Failed main bearing ?

freakystyley · Jan 5, 2007

1995 YZ250, with what I suspect are bad main bearings. At idle there is a metallic knocking sound that seems to be in sync with engine rotation. Sort of like a bad/burnt/flat spot on a bearing. idle is also very rough and labored - not smooth and "free." Clutch/inner hub/basket/cable/springs are all new. carb and air cleaner clean. Other than that, she runs great! :yikes:

I am going to split the cases to verify, but noticed on the parts view there are bearings and thrust washers in the crank itself. If the tranny ran extremely low on fluid (don't ask) and burnt the mains, can those crank bearings go bad as well? I have a top on order along with the mains...just wondering if she needs a crank rebuild too while its apart.

CRazy250 · Jan 5, 2007

it depends, if there is no play in the rod bearing and its tight then the crank shouldent have to be rebuilt. if there is wear on the shafts then you probably should, check all bearings to see if they are good or bad and replace them all because its better to have piece of mind knowing its new and making full power.

jrm · Jan 5, 2007

The crank bearings are not lubricated by the transmission oil, they are lubricated by the oil in the pre-mix your engine runs on.

The sounds you describe certainly fit the symptoms of worn crank bearings, but the only way to know for sure is to have the cases split and measure the wear in the connecting rod big-end and main bearings.

dirt bike dave · Jan 5, 2007

Crank bearings certainly wear over time, especially if the bike is overheated or run with poor lubrication.

It's a big pain to split the cases for preventative maintenance. But when the crankshaft and/or big end bearings go, you can easily destroy the cases, and the carnage can extend to the piston, cylinder and powervalve.

On an old bike, not servicing the crank bearings is a calculated risk. If you have a catastrophic failure, it may cost more to fix the bike than the bike is worth.

freakystyley · Jan 5, 2007

thanks for the help, I have a new topend and main bearings on order - just wanted to know if the crank itself can go bad do to overheat/stress. Sounds like as long as it is apart I should cough up the extra $200 for a crank and do it all now.
As for worth, $400 for new guts is cheaper than couple grand to get a new bike...even if this thing is going on 11 years old.
At least it will be completely fresh and last a while longer.

kingbrian · Jan 5, 2007

have you checked the power valve to make sure its not hitting the piston ive heard of that happening on these models of bikes all ya gotta do is pull the pipe and check the piston for scuffing....

MX86 · Jan 5, 2007

you have a brand new clutch in? did the sound appear before replacing the clutch? i know if you don't torque the spings down in sync you get what i call "clutch chatter"

verfiy that it is coming from below the head or if it is coming from the clutch side of the case..

Trashrider · Jan 6, 2007

Does the noise go away when the bike is revved? That would indicate the rod bearings are bad. Sounds like you have all the bases covered though. good luck.

Mark

freakystyley · Jan 6, 2007

Long story is, I do not have a skid plate and must have nailed a rock or stump on my drain plug. Didnt notice until it was too late...we were out and refueled a few times but didn't notice or hear anything odd, we were in a pretty loud area and I didn't kill it to fuel up. Soooo, at the end of the day when I shut it off, I noticed an unusual idle noise and the engine stopped instantly when I hit the kill button.
So I restarted when I got home in the garage to listen carefully and noticed the sound in the opening post. Checked oil and it was WAY low, topped it off, restarted and noise was still there. Checked the clutch and steel plates were cooked brown - so I replaced the entire clutch side- noise is still there at idle but goes away at rev and runs strong otherwise. I can only figure main bearings are bad...so time to dig in! I cannot find a crank besides OEM so may just try and have this one rebuilt. Love the bike, hate to chuck it for $400 in parts- and I cannot afford a new bike since I am saving for a PW50 for my son.

darringer · Jan 6, 2007

From the latest post's info, it sounds more like transmission main bearings got cooked from lack of lubrication. It really makes no difference though, as the cases have to be split anyway. Spend a few extra dollars and replace the transmission bearings when doing the crank. They are common bearings and not that expensive. When finished, it will be a bike that can run for many more years.

freakystyley · Jan 6, 2007

thanks - then my question is -does common sense point to the big end rod bearing is cooked as well? I obviously cant see it unless I press the crank apart. hate to replace something that may not be damaged.

MX86 · Jan 6, 2007

Hot rods sells a crank set :cool:

www.motosport.com << i dunno if they have em.. but they normally do.

freakystyley · Jan 7, 2007

I can't find a hot rods crank for this model - they offer a rod kit but not complete assembly - not even in their catalog. looked almost everywhere I could think of.

darringer · Jan 7, 2007

There's no reason to think the big end bearing is fried because of lost transmission oil. It won't be affected because premix lubes the entire crankcase/top end. Only the tranny, clutch, and associated tranny bearings were starved for oil when it was run low on oil. That being said, it only makes sense to do the crank when the cases are split, whether it "needs" it or not. Split the cases once, and only once.

freakystyley · Jan 7, 2007

thanks a bunch - just what I needed to know.
Just wierd the main bearings went bad around the same time it ran low on tranny oil - I am an old 4 stroke and ATV guy and new to 2 strokes didn't realize the tranny oil didn't affect crankcase. makes sense looking closer at the exploded parts view.

Failed main bearing ?

freakystyley

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kingbrian

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