uh oh possibly a big problem

so i was at the track today making some major improvements on my turning abilities. when i went off a jump and in the air the bike revved out of control all by itself so i pulled in the clutch shut it off when i landed. i tried starting it again and it was kicking back. i got a few full kicks in but they sounded empty and didnt feel normal. no visible damage. someone at the track said it is my valves. i think it might also be leaking anti freeze but its coming from the case. if you need pictures i can take some. plz help.

Might as well get us some pictures on where the leak is. You say when you kick it over it has no compression, or does it feel like something is locked up in the motor?


no compression. its not locked up. the leak seems to be originating from the oil filter cover.

Can you pull the cylinder head cover off and check your valve clearances and/or see if one is stuck open? That would cause a loss of compression.


Revving out of control, no compression.

Pull your carb and check to see if your slide is still in one piece and not broken or cracked.

When mine broke it did exactly what you say. The reason for the lack of compression was a small piece of the slide was stuck between a valve and the seat, hence the lack of compression because it was being held open.

I would bet money on the slide plate being broken.

let us know.

well i took it apart some and the valves seem to look fine but the carb has me worried this is what i found


i was going to check the valve clearance and i was getting it to TDC and i found this. it looks like the cam notches are in the wrong place when the timing thing is in the right place. either way i tested the clearance when the cam said it was lined up and when the timing was lined up. both were way to tight(i think) how do i fix this? check timing and such. plz help i really need to get this ready and running by friday. oh and the red dots are the notches on the cam and the timing mark. plz plz help


Your carb slide plate is broken as I suggested before. If you are experiencing a lack of compression you probably have a piece of that plate stuck in one of your valve seats. You will need to remove the head to check for this. You will probably find small pieces on top of your cylinder as well. Hopefully it didn't make a mess of the cylinder, check the walls by turning the engine over while you have the head off. With any luck you will only be buying a gasket and slide plate. These are the carb parts you should need but double check. This came off of the North County Yamaha parts findr for a 99 YZ400.


4FN-14546-01-00 (replaces 4FN-14546-00-00) $70.51


4FN-14997-00-00 $6.28

There is lots of info on here regarding your timing and how to set it and checking/adjusting valves as well.

lots of info here http://motoman393.thumpertalk.com/

Regarding the timing question, it's timed correctly, nothing to fix. You should toss in a new timing chain while you have it apart, but here's the question to ask: If you moved either cam one tooth from where it is in the picture, would it look better, or worse? The truth is they never line up perfectly.

Good call on the throttle plate, coop. Nothing like experience, eh?

Alright, just something I want to point out in this thread, seems like you have the problem solved, but...where is you're case saver?! :thumbsup::applause:

It's pretty hard to believe that you have antifreeze coming out of your oil filter cover. I dont' think the stuck throttle is in any way related to you finding antifreeze leaking out. Your waterpump is close to your filter cover, is it coming from there?

That's the first slide I have seen broken, it probably scared the crap out of everyone it happens to...

i read up on how to take the head off and it scared the crap out of me. i dont think i would be able to put it back in good working condition. and i leave for college this sat and wont have time or tools to fix it there. so thanks for the help. any simpiler way to check the valves? or is it inevitible that i have to take off the head?

No, not until you either find that piece of throttle plate, or look to ensure that it's not in the ports or the combustion chamber.

Tip: take the exhaust off and see what shakes out. You may get lucky.

If you have no compression I will bet money that there is a small piece of throttle plate holding a valve open (yes, from experience I know this). The only way I can think of checking for that is by removing the head and removing the head is not a big deal. If you want to trust the old gasket (not recomended) you coould lift the head up an inch or so and have a look but I think it will be coming off eventually. The worst part of this job should be timing the cams and that isn't hard iether. And yes it would probably be a good idea to replace the timing chain while it is opened up.

Good luck

bad news no metal piece in there and nothing in the valves everything looks fine for how old it is. there is some damage from the metal piece on top of the piston. i can get pictures if you need them but i have no idea what could be the problem. so if you have any ideas please help.



Good call on the throttle plate, coop. Nothing like experience, eh?

Yeah, I was thinking the same thing, good old TT, a wealth of info at your fingertips! That problem was solved way too easily!

The intake valves appear to have hit the piston (see the bright half circles in the cut outs?). You should have them removed and checked for being bent. Don't just solvent check them. If they are slightly bent, that will make them flex a little each time they close, which will make them wear faster, and may cause them to snap them stems off (very bad).

so i got to looking at it and it looked like there was no room to bore the cylinder. so i got a ruler(i know not supposed to be measured that way) and it measured 94 mm and the standard is 92. so i was all &%$#@!? do you think its just my bad measurement technique or that its been bored all the way out.

From the pics I think I would rebuild the head or have it rebuilt by someone. A new piston and rings would be a good idea along with a timing chain. Have a shop measure the cylinder for you to be accurate.

I had a valve break off and it cost me a rod, piston, rings, cylinder and a complete head. Much cheaper to do before it breaks.

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