Seized my motor today

[stnkbg1]

Long story short, I was riding my 99 YZ400 on the freeway for about 20 miles (it's a plated motard) and it started kinda hiccuping like it was out of gas. When I let off the throttle I got WAY more engine braking than normal, but it would still go if I twisted. I pulled off at the next available exit, and the bike stalled just after turning the corner. After checking the fuel and oil and not seeing any obvious leaks or smoke I tried restarting it. Fired right up, ran for a few seconds and STOPPED. Not died, stopped instantly. Tried to restart and could no longer kick it over.

After getting a ride to my shop I started taking the bike apart and found that it was very low on water. I'm pretty sure it overheated and seized the piston to the cylinder, though I wont be sure of the damage until I open it up. I'm pretty comfortable taking on the job of rebuilding it, but since this will be my first dirtbike engine work I'm lookin for any advise you guys can offer. Where should I look for parts? While I'm in there are there any good mods to do that wont add much cost? Can I swap in a 426 cylinder, or bore mine to 426 spec?

Biggest bummer is that this bike ran freakin tip top. It had TONS of power and flat out stomped on all my friends bikes. It used to be an MX race bike, and though it had no mods other than exhaust and jetting it was worn in just enough to spin hard. I had to replace the water pump seals recently and the inside of the motor is sparkly clean, like it's never been run. Oh well, guess that'll teach me to pay more attention to the little thumper than I do to my sportbike.

Dallas

[642MX]

I'm willing to bet that its not the top end. Most likely, its seized the big end bearing on the crank. Keep us updated.

[sirthumpalot]

Easiest way out is send the head and jug to a performance shop and let them freshen it up and send you a piston to match the resulting jug. But you need to figure out what seized and why. If you don't solve the why part then you'll have the same problem again. As stated above it seems the rod big end seizing is not unheard of. Check the timing chain lower sprocket while you have it apart.

[ThumperKid250F]

just wondering but were you holding it wide open on the road or just crusing like 5th gear half throttle?

take it apart and see what is going on inside there and post up some pics.

[stnkbg1]

just wondering but were you holding it wide open on the road or just crusing like 5th gear half throttle?

take it apart and see what is going on inside there and post up some pics.

Well I sure as hell wasn't wide open, cause this thing will hit the rev limiter in 5th pretty easily at about 95mph. I was probably around half throttle doin 70-75ish. I'm pretty sure the reason it seized is what I already mentioned, it had almost no water in the radiators. Maybe enough to fill the lower tanks and not much more. Oil is full clean and fresh, that's why I'm betting on the piston seizing to the cylinder. Unless there was another freak problem all the bearings, etc should have maintained oil pressure which would have kept them cool enough. We'll see when I pull it apart though. I was gonna pull the motor out this afternoon, but I rode my other bike a few hundred miles today so I'm tired.

Right now I'm pretty sure I'm gonna need at least a piston, rings and either a cylinder, or have the cylinder machined, plus any gaskets that are required to do the job. Any suggestions on where to look for this stuff? I'm clueless when it comes to parts suppliers for dirt bikes.

Dallas

[BC3]

Yamaha Dealer?

[642MX]

Yeah, a Yamaha dealer can probably get the parts.

[Birdy426]

TT Store for parts. Best prices around. If you breathe on the motor at all, upgrade the rod to a 426 rod and then go for a 426 barrel or a 444 big bore from Luke's Racing or Eric Gorr. The rod was a weak point on the 400s, and they made the small end bigger on the 426s.

[stnkbg1]

Now that's the kinda stuff I was lookin for. Obviously the dealer can get parts, but they are rarely the cheapest or best performance.

Just looked on lukes and they have this for $425 exchanged (assuming mine is good as a core)

http://www.lukesracing.com/luksr/yz400f.htm

What's the difference between the 2 compression ratios? I mean, obviously one is higher, but is it gonna make a lot more power? What's the stock CR? Also, since I ride it on the street, I'm not big on switching to race fuel, though I'm fine with putting in premium, which I always did anyways.

[grayracer513]

What's the stock CR? Also, since I ride it on the street, I'm not big on switching to race fuel, though I'm fine with putting in premium, ...

Stock is 12.5:1. Going up to 13.5 should be worth about 1hp or more, with a bigger boost to low and mid range than to the high end. Pump premium works in most cases.

[jayh300]

sorry to hear that Dallas....

I know at some point mine is gonna do the same thing and am sure not looking forward to it. Post up what you find when you get in to it.

jay

[Birdy426]

Stock is 12.5:1. Going up to 13.5 should be worth about 1hp or more, with a bigger boost to low and mid range than to the high end. Pump premium works in most cases.

I've got the 13.5:1 444 kit on my bike. In most cases I can get by on pump premium. Occasionally, I will get a bad tank from somewhere, and end up ruinning some octane boost or a mix of premium and race fuel until I can drain the shit-gas out. Pretty noticable difference all across the powerband with the higher comp., but as Gray said, more noticable on the bottom and mid...

[stnkbg1]

well you guys were right, the big end bearing is toast. The piston still looks pretty good, and the cylinder is only very lightly scored. I think I'll go ahead and replace em anyways though. I have the Yamaha service manual, and went through all the steps to remove everything in order to split the case. Balancer, oil pump, clutch, stator, etc are all off now, but I cant seem to get it apart still. I can separate the 2 halves about 3/8" but no more. I cant see what's hanging me up either. Anyone with experience have any suggestions. The book is really not very helpful. I took a bunch of pics and I'll post em up when I get some time.

Thanks.

Dallas

[stnkbg1]

Here's the pics from today.

[stnkbg1]

One last one.

Heres what the bike looked like originally

[642MX]

Take some pics of the motor as it sits ready to split. The cases come apart really easy on the YZF. My guess is there is something still in the motor not letting them split.

[grayracer513]

There is on sneaky crankcase bolt that people will overlook; the one under/behind the ignition area, just above where the return oil line bolts up. Be sure that's out. The bearings are held captive in the case, so the shafts have to slide through them.

[stnkbg1]

The motor looks pretty much like the 2nd to last pic, but I'll take a couple more pics today if I cant figure it out.

[grayracer513]

Here's what I see wrong with the situation as shown in the picture below:

> The transmission lube rail should be removed. That's the small item above and between the trans shafts, held in by a single bolt.

> The shift shaft needs to come out, and and the shift cam retainer plate assembly has to be removed unless you're going to try to get the whole trans to stay in the right case, which it won't want to do. It's much easier to back the shafts out of the right side, since the ends of both shafts and the cam are exposed on that side.

[642MX]

The motor looks pretty much like the 2nd to last pic, but I'll take a couple more pics today if I cant figure it out.

Grayracer is right about removing the shift shaft and retainer plate. Becareful when you remove the retainer plate. There a few very small parts that will fall out when you do.

The tranny lube rail doesn't have to removed, but it will make the job easier. I left the one in on my wifes bike, but later removed it when I got the cases to split.

Once you remove the shift shaft, the cases should split fairly easy. I used a rubber hammer to influence the 2 cases.

The crank also slides right out once the cases are split. Use a rubber hammer to help it if it doesn't cooperate.

The transmission will need some help coming out of the case too. The book says to use a hammer on the countershaft while holding the tranny shafts. You may need an extra set of hands to help hold the tranny shafts. Take a picture of the transmission before you remove it. When you reinstall it, you may find the photo helpful since the 3 forks, 2 shafts, and shift drum all have to be aligned & installed at one time (its a real pain, it took me 15 attempts to get it right).