Engine died... valve or timing chain or ??

Out riding yesterday the motor died and made a noise similiar to the drive chain coming off the sprockets and binding up, but not quite as loud. I looked down to see the chain stillriding on both sprockets and then tried to restart the bike. The bike would not hit TDC, just kept cranking over. It feels like there is still decent compression.

I did a couple of things the day before and I'm not certain they had a role in all of this or not. 1. put on a FMF powebomb and Q silencer (162 main, 42 PJ, needle in 3rd & 2.5 turns on the fuel screw) and 2. removed the screen from the frame oil reservoir. I took it out to clean it for the first time and it was trashed from the previous owner and wasn't filtering anything, so I removed it.

Could the jetting have anything to do with this? Or the oil screen in the frame?

Any guesses on what happened?

I cant tear into it until Wed.

:naughty:

It sounds to me like your cam chain jumped a couple of teeth. I would take the valve cover off and check that your timing is still correct. If not, It might be time for a new cam chain and tensioner.

I highly doubt the chain jumped teeth. It sounds like you dropped a valve. Has it been long since you checked your clearances? If they get too wide, the keepers will wear thin and pop out, thus dropping the valve.

Its been a while since they were checked. Whats the damage report with dropping a valve?

Out riding yesterday the motor died and made a noise similiar to the drive chain coming off the sprockets and binding up, but not quite as loud. I looked down to see the chain stillriding on both sprockets and then tried to restart the bike. The bike would not hit TDC, just kept cranking over. It feels like there is still decent compression.

I did a couple of things the day before and I'm not certain they had a role in all of this or not. 1. put on a FMF powebomb and Q silencer (162 main, 42 PJ, needle in 3rd & 2.5 turns on the fuel screw) and 2. removed the screen from the frame oil reservoir. I took it out to clean it for the first time and it was trashed from the previous owner and wasn't filtering anything, so I removed it.

Could the jetting have anything to do with this? Or the oil screen in the frame?

Any guesses on what happened?

I cant tear into it until Wed.

:naughty:

If you find that it did jump teeth, I have a brand new chain still in the package I'll sell for $25. I paid more for it, but didn't need it.

Most of the time, you've got to replace the valve, retainer and keepers, head gasket (you'll have to reshim it too) and sometimes the piston. Since you heard a noise, it doesn't sound too promising. I got lucky once when it happened and had NO damage. I just happened to be off the throttle when it died. You can pull your valve cover in about 20 minutes to find out the cause, but then you'll have to pull the head to make sure the piston is OK. You may be able to stay under $200. You might as well rebuild the top end while you're at it. $159 for a Wiseco kit.

biznet1,

thanks! I'll keep that chain in mind... in fact I'll take it. You got a paypal acct?

I am going to do a rebuild while its off. I've been wanting to and I guess now I have a reason.

Yes I have paypal. You may want to wait until you find out what the problem is though. My email is sjohnston@cdpemail.dhs.gov if you do want it though.

I'll let you know Wed what I find. I'll probably replace it either way though, piece of mind....

Thanks.

A quick check of your cam timing will tell you if it jumped or not. But if it has, you'll want to know 2 things; why, and what happened as a result?

As to why, if the chain has never been replaced, it's possible that the crank sprocket may be severely worn. This sprocket is unfortunately integral with the left crank axle, and not replaceable separately from the rest of the crank. Also, check the chain tensioner carefully. Retract it all the way, then push it up against the edge of the workbench and let it out a little at a time, trying to push it back in at each step. If you can cause it to skip, ditch it. If you're not sure, ditch it anyway.

In the what happened column, you will mostly be concerned that it didn't cause the valves to hit the piston. If the compression is down after you re-time the cams, it's a bad sign. If it's good, pull the seat, hold the throttle open, and listen for a compression leak into the intake or the exhaust while someone puts pressure on the starter. Any valve that has made contact, even if it's still straight enough to seal, needs to become a paperweight before it turns into a grenade.

That looks like the teeth on the crank gear are shot and it jumped them. That means total tear down and a new crank if they are history.

To check them pull the flywheel and stator baseplate off. You can see them easily then.

You're sure that's TDC on the compression stroke? Take pics of the cam lobes (the gears can spin on the shafts)... The scary part that I see (assuming the pics are correct) is that both cam gears are correct relative to eachother, which would point to a bottom end problem (if the gears didn't spin on the cam shafts).....

However, this looks to be almost exactly 180 out (TDC on the non-compression stroke)

That's really odd. It is exactly 180 out. Is your chain still tight?

To get to TDC on the compression stroke, turn the engine over in the normal direction of rotation. Watch the intake valves open and close and then continue turning until the flywheel marks are lined up. All valve lobes should be roughly facing away from the valves. It looks like you are at TDC on the exhaust stroke in these pics, or something is very wrong.

Do a search for YZ manual in TT. You can download a pdf service manual. You're gonna need it.

That's really odd. It is exactly 180 out.
So? Turn it around once.
To get to TDC on the compression stroke, turn the engine over in the normal direction of rotation. Watch the intake valves open and close and then continue turning until the flywheel marks are lined up. All valve lobes should be roughly facing away from the valves. It looks like you are at TDC on the exhaust stroke in these pics, or something is very wrong.
:naughty:

Its probably TDC on the exhaust stroke, I'm a 4-stroke newb so chances are good that it was the wrong one. I cant go verify now because I've pulled the head and cylinder since my last post. Looks like I might have some new problems....

Look at the blueing of the crank

IMG_5661.jpg

and scoring on the cylinder wall

IMG_5665.jpg

Here's some pics of the combustion chamber, piston and another one of the cylinder wall. Please have a look and advise what the hell I've got on my hands here

http://img.photobucket.com/albums/v420/BrandonV/426/IMG_5663.jpg

http://img.photobucket.com/albums/v420/BrandonV/426/IMG_5662.jpg

http://img.photobucket.com/albums/v420/BrandonV/426/IMG_5666.jpg

Thanks for all the help everyone! I will learn this damn motor if its the last thing I do.

you definitely need a new jug. take a pick of the crank gear when you get a chance.

I don't think this has anything to do with your cam timing. Since you have the head off, tip it up and pour a bit of gas in the intake port and see if any runs out past the valves. Then turn it over and pour gas into the exhaust port and see if it leaks past the valves. I'll bet the valves are not bent. My crank is a bit blue but not quite as much as yours. The marks in the cylinder could explain the noise you heard.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now