Yz 400 Chain

[jay3and88]

Out the other day and the bike ran great for a few hours, 2/3rds of the way back to the truck I let off the throttle to go through a dip in the trail and when I got back on, it was bogging real bad for about 30 seconds then died.

I have come to the conclusion that the timming chain has probably skipped a tooth or two on the crank as the chain was a bit noisey to begin with, I have ordered the new chain and will take the bike apart this week to confirm that it no longer has proper timming.

Has this ever happened to anyone before and are there some other area's that I should be looking at while I make this repair. I have searched the site for the same problem and have gotten some info that has helped but more the better.

Thanks for any reply I get with this,

Jay

[642MX]

If it is out of time, you need to remove the mag cover and flywheel next. If it jumped time, chances are the lower gear is toast. If its missing teeth or has excessive wear, you'll need a crank half.

[jay3and88]

Thanks man, I will have a good look at that gear, really hope I don't have to change it but I guess I'll find out.

Thanks

[matt4x4]

Typically - jumping time is due to an overly stretched chain or bad cam chain tensioner, not the gears.

[grayracer513]

Another occasional likely cause is camshaft seizure. The lower gear can get badly worn by running a worn chain for too long though.

[642MX]

Typically - jumping time is due to an overly stretched chain or bad cam chain tensioner, not the gears.

If the chain is so bad that the tensioner is fully extended and then it jumps time.... 9 out of 10 bikes will need a crank half. I see them all day long. The cam gears are pretty much bullet proof, but the crank gear is the weak link on the YZF. ?

[grayracer513]

That could be true on older bikes, and of course we are talking about one here, but I have never seen a chain worn so long that the tensioner fully extends.

On the later bikes ('06+), especially those that jump time at low hours, it's usually a tensioner failure. Cam seizures brought on by incorrect cap torque are also very common.

The YZ250F we bought used had never had a timing chain replaced until I did it in late '04, and even that freakishly high revving beast had very little wear at the crank gear. I'm not saying it won't happen, it will. I just haven't found it to be that common.

[rufusz]

Cam seizures brought on by incorrect cap torque are also very common.

Are you sure it seizes because of over tightening of the cam bolts??? I don't believe over tightening could have anything to do with it, because the control surfaces for the cap being so large, that those M6 screws can't over tighten the cam. Hope you understand what I've meant. The torque values for the camshaft bolts are specified only for people who are used to strip bolts out of aluminum or are afraid of tightening it too much.

The sleeve bearings, formed by the head and cap, are converging to the perfect circle by tightening the screws. Simpler: When the screws are not tightened enough, those 2 parts form a shape like an egg, by tightening it they start to form a circle. You can also check it by hand: install a cam and cap, and don't tighten the bolts and rotate the cam by hand. now tighten the bolts and you will notice that the cam turns more easily.

[grayracer513]

Are you sure it seizes because of over tightening of the cam bolts??? I don't believe over tightening could have anything to do with it, ...

They can seize for other reasons, too, of course, but yes I am. Often, though, it has more to do with tightening them unevenly than simply over tightening. Also common is the mistake of pulling the caps down with the bolts instead of seating them manually before tightening them. In either case, the caps become distorted, and the bore goes out of round. The cap to shaft clearance is only 0.028 ~ 0.062 mm (0.0011 ~ 0.0024 in), so it doesn't take a great deal of distortion to cause trouble. It's also fairly telling that this frequently occurs only after someone has serviced the camshafts in some way.

Over tightening does two things: it pulls the cylinder head material upward at the the threads in the hole, distorting the flatness of the surface, and more importantly, compresses the cap material immediately around the bolt bores. This compression forces outward on the surrounding area in a barrel shape and contributes to cam bore distortion.

86 in/lb (7.2 ft/lb, or 10Nm) is given as the torque for the cap bolts. This is the standard torque for an M6x1.0 bolt, but the standard torque is for clean dry threads. The manual specifies oiled threads for the cap bolts, and even if one missed that point they normally are oily anyway. Oil on the threads increases the actual clamping force at the bolt by as much as 20-25%. I personally use 75 in/lb on all my YZF cam caps, and have for several years.

[jay3and88]

Thanks for all the info from you all, I know if I have a question I can serch this site and get what I need, or just ask.

Thanks again,

[rufusz]

They can seize for other reasons, too, ...

Yes that's more to it:

- pulling down the caps using the screws (sincerely, a person who does such a thing, should stop working on bikes)

- tightening them unevenly

Did you actually see an overtightened bolt that distorted the cylinder head instead of just pulling out the threads? And distorted it so much that the cap was not perfectly aligned?

[grayracer513]

Did you actually see an overtightened bolt that distorted the cylinder head instead of just pulling out the threads? And distorted it so much that the cap was not perfectly aligned?

It won't distort the cylinder head except by pulling the edges of the threaded hole up, which of course, means that the clamping surface is no longer flat.

It's the cap that gets bent, but the bore is in both parts, so it's distorted either way.

And yes, I have.