YZ 426 Lower End Bearings.....??

I just rebuilt my entire engine. Top and bottom end. All new bearings and seals in the bottom. New wiseco top. Before i rebuilt it, it seized up. Something in the bottom end was stuck. The oil filter was covered in metal.

So i tore it down and replaced all but the left side primary drive shaft bearing and the shift drum bearing on the left as well. finished it and it ran great. i still saw a few flakes in the oil but i thought this was typical of an engine after catastrophic failure and rebuild such as this. i rode very easy and broke the motor in completely.

after about 3 rides (changing the oil every time) the flakes started to appear more and more. the bike started to lose power which was VERY noticeable. still seems to run fine and i checked everything else. even did a leakdown and came out fine.

my question is could it be a bad bearing in the bottom. maybe something w/ the crank ( ie. bent, bad rod bearing etc..) that would cause it to lose power and metal flakes to appear. how can you tell that a bearing is bad. they all turned freely and smooth and the rod had no play in it???

any input??

Try this. Pull the clutch cover and remove the pressure plate and clutch plates. Turn the clutch boss and see if it turns freely and smoothly. If not, that left main shaft bearing is probably bad. They are one of the most likely transmission bearings to fail out of the set.

will do. i just dont understand how it could have failed that quickly. i checked everything twice. i took my time and stretched out the reassembly for a week so i could make sure it was right.

it ran like a bat out of hell for awhile. are the flakes typical after a rebuild such as this? and if so, how long can i expect to see them before they are eventually flushed out?

If the flakes were in the oil filter, they were in the frame, and if you didn't flush that, you can expect to see more until they're finally gone. Remember that the oil is picked up by the return pump and sent to the tank (frame), after which, the feed pump draws it back top the engine and sends it first to the oil filter.

A bearing race can start spalling off (flaking) at any time, which is exactly why any bearing that requires that much work to replace should be replaced at the same time the rest of the crankcase bearings are when rebuilding a high hour unit; you simply don't have a way of knowing how much longer the aparently healthy bearing will last. It might not be bad, though, so don't panic just yet.

could a bad bearing like that be the cause of such dramatic power loss?

"Dramatic power loss" would be asking quite a bit from a bearing, frankly, short of a full siezure, especially when we're talking about a 426. Actually, the first symptoms of a mainshaft bearing that drags a lot would be difficulty shifting at high RPM.

Otherwise, power loss is the fault of incorrect timing (flywheel key, cams out of time), loss of compression, stuff like that.

Before i rebuilt it, it seized up. Something in the bottom end was stuck. The oil filter was covered in metal.

The above quote is exactly what I had happen to my 426.

Although my problem was a piece of my crank was being shaved down and eventually broke off and logged in between some gears, causing the engine to seize.

Im not the most mechanically inclined when it comes to 4T bikes otherwise i would try to explain alittle better.

But alot of the signs you mention are the same that I saw.

Hope this helps...

i checked the bearing behind the cluch boss, seemed fine. checked the flywheel and key...fine. checked timing and did the leakdown again. all seemed fine.

just dont understand what could be wrong. could it be electrical??

anyone think this could be electrical

Unless it's misfiring, the only thing electrical that could cause a power loss would be timing. Theoretically, it's possible that the CDI could cease to correctly map the ignition timing correctly while remaining otherwise functional, but it seems very unlikely. There could also be a problem related to the TPS, which you could verify one way or other by trying the bike with the TPS disconnected. It should run the same at full throttle as it would with it connected, and if it was causing the trouble, the bike should run in a reasonably normal manner under a heavy load.

Otherwise, an ignition timing problem is a matter of a sheared flywheel key.

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