00 CR250 motor seizure

[Cotton]

I just bought a "clean" 2000 CR250 last Thursday. After an hour of riding Friday afternoon and another hour Saturday afternoon, the motor locked up. I haven't been in the motor yet, but judging by the way it seized, I think it broke the piston skirt and hung up in the cylinder. What are the chances a simple topend rebuild(piston, cylinder hone, etc.) will fix it? How often do broken pistons in late model CRs require major motor work, like a new bottom end or a whole new cylinder? I have rebuilt several two strokes(250R fourwheelers) in the past but never one this new. The guy I purchased the bike from said he would pay for parts for a new topend(piston, gaskets, etc.), but if it needs more, I am going to be pretty dissappointed. Thanks for any help/advice. Cotton

 

[MXFastGuy]

My rod bearing let go when the bike had 40 hours on it. The shrapnel came up into the top end and trashed the piston AND cylinder. I know of several other guys who also had the same thing happen on their '00 or '01's.

You may want to check into having the cylinder re-sleeved. It may save you a few bucks vs. a new one. Also insist on an aftermarket rod kit, it will save you some $$.

 

[RM_guy]

If the plating is toasted, do not resleeve it. Have it replated. US Chrome is the best for replating.

 

[Cotton]

Thanks, guys. The bike is at the shop now, and my mechanic should let me know the damage by tomorrow. Hopefully it won't be too bad.

 

[mx547]

Originally posted by MXFastGuy
My rod bearing let go when the bike had 40 hours on it.

upper or lower bearing?

 

[MXFastGuy]

Lower :whiner:

 

[mx547]

Originally posted by MXFastGuy
Lower :whiner:

any clue why the failures? i've probably got about 50 hours on mine. i asked which bearing because i'm getting ready to do another top end and i was going to reuse the upper bearing this time.

 

[MXFastGuy]

I don't know why. I think it was just worn out, and it decided it was time to quit. If you've ridden your bike very hard, you might want to think about checking-out the lower-end. Especially if you're going to run it all year. As far as the top rod bearing, you're probably okay only changing it every other time you replace the piston. My rule of thumb with 250's was top-end 10-15 hrs and lower-end 30-40 hrs.

 

[Mxracer510]

dang..you all change your stuff often. I have a 2000 CR250 as well. I race maybe 60 hours before changing the rings. And prolly another 60 before i put a new piston in it. Just have always used good oil and such. Never had a piston be out of spec. either. The only problem is once I crashed and sprung a leak on a radiator and ended up overheating and blowing up a piston that way. But that wasnt the fault of a piston or ring.

 

[dale williams]

The parts manufactures must really like you. There should be virtually no wear on a well-lubricated part. The difference between poor lubrication (parts making contact) and good lubrication (parts never making contact) is like the difference between 10 hours run time and 1000 hours run time. Cars run over 2500 hours with less top lubrication than 2 strokes get. Yes bike engines run faster and wear is a function of speed squared so 2 stroke 250s should get something on the order of 500 hrs. run time. The only bearing failure I've experienced was on a 1970 version of a G31M. This was Kawasaki's idea of a 100cc flat track racer. Using regular mineral oil at 20:1 we only got 30 minutes out of a rod lower bearing. Changing to Castrol R (vegetable oil) at 32:1 these engines would run forever. I'd say real short life is due to poor quality oil or manufactures defects. Reduced life is due to dirt. Otherwise a 2-stroke engine should have a long and prosperous life.

 

[2001yz250]

Don't ride a 2-stroke do you dale? This kind of thinking cost me $600 in parts alone last year when I broke a piston skirt and shrapnel fell down in the crank and cracked both cases and cracked the skirt on my cylinder. I use Bel-Ray oil which is high quality. My mistake was thinking I could go three summers on one piston even if it was mainly woods riding. Even the manufacturers suggest replacing the piston every 4-5 races and motocross races don't last very long if you think of it in actual minutes on the track.

 

[parkerCR]

This is a great topic. I only ride my 01 CR250 for recreation sometimes hard but I always thought along the lines of what dale said. I never thought I would have to change the piston every 20 hrs. I don't race and take good care of my bike. This is an interesting discussion and I am interested to see where it goes and what the consensus is.

Thanks

 

[Mxracer510]

The manufactures are overly excited to put those times down in the owner's manuel. But think of it this way. In most of those owner's manuels you are changing the oil at the same intervals as the piston eh? Maybe if your at a national level I can see tearing it down that often. But...ya know..thats alot of good parts that still have alot of life left. Money out the window in most cases in my opinion.

 

[Cotton]

My mechanic tore the motor down yesterday and the crank is screwed. The piston skirt broke and pieces of it destroyed the rod and main bearings before the motor stopped. I have a good friend at a local Honda dealer though, so I will get a deal on the parts. This is the first late model Honda 2 stroke I have owned, but I have a 1987 Honda 250R fourwheeler. That thing is pretty much indestructible. I ride it only on sand dunes where it sees extended high RPM use, and it has never broken on me. I have never ran a piston in that thing any less than 1.5 - 2 years. It doesn't make sense to me for a motocross bike ridden on a track by someone with less than stellar skills to need a piston more than once a year. It sees a lot of on/off throttle and not a whole lot of extended high RPM use. If you don't race extremely hard all the time, I think a well maintained 2 stroke shouldn't need a piston replacement nearly as often as the manual recommends. But, that is just my thoughts, and I am not the most knowledgeable person on the subject.

 

[Buzz Bomb]

My guess is that the guy who sold you the bike had teh stock piston in it when he sold it to you. A 250 motocrosser doesn't need a piston more than once a year unless you are racing in the pro classes. Mine was used all summer last year and it still ran fine, but I replaced it because I dunked the bike in my friends pond by accident. These bikes are stone reliable! I just cleaned my engine out with kerosene, poured the kerosene out, put in a new top end, and the bike ran like new again, no problems.

 

[scotts250]

Cause for bearing failure in CR..

My '94 CR250 had parts of the ball retainer break off and lodge themselves in the cylinder.
When I replaced the crank and bearings you could see some surface rust on the old bearings. I believe that I may have caused this by starting my bike after I would wash it. I wasn't covering up the holes in the airbox and water was getting on the filter and being sucked into the motor.
I had US Chrome replate the cylinder. They weld the gouges and replate. Came back looking new.

 

[dale williams]

I have had several two-stroke engines in my life and I currently ride a 2001 CR250. Parts breakage is not caused by wear unless the wear is reducing the thickness of the part like a piston skirt such that excessive slop allows so much rattle that fatigue causes a cracked skirt. Two stroke pistons are more immune to piston rattle because the height is as much or more than the bore. Four stroke pistons have very short and small skirts and are much more susceptible to fatigue just by their design.
Wear is caused by dirt which could easily cause excessive slop in a few rides if an air cleaner is leaking because there is a infinite amount of grinding compound and a finite amount of engine metal.
The other source of wear is not really wear but scuffing caused by oil film violation. This is usually due to oil dilution by fuel. The engine must be warm enough to evaporate the fuel in the crankcase and leave the oil behind. If not you get a lot of smoke and piston scuffing. I’ve seen this on a weed wacker that had such low compression that it wouldn’t start.
I’ve seen bearings on a chain saw that wore flat when running Pennzoil multi purpose two-stroke oil but everything else was fine except for some minor scuffing on the cylinder.
As I mentioned before I’ve seen the bearings on a G31M burn up running 1970 vintage Unimix.
The above situations were caused by oil film violation, although dirt could have contributed to the chain saws problem. I hope that Belray is a good oil because that is what I am currently running, although I find the engine runs better at about a 30:1 ratio than at a 40:1. Probably better ring sealing. It also ran good at 20:1 on mineral oil with not much more spooge.
I would suggest that the failures you’ve mentioned, broken piston skirt and fly ball were due to fatigue resulting from a manufacturing defect or sloppy installation that resulted in a small nick that started a crack. If this is the case replacing parts that are working will not necessarily result in avoiding a problem because there is no time schedule for a defective part to fail. It is most likely to fail early in life but that is no guarantee.
As far a how long a two stroke engine should run without failure just think of all the two stroke engines out there, outboard motors, chain saws, leaf blowers, generators, water pumps, scooters, airplane engines. All of these engines enjoy lives in the range of 100 to 1000 hours before needing a top end.

 

[dale williams]

Check out this web site.

http://www.ultralightnews.com/features/oiltest.htm

 

[bwalker]

is like the difference between 10 hours run time and 1000 hours run time. Cars run over 2500 hours with less top lubrication than 2 strokes get

A auto engine with a pressurised lube system cannot even be compared with a two stroke total loss system. Two strokes are poorly lubricated because of fuel dillution and many other factors. This is why they have ball crank bearings vs plain shell bearings that are found in auto engines. BTW the ultra light link has very little useful info that relates to bikes and all the oils that where tested in the article showed high level of deposits IMO. Seems to me like a Penzoil sales pitch.

 

[dale williams]

bwalker, I agree it is a sales pitch but the point is that they got 250 hours using 50:1 with a crummy oil. I consider most bike oils to be far superior to Pennzoil. The big difference between 4 strokes and 2 strokes is the dirt doesn't go directly into the oil, and some even have filters to remove the dirt, so only clean oil goes directly to the bearings. Hence it is VERY IMPORTANT to maintain a CLEAN air filter on a 2 stroke.
Two strokes do also have the problem of oil being diluted by the fuel. That is why it is unwise to "give it the gas" before the crankcase has warmed up some. The fuel is supposed to evaporate in the crankcase leaving the oil behind, although I placed a thermometer on mine and on a 45 degree day it never got over 80 degrees so it is very important to not run a fuel rich mixture that would over cool the crankcase.
I also find that a carburetor adjusted to give the proper mixture on acceleration will go rich during constant speed operation due to the increased airflow and vacuum. I wander if this could be caused by the pilot jet. Maybe the TM carburetor with the deleted pilot jet addresses this.

 

[bwalker]

Hence it is VERY IMPORTANT to maintain a CLEAN air filter on a 2 stroke.

I agree 100%. Thats why I run twin air filters plus a twin air dust cover over the filter.I also clean the filters after every ride and use a generous amount of filter oil(not No Toil). Nothing will lunch a two stroke faster than sucking a big load of sand.

I also find that a carburetor adjusted to give the proper mixture on acceleration will go rich during constant speed operation due to the increased airflow and vacuum. I wander if this could be

A two stroke requires more fuel upon excelleration than at constant speed as you have observed. This why mfg's use power jet carbs that add fuel to the midrange. I believe this is caused by engine load more than anything else. Is the tm carb you are refering to the one with only a main and a needle jet? If so I have heard this carb was not widely used because it was a nightmare to calibrate. Trying to find the right needle for this type of carb is a nightmare. Been there done that with Lectron carbs on a snowmobile.Had me ready to tear my hair out.

 

[Marcad]

I have a 2001 CR250. Word is that the stock piston for this bike is prone to having to skirt failures. I ride a lot and usually very hard. I freshen my top end about every 4 to 5 months. The last time I had to do a top end was from a seizure. I suspected an air leak. After inspection I noticed that the center case gasket stuck out into where the mating halfs of the cylinder and case come together. Don't know for sure if that was the reason for the seizure but after cutting the excess gasket material and using a sealer on the base gasket, I haven't had a problem.

As far as bottom ends. I have seen guys go years without having a bearing failure on Honda CR250s. I wish I could tell you the same for my last Kawie.

 

[spanky250]

Originally posted by dale williams
Reduced life is due to dirt.

You aren't allowing for the added stress of crossing the port openings. And car engines aren't run flat out for most of their lives (well, not most of them). Most car engines don't have aluminum pistons, either.

500 hours on a two-stroke? I wish...