2008 Yz 450 having issues

I am new to the forum, but have been reading a lot of threads over the last year that have really helped me out. That is why I am here.
I have a 2008 Yz 450 that will start, run for a second, and then die like you just hit the kill switch. Then, it is phisically hard to kick start, like it is seized up. But after you try to kick it a couple of times, it frees up and starts again. Then it dies and does the same thing again.

I have checked everything I know that I think would cause this.

It is getting fuel and good spark
Valves are in spec
Cams are timed correctly
Carb is cleaned
Cylinder is not filling up w fuel
Piston, rings, and cylinder are visually fine and mic within spec.
I have it torn down to the lower end now, the rod has no excessive play in it(which I thought might be making it hang up, due to bad bearing)
I am just looking for suggestions before I tear the lower end apart.
Also, this is a low hour bike with the original tires still on it.
Thanks in advance.

Edited by Treeyz450

If the bike is stalled, it will almost always stop against the compression stroke.  If it stops quickly enough, it will get up against pressure before the auto decompression mechanism can extend, you get the full compression stroke instead of the shortened one you're used to:




If the hard to kick situation comes up again, try putting the bike in gear, clutch lever out, and pulling it backward about a foot.  If the kick feel returns to normal, that was it; the engine just stopped quick.


If that kind of thing is happening while the bike is just idling, then it could be that your idle speed is just too low.  You might also want to try draining the float bowl to be sure it's not water in the bowl. 


If it isn't that simple, there are lots of things it could be.

It won't even run long enough to let it warm up to set the idle..........the float bowl is full of fresh fuel. I can take a drill and turn the rotor nut while letting the cam chain ride on a screwdriver and get it going pretty fast without hanging up or catching on anything. Would you tear the lower end down, or put the topend back together w new gaskets and see what it does?


Take your wash plug out of the exhaust.

Take your wash plug out of the exhaust.

That was it!!!!!!! Thanks a lot bud!

Seriously though, need some help

Ok, well I think I am going to order new gaskets and put the top end back together. Double check the timing, valves, and carb, then try to start it and see what happens.

If it were me I would be most suspicious of the carb. Remove and clean all jets set the float height inspect the rubber tip on the float valve. Next I would look at air, check the airbox and the intake carb boot for cracks. It was starting and running so that takes the odds of major internal engine issues down some. Also remove the header, midpipe, and silencer and check for any obstructions.

Edited by DirtyMan

I have the engine out of the bike, torn down to the cases. I am going to wait until this weekend for better temps here in Missouri, then take my time, inspect and install every piece slowly and by the manual, go through the carb, make sure everything is right. Then make sure it has fresh gas, and a new plug in it.

Thanks for the input, I will let you know what happens.

Ok, got it all back together, it starts and runs and idles for a few, but then it starts to stutter and if you hit the throttle it will rev fine and then idle again. It did the stall and physically hard to start once. I have the fuel screw 1-1/2 turns out and kept trying to bump the idle up, but it seems to do no good. I think it is a carbuerated issue but not sure.........any ideas?

Are your floats set at the correct height. What is your current jetting?

Stock jetting stock pipe, the float is set correctly, the more I messed w it in the garage, the better it got. It idles high for a few, then drops to where it should be. When you hit the gas it revs fine, but comes back down to a little high of an idle for 30 sec or so, then drops back down to norm. It is def carb related. It doesn't seem to stall now at all. I am going to check all the jets again, then take it out and mess w the idle and fuel screw and see what happens. May have to need w the jetting.


Well, got it running right and took it for a little ride( running like a champ ) and about halfway through the ride(a mile or less), it locked up in 3rd gear and came to a skid stop on pavement. Rolled it backwards in gear and was able to kick it again, but now it feels like there is more resistance than before. So, it's back on the lift and coming apart again..................F$&K!!!!!!

Edited by Treeyz450


Good call grey, Rod bearing seized up

Edited by Treeyz450

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