sad beginnings...

[Moshi]

Hi, I just bought myself a secondhand cr250 '99 model and rode it for a couple days before the engine locked up on me. Now I am not sure what happened and why. I live and ride in Tanzania (East Africa) and so I do not have ready access to good mechanics...
The story goes like this. The bike was using a 1:40 oil mixture (was told that the reckomended was 1:50). I was riding on tight paths and was going really slow, almost idling for a maybe 4min, then the bike died on me and I couldnt start it again. Pushed it to the nearest village and opened up the spark plug. it was very fouled with oil. I cleaned it out with some rough sandpaper, petrol from the tank and re-inserted it. I think the spark gap might have changed in the process. The bike fired up again but ran a little more uneven compared to before. I raced back to home (10km), keeping the revs high so not to foul the plug again. The bike sat for a few minutes and a friend took it for a ride. and after 5min the engine locked up. it was impossible to turn the kick starter. we tried to push the bike backwards in 3rd and it worked (we could hear the piston moving up and down with the sparkplug out, but it was impossible to push it forwards. The spark plug was nut brown when I took it out. it is a racing plug. The next day we were able to push the kick starter around and heard the piston moving up and down, but through the kick starter it felt like the motion was not smooth, asif it was scratching against something. Then it locked up again. we opened up the top and found that the cylinder edges were smooth and the piston was at its lower position. after closer inspection there were some scrathes on the rear end of the cylinder (towards the rear tyre), but these felt pretty smooth aswell, asthough they had been there for a while. We opened up the side covers and found that nothing seemed to be wrong there, except the oil was milky-grey. Can somebody please advice me what might have happened and how I should proceed?
Moshi

 

[Moshi]

...and when we looked at the cylinder head, it was corroded around the cooling ducts, not on the cylinder. should i get a new cylinder head, or should i have it machined smooth again... i can only buy pump gas here (incase the compression is increased by machining the head),
moshi

 

[Morvo]

Milky grey oil indicates that you have water in your oil, so check your head gasket for leaks and the cylinder head surface for 'flatness'.

 

[dirt bike dave]

I blew up the bottom end on my CR by riding it hard on a long, straight road, then backing off the throttle. No throttle = no lubricating oil going through the hot motor. Without pulling in the clutch, the motor was still turning fast. In my case, the lower rod bearing dis-integrated very quickly and the bike locked up. If the big end bearing is gone, the rod will have excess play and it may lock up against the crankcase. I needed a new crank assembly and crankcase half, as well as piston, gaskets, etc... I strongly suspect your lower rod bearing and/or crank bearings are in bad shape.

On a two stroke, the water in the head does not ever come near the transmission oil. If water is getting into the oil, it is probably a bad seal at the waterpump.

With Hondas, an entire crank assembly is actually not too badly priced (I paid $170 or so at Service Honda).

The lesson is don't roll of the throttle after lots of road riding on a CR250 or any pre-mix fed two stroke. Pull in the clutch when you need to slow down after sustained high speeds. Oh, and carry a spare plug, too!

 

[nickyd]

what kind of oil were you using the gas?

 

[Moshi]

thanks very much for input. your advice seems to check out with what my friend might have been doing - going fast down a long tarr road... plus the oil:gas mixture may have been on the lean side...
I used castrol tts oil, is this suitable?

If I order parts in the next day or so I can have them here in a week or so (a friend is coming out). would you reckomend I get the whole crank assembly, crank case, new piston+rings, gaskets, head, water pump seal... anything else..
I am not sure that I can have it opened in time to have my friend order the parts.

thanks again,
Moshi

 

[viking20]

You need to look at the bottom rod bearing , see if there is any up-down play in the rod , or if the rod moves roughly around the bearing. If so , you will have to split the cases , and install a rod kit or a new crank , and preferrably new main bearings and seals.
The head gasket area on the cylinder head will look like you describe , if pure water is used as coolant.
The correct mix of anti-freeze and water is not just recommended because of lowtemperature riding , but because the antifreeze prevents corrosion inside the engine.
This is the case if the chipping is visible around the waterholes in the head.
If its around the combustion chamber , the head gasket has been leaking coolant into the chamber.
The milky looking gear oil , can be because of condensation of the oil ( probably not where you live !) , or the seal on the waterpump axle allows coolant to be mixed with the oil , as said earlier....

 

[Peer Lovell]

I wouldn't order any parts until you get it apart.
What you describe sounds a lot like the kickstarter return spring locking up. I would pull that side of case before digging into the engine.

 

[Moshi]

i have opened the side cover and the kick starter seems ok, will open the rest this evening. I found a lot of oil and carbon inside the exhaust valve mechanism on the right hand side of the cylinder and also inside the adjustment, is this supposed to be?

 

[viking20]

It will always gum up more or less , low revs and to rich jetting will make it worse...

 

[Moshi]

it has all been opened up now and the damage has been identified. A new crank assembly and bearings, new piston and seals are innhouse and ready to be installed. But first a crach in the crank case needs to be welded... it has set me back close to 800 dollars.
My question now is, because I really really do not want this to happen again, was it bad luck that it happened, or should you never ride far and fast on a performance 2-stroke? Is it likely that it was already worn, and thats why this happened, or could a perfectly good engine seize up like this without notice (just an ignorant rider)?

 

[dirt bike dave]

A perfectly healthy, pre-mix fed two stroke can do the same thing. In normal offroad riding, you are at virtually no risk if the bike is jetted correctly. The motor killer is sustained, full throttle high speed road riding, then rolling off the throttle while the bike is still turning high rpm (coasting into a corner, or down a hill). There is a reason the factories put oil injection on their road legal two strokes.

Just remember to pull in the clutch, and don't use engine braking after a long stretch of hard road running and you should be fine.

Oh, and check your powervalve bearings on your CR, too. One of mine overheated when I blew my bottom end.

 

[Colorado]

If you pull the cylinder and piston you should be able to rotate the crank with the connecting rod. You can get some idea about the lower rod bearing that way. If it turns smooth with no lateral play, then check everything else closer before splitting the cases. At least that's what I'd do.