motor problems--98yzf

I have a 98 yzf and I was racing it when the motor instantly started ticking and lost about 20 percent or so power. I shut it down and looked it over. Oil was ok. I was out in the middle of the loop and had to get back, so i rode it back gently. When a friend of mine tore it down, the valve clearance and everything else checked out. He did notice that the piston had a few valve marks. The bikes timing marks were aligned properly though. I am wondering if the cam could have turned on the gear or is that possible? What else could be the cause. The bottom end is in great shape as well as the topend. Just though I would ask you guys. Thanks in advance.


Possible valve float? If you overrev, the spring pressure isnt strong enough to reseat the valve quickly enough and the piston returns back to the top of the stroke and knicks/damages the top. I don't know if the YZ 400 is prone to valve float or not, and it shouldn't rev that high anyway. If your valve train is o-kay, I don't see how you would get this situation and hear a ticking unless the spring failed and it(the valve) was slapping the piston. In that case youd hear alot of noise. Backfiring through the valves. Etc. Anybody else?

[This message has been edited by JPNTHUMP (edited 08-18-2000).]

Valve float is pretty extreme..

You'll hit the limiter before valves start floating.. though in a freak situation with the right harmonics etc its always a small possiblity... small being the word though! :)

You said the timing was ok.. that meaning the cams were lined up ok or the whole lot including the crank etc?

Possibly the timing chain jumping a tooth on the crank gear.



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

Discounts for ThumperTalk members.

How about your decompression lever? Could it possibly be stuck open slightly? I've heard before that if you hold the lever all of the way in the valve could hit the piston.

I'm with npuza, check out your compression release cable and make sure that when you turn the handlebars from side to side it doesn't tighten up too much. If you pull in the compression release when the engine is running it will make a clicking sound and is certainly not good for the engine.

First of all, thanks for the responses.

The first thing I did was to check the decomp lever and assembly. It's working correctly. My friend said all the timing related marks were lined up correctly. I asked about the t-chain and he said it was correct also.

The decomp lever opened when runiing the engine won't damage it.

If it would then holding it open and kicking over the bike to clear a flooded engine would damage it.



**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

Discounts for ThumperTalk members.

There is a huge difference between kicking a YZ over with the decompression lever pulled in and puliing it in at high RPM...

You should be checking the simple things, like spark plug, or even better, water build in the plug well.

I'm with derwud,

Running and kicking over- two different things.

I can say that running with the decomp lever in is not good. I had a spill one day and jammed a small stone in between the decomp lever and perch (punched a hole through the cover). Rode briefly after without noticing. Once the engine cooled completely, I had no compression. The small cam that presses on the bucket had rolled a burr on the edge of the bucket from contact. Thrashed the bore that the bucket rides in and stuck the valve open. Saved the head, but only after it was quoted to me at $700 USD.


[This message has been edited by Ken (edited 08-21-2000).]

I own a 99YZ400 that I boght new in Dec 98. I ride/race it like I stole it and want to know how long these motors hold together. I change the oil frequently. I spoke with Frank at Engines Only in Cupertino about freshening the motor he recomended replacing the intake valves and timing chain because the chain tends to stretch and the valves wear. At the tune of $750. The bike makes good power although with minimal low end. I run a ti header and E-series ex. with stock jetting/gearing. Is there anything I need to do to this motor before I resume racing Dist-36 Hare Scrambles Oct. 1st. Monty I really enjoied riding with you at Stonyford and look forward to our next ride hopefully in Georgetown. COME ON MOAB :)


You probably already know this, but you should check/adjust your valve clearance. It's easy. I've had my 426 since late January and have checked my clearance 4 times. I had to change some pads out the last time. The clearances got progessively tighter, in predictable increments, each time I checked them.

So I suspect your 99 MAY need some adjusting.

But I do, IMO, ride my bike pretty hard.


That is a frightening story about the comp. release, thanks for sharing it. I always knew it was bad but did not consider it THAT bad. :)

[This message has been edited by Hick (edited 08-22-2000).]

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