to-side movement. Is this normal?
A small amount of side to side movement is acceptable. The up and down movement is what you really need to worry about. Do you have a service manual by any chance? If so, you shouldn't have any problems at all with checking tolerances/clearances along with re-assembly.
Has the cylinder been sleeved? or is it plated, as stock? Personally, I'd stay away from the hone if the cylinder's plated. It can be done if you're very careful, but even piston manufacturers instructions warn against honing plated cylinders, unless done carefully with a ball hone. Totally up to you though. :)
Has the cylinder been sleeved? or is it plated, as stock? Personally, I'd stay away from the hone if the cylinder's plated. It can be done if you're very careful, but even piston manufacturers instructions warn against honing plated cylinders, unless done carefully with a ball hone. Totally up to you though. :)
Was the melted piston on top, or the skirt?
- Thread starter
- #6
Honestly, I'm not sure how to tell the difference between a sleeved cylinder or a plated cylinder. I'm assuming it's stock. So if I don't hone the cylinder, is there something I should do about the scuff marks in the cylinder? The cylinder feels completely smooth. I did use a mild scrub pad to clean it up. The piston was melted along the side. The rings were melted into the piston. Is this significant? Oh, and I noticed a dark spot (burnt oil?) on the underside of the piston, at the top. Is this another indication of overheating?
If you're unsure of the condition of the cylinder lining, you should probably get the guy at your local bike shop to have a look at it or even post some pics here. Most people just use a scotchbrite pad to clean up the cylinder walls, so it sounds like you did the right thing. :cool:
There's no point spending a lot of time and money on something only to find the new parts have worn prematurely only a short amount of time down the track, due to the problem not being fixed properly the first time around. Better to be safe than sorry.
I'll post some photos of my 200's cylinder with extremely worn plating, and some of after I had it sleeved if I can find them. Hopefully I'll be able to get around to it sometime tomorrow.
There's no point spending a lot of time and money on something only to find the new parts have worn prematurely only a short amount of time down the track, due to the problem not being fixed properly the first time around. Better to be safe than sorry.
I'll post some photos of my 200's cylinder with extremely worn plating, and some of after I had it sleeved if I can find them. Hopefully I'll be able to get around to it sometime tomorrow.
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dirt bike dave
Sponsoring Member
- May 3, 2000
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Since it did this under a sustained high speed run, it might be something related to fuel starvation or overheating caused by something else besides jetting.
Float level too low, gas cap not venting, clogged fuel filter, clogged radiator, etc...
Float level too low, gas cap not venting, clogged fuel filter, clogged radiator, etc...
EYY said:
many boats are 2T and they have a throttle that can be positioned and held thereby te way the thottle stick is made...
but we are taking bikes I know right... well in that case 4000rpm is kinda in the lower portion of the rpm range..all things being proper I would not be afraid to run for 10-15 minutes at that rpm, but never do the trails have to many variables. Desert racers may have that oppertunity though
my guess is that he did no prep work to run wide open...
JUst for fun http://www.youtube.com/watch?v=Vz_r_3yqqR4
dirt bike dave
Sponsoring Member
- May 3, 2000
- 5,348
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In my experience, it is not the holding it WFO on the road that is the problem. That does generate tons of heat, though.
The problem is rolling off the gas at the end of the top speed run without pulling in the clutch. Lots of heat + high rpm without benefit of lubrication in the fuel can result in big problems. I blew up a CR250 that way. Took out the crank, one of the crank case halves, piston, etc...
The problem is rolling off the gas at the end of the top speed run without pulling in the clutch. Lots of heat + high rpm without benefit of lubrication in the fuel can result in big problems. I blew up a CR250 that way. Took out the crank, one of the crank case halves, piston, etc...
I see where you're coming from, but I was talking specifically about dirtbikes in my previous post and probably should have mentioned that. They just aren't designed to be ridden at one RPM for any amount of time. Although, jetting may be richened to help reduce the risk of such failure dramatically. Engines found in 2 stroke road bikes, boats, and lawnmowers are designed for long periods of use at a single RPM and so will extremely rarely face such a problem.
As for disengaging the clutch after a full throttle run, this is likely to be the cause of damage rather than prevention. As for the lack of lubrication, enormous amonts of air/fuel mixture are being circulated throughout the crankcase and top end during such rpm's. More than enough oil is in the crankcase being thrown around onto the cylinder walls and throughout the bottom end whilst deceleration is taking place for a significantly short amount of time.
While the lubrication is very likely to be more than sufficient, the efficiency of the cooling system is also at its greatest when decelerating. The engine is not producing any heat due to combustion, and at the same time the waterpump is working overtime circulating coolant. The speed of the bike also allows massive amounts of air to flow through the radiators and thus also rapidly cooling the coolant.
As for your case, a combination or one of the following may have been an issue;
-Lean jetting
-Maintainance issues
-Service intervals overlooked
Your bike didn't blow up because you let go of the throttle without pulling in the clutch. By that time, the damage would have already been done, and the repair bill would have been just the same.
As for disengaging the clutch after a full throttle run, this is likely to be the cause of damage rather than prevention. As for the lack of lubrication, enormous amonts of air/fuel mixture are being circulated throughout the crankcase and top end during such rpm's. More than enough oil is in the crankcase being thrown around onto the cylinder walls and throughout the bottom end whilst deceleration is taking place for a significantly short amount of time.
While the lubrication is very likely to be more than sufficient, the efficiency of the cooling system is also at its greatest when decelerating. The engine is not producing any heat due to combustion, and at the same time the waterpump is working overtime circulating coolant. The speed of the bike also allows massive amounts of air to flow through the radiators and thus also rapidly cooling the coolant.
As for your case, a combination or one of the following may have been an issue;
-Lean jetting
-Maintainance issues
-Service intervals overlooked
Your bike didn't blow up because you let go of the throttle without pulling in the clutch. By that time, the damage would have already been done, and the repair bill would have been just the same.
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- Thread starter
- #14
I'm guessing it was a combination of jetting and maintance. The bike is 20 years old and the previous owner couldn't tell me the last time the top end was done. I did have the bike jetted per the recommended specs found on this web site. So I dropped down one on the main and the pilot. I'm still running the stock needle, I'll drop that one clip also. I'll lose performance but I'll hopefully gain more reliability.
dirt bike dave
Sponsoring Member
- May 3, 2000
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EYY said:
Not after the throttle is closed.
EYY said:
I disagree. Most bikes do not have a pool of oil laying around in the bottom of the crankcase. A red hot two stroke bike at top speed could take a long time to slow down using engine braking only.
EYY said:
But the same rpm that is making the waterpump 'work overtime' is also generating friction in an already hot cylinder. I've heard it is also possible to circulate the water too fast for maximum heat transfer.
In any event, it takes time for the radiator to transfer the heat. The heat added from friction by the high rpm engine braking builds very rapidly. No lubricant and fuel to cool the hotpots can quickly result in failure.
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It was never stated that there was a "pool" of oil in any two stroke crankcase, but merely residual oil. And yes, if you are using techniques of engine braking on your 2 stroke for extended periods of time such a failure is very likely to result, as mentioned by yourself.
As previously stated, the bikes are made to be ridden at varying revs and throttle positions. When decelerating under engine braking, if the throttle is completely closed for the duration of deceleration, this is when failure will occur. That's why many riders open the throttle in short bursts during deceleration whilst completely backing off.
I know of many riders who have experienced piston seizure during high rpm operation, none of which had occurred during deceleration. No matter how you put it, the heat of combustion constantly heating the exhaust side of the cylinder and the piston will eventually result in top end failure. That is undeniable.
It's all about riding technique and the presence of regular maintainance, or lack thereof. As a rider, you also should also know of the importance of engine braking under slippery conditions or loose terrain in relation to the control of the bike.
I'm not trying to say you're completely wrong, I'm just saying that what I previously stated is a common cause 2 stroke seizure. From the information given, I had assumed the failure had occurred during high rpm operation and not during deceleration.
As previously stated, the bikes are made to be ridden at varying revs and throttle positions. When decelerating under engine braking, if the throttle is completely closed for the duration of deceleration, this is when failure will occur. That's why many riders open the throttle in short bursts during deceleration whilst completely backing off.
I know of many riders who have experienced piston seizure during high rpm operation, none of which had occurred during deceleration. No matter how you put it, the heat of combustion constantly heating the exhaust side of the cylinder and the piston will eventually result in top end failure. That is undeniable.
It's all about riding technique and the presence of regular maintainance, or lack thereof. As a rider, you also should also know of the importance of engine braking under slippery conditions or loose terrain in relation to the control of the bike.
I'm not trying to say you're completely wrong, I'm just saying that what I previously stated is a common cause 2 stroke seizure. From the information given, I had assumed the failure had occurred during high rpm operation and not during deceleration.
- Thread starter
- #21
So I finally got around to putting the bike back together. I had a mechanic at my local shop check out the cylinder/piston, to see what he thought caused the failure. And sure enough...
Anyways, got the bike back together, warmed it up, and preceded to ride the hell out of it, as per the break-in procedure on some of the threads here. I usually break-in my engine's the way the manufacturers suggest, but I figured I'd try something different this time. I gotta admit, I like this way better. The bike actually feels stronger. I've already done about 200kms of highway riding without a problem, plus the off road stuff.
Tom68 said:
Anyways, got the bike back together, warmed it up, and preceded to ride the hell out of it, as per the break-in procedure on some of the threads here. I usually break-in my engine's the way the manufacturers suggest, but I figured I'd try something different this time. I gotta admit, I like this way better. The bike actually feels stronger. I've already done about 200kms of highway riding without a problem, plus the off road stuff.
GREAT to read, thank you for updating!
