KX250 Bottom end knock

[KX250newb]

New to motocross and just purchased a 00 KX250. Purchased it from an aircraft mechanic so I figured it was well maintained. I need to call him to confirm but as I recall he had rebuilt the top and bottom end about 20 hrs ago. Bike runs great.

Was out this Friday riding with some folks that have much more experience than I. We noticed an engine knock that seemed to be coming from the bottom end. Not real bad but was noticeable. Two opinions were given by the experience MXrs and one felt it was a bad crank bearing and the other thought it was just a detonation issue.

Knock disappeared once the bike was in gear (seemed only to happen when in neutral - idle gear??) and felt strong all day. I plan to clean the air filter and change the oil this week but wanted to see if anyone had any ideas on how to trouble shoot this one. Premium gas is what i used for the mixture.

Thanks for the help!!

 

[ruffryder19]

If the bike has had any cylinder mods in the past, it might of raised compression , then it might be detonation. and also cold weather makes it run leaner. but if the cyclinder is stock, then its definatly not detonation.

 

[QKENUF4U]

does the knock go away when you pull in the cltuch lever ? sounds more like a wear noise in the cltuch setup (notchy basket etc)

 

[rm_racer]

Maybe a rod bearing? What octane fuel are you running? Did he have to run race fuel? Considering he was a aircraft mechanic, maybe he had access to and used AVgas because of high compression.

 

[KX250newb]

No engine mods were made so I don't think that gas is the issue. I just started it and listened for about five minutes.

Does not go away when the clutch is engaged - gets a little softer though. Sound as if it is coming directly from the crank case area (louder on right side near bottom).

Are they normally noisy? Will have my friends take a look when they get back from the holidays. Bummed.... :bang:

Thanks for the help!

 

[rm_racer]

Maybe your right side crank bearing is going? Does it lose transmission oil, or foul out alot of plugs? Take a long screwdriver or chisle or something, plug one ear, and put the head of the driver where you think the noise is comming from. put your ear on the handle. You will hear any bearing noise alot more clearly like this way. I dunno how loud the engine noise will be that close, Ive only done it on four stroke heads. I would say you would want a really long screw driver, as the expansion chamber will be right there if its short.

 

[KX250newb]

I need a longer screwdriver...lol. Yeah I tried it earlier but need a longer one I think. Once I see some of the diagrams it might make it easier for me to try an focus on an area.

I assume if it were going bad it make a knocking type noise?

Is replacing the crank bearing difficult to do?

Thanks for the help rm.

 

[darringer]

Using a long screwdriver may work. Sounds in the motor have a tendency to "travel" through the cases, making it difficult to pinpoint. Pull the ignition cover off and grab the flywheel. If there is any up-and-down or side-to-side play, the crank bearings are worn out. I have had a severe knock when the crank bearings were failing and other times no noise whatsover. To check the rod big end bearing you must remove the cylinder and try pulling up on the rod/piston. A bit of side-to-side play is ok here, but should have ZERO up-and-down play. If any of these bearings are bad, the cases need to be split. Major surgery. Not terribly difficult if you have mechanical ability and the correct tools.

 

[karter55]

You might also want to check the center bearing in the clutch basket. Remove the clutch cover and see if the clutch basket will rock back and forth.

 

[KX250newb]

Santa finally delivered my maint. manual... From the posts that I have read and from the manual everything seems to point to the clutch (my guess is needle bearings or similar). Karter, this is what you are thinking by checking the clutch for play?

Is this a difficult repair for a newb? (with some mechanical ability)...

IF it turns out to be the needle bearings (and somehow I am able to confirm this) am I ok to replace JUST the worn parts? Or should I replace more?

According to previous owner (aircraft mech) he rebuilt the clutch not too long ago.


Thanks for all of the help and Happy New Year!

 

[darringer]

It is a fairly simple job to replace the clutch bearings. If there is in fact up-and-down play at the clutch basket, replace the needle bearings and also the bushing they ride on. The previous owner probably replaced only the plates and springs.Sometimes there is no wear marks on the needle bearings or bushing, so it's hard to tell by looking if they are bad. They aren't expensive parts anyway, so it's worth a try.

 

[Rcannon]

Dont worry about this repair at all. An air powered impact gun makes the repair much easier. The nut that holds the clutch basket in can be a real bear.

Otherwise, no big deal. Figure 2 hours taking your time.

I have been inside of the clutch area on my yz 250 many more times than I care to admist. I imagine I could change the bearing in under 30 minutes.

 

[KX250newb]

Do I need the clutch holder tool then? or can I manage with out it? Could I wrap a towel around it and twist?

Please advise. Thanks.

 

[KX250newb]

I assume that I should not ride with the bike in this condition? My buddies are riding tomorrow....oh the pain......

 

[darringer]

Using an impact wrench on the clutch hub nut will work without the need for any special holding tools. Also, riding the bike in this condition will not hurt it. This is actually a fairly common KX problem. It will make noise, but won't cause any other real concern.

 

[KX250newb]

Doh.....they just left. Oh well. Should have stayed up late last night lol.

 

[KX250newb]

Alright, I finally got some time to work on it today. A breeze getting it broke down, but I haven't gotten the clutch nut off yet. I am not looking forward to it as I do not have an impact wrench. What about a hand impact driver? -where you use a hammer?

I will attempt with a cheater bar to see if I can get it off first but have a feeling that im going to break my knuckles.


There are some pretty good notches in some (not all) of the basket fingers. I have a pic but don't know how to add it to the post?

If its notched does it need to be replaced?

When I rotate the clutch hub quickly back and forth I hear the same type of knock that I heard with the engine running. I'm pretty sure that this has got to be it. The clutch basket has some play in it as well.

 

[darringer]

I don't recommend this way to lock the clutch hub, but you can put the bike in gear and lock up the rear wheel to get the clutch hub nut off, and also to put it back on.

If the clutch basket is notched, it can be draw filed in one direction to just remove the notches.

There will be some gear lash noise that comes from the tranny, and it can be quite loud when the clutch cover is off. So that may or may not be your problem.

 

[karter55]

The nut is really easy to get off with an impact wrench but will be much tougher without one. Try the idea of leaving the bike in gear and having someone hold the rear brake on. If that doesn't work, you might want to see if you can borrow the clutch basket tool from your local shop (since you'll be buying the bearings from them. As for the cost of the bearings expect to pay about $25 to $30 for the two of them. I'd agree that the noise you hear when rocking the clutch back and forth is probably pretty normal.

 

[KX250newb]

If I jiggle the clutch basket and it has some play...does that mean the needle bearings are it?

I would hate to replace the bearings and start the thing up only to hear that knock again....lol. :bang:

 

[karter55]

You're actually not looking for play rotationally (like when you turn it clockwise and counter clockwise) You're looking to see if you can wobble the basket on the shaft. This is kinda tough to explain in writing, but a little bit of rocking is okay a lot is of rocking is a problem.

 

[KX250newb]

I need someone with more experience to take a look or take it to a shop. The basket wobbles a little when I grab and shake (axially). I would think that it shouldn't move at all or at least feel fairly firm.

 

[darringer]

If you grab the basket it will move in and out a slight amount. When you grab it and it moves up and down is when the bearings are worn. Since you are in there anyway, replace them and the bushing. They are probably worn, even if that's not the source of the noise. Did you ever check the crank for play? Since you have the primary side off, try to pull the primary gear in and out and also up and down. Any movement is bad news.

 

[KX250newb]

I haven't taken the crank case off yet wanted avoid the extra labor (ie. increase the odds of me messin somethin up). lol

 

[darringer]

Pull the ignition cover off and push/pull and lift up and down on the flywheel. That will yield the same results to find out if the crank bearings are worn.