2007 YZ450F jumped timing

My bike was giving me a lot of trouble trying to start it today, it finally started and when I shut it down and tried to restart it, I couldnt get it to start back up. I pulled the valve cover off to check valve clearences and noticed the punch marks had 14 pins in between them. I lined everything back up, the exhaust cam had jumped 1 tooth. I put it all back together and it fired up first kick. Shut it down a couple times and it restarted first kick every time. Im going to replace the cam chain and was wondering if I should replace anything else while its apart?

cam chain tensoner

im lookin to do this on my 426, are there any diagrams that show exactly how to set timing?

maybe check the timing chain guide. maybe well worn by now.

Typically, the cause is in the chain itself. Most of the '06 and later models I've seen that have skipped time had a kinked chain with binding links. The tensioner doesn't have enough spring force to push the kinks out on the back side run, leaving the chain with less real tension than it should have. As soon as the right conditions occur, the chain slips.

Typically, the cause is in the chain itself. Most of the '06 and later models I've seen that have skipped time had a kinked chain with binding links. The tensioner doesn't have enough spring force to push the kinks out on the back side run, leaving the chain with less real tension than it should have. As soon as the right conditions occur, the chain slips.

Thanks Gray thats exactly what the problem was the chain was pretty kinked up. I put a new chain on this morning and rode all day without a flaw. Starts on first or second kick again.:busted:

Just had my timing chain skip today. I was riding a MX track. I wasn't wide open, on the limiter, or lugging the motor and all of a sudden it started to make a hellacious noise. It took about 5 seconds for me to get off the track and shut it down. It started back up with the same noise, so I pushed it back to the truck and there I wasn't able to restart it. I could kick it through a full cycle but just barely, as it would stick very hard at TDC.

Pulled the valve cover and the timing plugs and both cam shafts were lined up together, but they were 3 or 4 teeth off from the crankshaft. We realigned everything right there in the parking lot and it started right up.

I picked up a new cam chain on the way home and I just got done installing it. The old cam chain was maybe a little bit stickier than the new one, but not much different. The bike seems to run great now, even though I'm pretty sure there was some piston to valve contact due to the "clacking" noise when it first skipped and the sticking kickstarter.

I'm racing a harescramble with it tomorrow. I'll let you know how it holds up.:busted:

I too had the same problem exept i wasnt so lucky. I was ridding hard up in the rpms and my piston bent my exhaust valves, breaking one of them into the cylinder. Big boom. i am almost through rebuilding my motor with a new head valves springs piston timing chain, the works, i put a new timing chain in it,as i suspected that to be the problem. should i replace the tensioner as well or do those not typicly wear out. It is an 07 yz250f with about 100 hours on it

Ya, I dropped a valve in the race on Sunday. Replacing the head, valves, and piston. I'll also be replacing the cam chain and tensioner just to make sure.

Same problem on the 06. Bike was idleing and stalled out. Turned out to be the chain. Tensioner was good, but replaced it anyway along with the chain.

I know its a old thread. Just want to share my findings on my 08 yz450f with low hour.

I bent valves not to long ago. Waiting on parts and then on the machine shop to reassemble, The spring inside my cam chain tensioner collapsed . The chain looks ok. Havent compared it to the new one yet . after it died on me and checked timing ,I pulled the tensioner out and found it almost retracted, the spring was not working at all, no spring back tension after I would recoil the inner screw. It was acting almost as a mechanical cct.

Im a profesional mechanic for Vw , im not new to 4 strokes . I want to see a detailed picture of how the cam chain tensioner works inside. How it is design.

The only thing that is fool proof is a manual/mechanical cct. Grayracer can you help me find a overexploded view of the inner workings of a OEM cct?

Edited by Gus801

Manual tensioners aren't foolproof at all. They can be adjusted wrong, come loose, or be forgotten about, and are unable to compensate for changes that occur during a ride session. The only time they should be used is when the engine is equipped with very aggressive cams and heavy valve springs.

If you're a professional mechanic, you'll have no difficulty in taking apart your old tensioner and analyzing its function. It's a magnificently simple device, and not prone to failure at all under normal circumstances. The screw that you turn at the back end is a worm gear driven by the spring, which is anchored at the other end in the outer case. The plunger is keyed to the case so it can't rotate, and is internally "threaded" to match the worm gear. As designed, the spring puts relatively little tension on the chain, but will immediately extend the plunger if the slightest slack appears on the unloaded side of the chain. The plunger cannot be pushed back any more than you can push a bolt into a threaded hole (although I know that as a pro wrench, you've probably seen more than one person try that), just one of the fundamentals of worm and pinion gears.

The only three things that will reliably cause the tensioner to malfunction are contamination by debris, jamming it, the spring breaking (never seen this), or, most commonly, a stiffened cam chain. In the latter case, the stiff chain will run kinks down the slack side of the chain, holding the tensioner back. When the engine is revved hard enough to pull these kinks out all the way straight on the taut side, that creates a dangerous lack of tension over the cams. Also if the engine kicks back or bounces back off the compression stroke while starting or coming to a stop, this makes the normally slack side the driving side of the chain run, and as that load pulls the kinks out of the chain, uncorrected slack appears on the front side, and the engine jumps time.

The only thing like a flaw the design has is that the tensioner can't forcibly push slack out of the chain, but again, if the chain is healthy, that's not an issue.

I know this thread is old but... I recently bought a clean 07 YZ450. Second time out with it and an hour into my ride, I stalled it and couldn't kick it over. Got it home and found the exhaust cam jumped about 40 Deg. Getting ready to order a new cam chain, tensioner, and guide, but a buddy who has torn down several 99-04 YZ's said he's never been able to set timing with the cams bolted down, which I was able to do relatively easy. Is it a characteristic of the newer motors that the tensioner takes up that much more slack, or is there something more than meets the eye?

If you can get the chain over both cams with them bolted down, I would guess it is definitely time for a new one, and that close inspection of the chain guides is in order.

That's what I figured. Hoping the rear guide ok, but there were 3 small chunks of plastic stuck to the tensioner when I pulled it out, so that's probably a lost hope. Thanks for the reply...

New cam chain was roughly 1/4" shorter than the old one. Didn't think they could stretch that much!

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now