Posts
12
Joined
11/26/2019
Location
Jamul, CA
US
Edited Date/Time
12/4/2019 10:57am
Little bit of info: I was riding my YZ250 this weekend in Baja and on our way back to camp, we were on a long stretch of highway when my bike died. It sounded like I ran out of gas but when I went to kick it over, it was seized. I also realized after it happened that I was low on coolant.
I took off the top end (it looked fine) but the rod doesn’t move on the crankshaft. I can tell something melted around where the rod meets the crankshaft.
My question is: what would cause this type of failure? I originally thought that it was because I was low on coolant but looking at it, it doesn’t look like the coolant is even able to cool the crank. I was thinking I may not have had enough premix in my gas (I had 40:1 but I know it asks for 32:1). I just want to prevent it from happening again if I can.
I took off the top end (it looked fine) but the rod doesn’t move on the crankshaft. I can tell something melted around where the rod meets the crankshaft.
My question is: what would cause this type of failure? I originally thought that it was because I was low on coolant but looking at it, it doesn’t look like the coolant is even able to cool the crank. I was thinking I may not have had enough premix in my gas (I had 40:1 but I know it asks for 32:1). I just want to prevent it from happening again if I can.
Also, motocross engines are not designed for extended high-speed operation.
What year YZ250 and what size main jet?
I've known a few old school desert guys who will go so far as rigging a cable for a handlebar reachable choke on their two strokes to enrichen their mixture on long straights.
The Shop
If you are spending more time in the desert and less on a track, consider doing the 3rd-5th WR gear swap while you have cases split for the crank. You can find them used on Ebay if needed on a budget. Basically the new 250X transmission spread and making long high speed runs in the desert much easier (and the 250F's will no longer walk away from you on top.)
Another thing you can do instead of adding a choke cable, is to remember to hit your kill switch every now and then with the throttle open. You'll interrupt the spark for a few revolutions and the unburnt fuel/air mixture will cool down the whole works. (Only do this while you are riding down long straights for longer stretches of time, otherwise you'll just make a lot of excess spooge.)
Injector oils usually have a thinner in them so they don't clog injectors. That, or they have unusually low viscosities. Either way, the film strength isn't as high as it could be. This is why I recommend a heavier ratio when mixing them, but even that doesn't really fix the film strength; it just (hopefully) makes up for it with "more" film.
X2 on the heat diagnosis. That bottom-end heated up and seized.
For reference, Bel Ray MC1 is 208 F, Bel Ray H1R is 435 F, Maxima Super M is 280 F, and Motul's 800 has a whopping 485 F flashpoint (slow trail riders running the Motul would have exhaust drool like a 6 month old baby with a mouth full of Oreo's).
Pit Row
When a crank wears out and the rollers start to fall out of the cage, naturally there is a lot of grinding and extra heat. That blue color on the crank halves is from the factory (don't worry about that), but the melted plastic crank stuffers need to be replaced and show some signs of being lean.
Good/lean jetting has the bike running really good, but long stretches of straight away are really hard on the bikes. I'm not saying your last run was what killed it.
It might have been that the crank was waiting to fail when you got the bike.
That's what I did last time and then at my leisure, when I had a few extra bucks, I sent the old crank off to Crankworks to get rebuilt. Now I have a spare ready to go!
BTW, if you decide you don't want to get into dealing with fixing the bike, maybe let me know. I'm looking for a project fixer.
Call up Crank Works to see what they would charge to rebuild your crank. My guess with labor and shipping, you are near a new OEM crank in cost. If you have someone local, that will save you some money. Generally about $100 labor in my experience in CA.
I would strongly suggest you check your engine vin number. Your dealer or an internet source can verify the year.
The cranks changed very slightly from 99-01, 01-02, 03-current. Taper on the flywheel side. It's not uncommon for older YZs to have an engine from another year in the frame.
Good luck with your rebuild!
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