06 YZ450F won't kick over but not seized???

[SchmidtC194]

I was riding my 450 for about an hour. We stopped to take a break for 10-20 mins. When I got on it to ride again I tried roll starting like I sometimes do. It fired but didn't start. When I got to the bottom of the hill I tried kick starting it and it wouldn't kick over. It was like trying to start an older 450 with out the decompression release. After standing on the kick starter it would gradually turn the engine. It never fired back up though. got it home took the plug out and it kicked over fine. So I know the motor isn't locked up. I am thinking valves, or automatic decompress. Anybody ever heard of this????????

[southerlin55]

You may have over heated your bike my 80 used to do that all the time..

[SchmidtC194]

what happend to it when it overheated, and how did you fix it???

[grayracer513]

Occasionally, the engine can come to an abrupt stop, like when it's stalled in gear, just as it hits the compression stroke. In this case, the cams may not have ever been turning slowly enough to allow the auto decompression flyweight to drop back and extend the decompression pin. When this happens, the pin is held back by the valve lifter, and the flyweight spring is not strong enough to extend it in opposition to the valve spring. To get past this, you either push it through to the next compression stroke, where it will work normally, or put the back in gear and back it up off of compression, which allows the AD mechanism to reset to its normal cranking speed mode.

If, however, you've done either of those things, and the engine still acts as if it has no compression release, one of two things may be wrong:

> The AD mechanism is jammed. (this is actually quite rare)

> The cam timing has slipped and is retarded. ( This causes the AD system to lift the exhaust valve too late)

Hopefully you just fouled a plug and got the engine stopped in an odd spot.

[SchmidtC194]

Yeah I have tried all those things... I've been talking to some buddies and we are to the conclusion that I slipped a few teeth on my cam leaving the exausht valves closed. Cause it happend when I was roll starting down a hill. Maybe put to much of a strain on timing chain causing it to slip. Hopefully easy fix....

[grayracer513]

Roll starting puts no unusual strain on the cam chain or any other engine component. None.

[stroker101]

my 06' yz450f does this on occassion. what i do is just keep pressure on the kick starter untill the piston passes tdc on the compression stroke.

good tip grayracer513 on putting the tranny in gear and rocking the engine. i'll try that next time "it" happens

[PK]

If you pulled the plug and kicked it through, it should have reset the compression release. Since it doesn't sound like it did, you should pull the valve cover and take a look at the compression release mechanism and you can check your valve timing in 5 minutes while your valve cover is off. You'll have an idea of what your dealing with after that.

[SchmidtC194]

Thanks I will try that...

[____Kurt____]

that must be a scary situation, i hate it when something happens and you out riding.

hopefully the 07's don't do this and if they do not as much

[live4speed]

this may sound stupid and obvious...but did you remember to pull the hot-start? is the engine still refusing to fire?

[toombstone]

what ever happened? my 03 just did this and I really don't want to pull the head to find out what it is.

[youngr24]

It is your cam chain. same thing happened to me. Your cam chain has jumped time so you decompresion in not in the right spot so you can't kick it though but once it falls through it feels like it will start but does the same thing again. Replace the cam chain and reset everthing to top dead center and the bike will fire right up. but replace the cam chain it will jump again trust me.

RY

[ridder112]

my 2007 yz450f does that i with i was riding it and it stalled and i tried to kick it over and it would just not move so i was like f%#k its seized but then i held my foot there with all my body weight on it and it dropped down like normal but stopped dead right before the foot peg and thats all i get idk whats wrong HELPPPP!

[grayracer513]

Sounds like cam timing. Check it. If you find it off, replace the chain AND the tensioner.

[ridder112]

also ill put it in gear and hold the clutch and roll it back wards it stops dead

[Yamaryder29]

Occasionally, the engine can come to an abrupt stop, like when it's stalled in gear, just as it hits the compression stroke. In this case, the cams may not have ever been turning slowly enough to allow the auto decompression flyweight to drop back and extend the decompression pin. When this happens, the pin is held back by the valve lifter, and the flyweight spring is not strong enough to extend it in opposition to the valve spring. To get past this, you either push it through to the next compression stroke, where it will work normally, or put the back in gear and back it up off of compression, which allows the AD mechanism to reset to its normal cranking speed mode.

This is interesting. I had it happen a few times when i had my yz426 and its happened about 2 or 3 times with my 08 yz450f. With both bikes and was always while stalling in gear and the first couple times I though I seized my bike up, but this makes complete sense. Thanks Gray

[grayracer513]

This is interesting. I had it happen a few times when i had my yz426 and its happened about 2 or 3 times with my 08 yz450f. With both bikes and was always while stalling in gear and the first couple times I though I seized my bike up, but this makes complete sense. Thanks Gray

It only makes sense on a 426 if someone added automatic decompression to it.

[Yamaryder29]

It only makes sense on a 426 if someone added automatic decompression to it.

I forgot to mention the 426 had a auto decompression hotcam.

[grayracer513]

I forgot to mention the 426 had a auto decompression hotcam.

Figured as much, but just wanted to clear that up.