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Author Topic: YZ 450 starting/decompression problem  (Read 5567 times)
Alex641
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« on: April 12, 2012, 06:48:57 PM »

Rode a good amount today on my 08 yz450, then went to start it for one more ride and this happened....kicked through a few times...no fire,no sweat I pumped the throttle a couple times,hot start then choke still nothing. Then it got hard to kick through like all of a sudden the compression raised. Can kick just past TDC but still hard to kick through. Any ideas like decompression malfunction or jumped time somehow?Thanks
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flipside
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« Reply #1 on: April 12, 2012, 09:59:20 PM »

It sounds like it jumped time to me. I haven't seen to many problems on the decompression.
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nickvalve
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« Reply #2 on: April 13, 2012, 08:20:24 AM »

Good possibility Flip , also , I suggest you pull the oil  filter and look for a disappointment , especially if you feel a drag on the kickstarter all through the stroke. She just might have eaten a crank for lunch.
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Alex641
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« Reply #3 on: April 13, 2012, 11:55:15 AM »

I hoped it would be flipside and Nick,thanks for the responses.Pulled the valve cover and checked decompression...seems to be working. Thought I'll check the valves but the punch marks don't seem to line up opposite of each other with the head @ TDC. Jumped time maybe? The cam chain doesn't seem loose. BTW I've never attempted a valve clearance check before...seems pretty straight forward according to the manual. Also Nick it cranks through by hand freely till TDC then it gets hard even without the plug in. Is that normal? I hope these are valid questions guys. Top end has 71 hours w/every 5 hour oil change,crank has 171 hrs. Thanks! I'll check back later.
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flipside
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« Reply #4 on: April 13, 2012, 12:39:21 PM »

I've seen the cam chain tensioner fail on occasion. Set the bike to TDC using the ignition inspection cover. Remove the cam chain tensioner. Engage it and try to push the plunger in.(push hard and wiggle it)

If it goes in replace it and the gasket as well as the cam chain. If you have a valve out of spec then the piston may have kissed it. Time for a tear down. At 200 hour the cam chain gear on the crank is probably about done. This gear/sprocket is not replaceable.
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Alex641
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« Reply #5 on: April 13, 2012, 04:21:28 PM »

I'll try that flipside. Shoot even if it is fixed I won't have confidence in it....time for a teardown I guess.BTW I put her back together and tried to kick her over..the compression is so great that I had to put a moto boot on. That's not normal because regular shoes have always been fine in the past.Weird! Anyway thanks a lot for your help and I'll let you know the outcome.
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langhammx
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« Reply #6 on: April 14, 2012, 12:48:25 AM »

I'll try that flipside. Shoot even if it is fixed I won't have confidence in it....time for a teardown I guess.BTW I put her back together and tried to kick her over..the compression is so great that I had to put a moto boot on. That's not normal because regular shoes have always been fine in the past.Weird! Anyway thanks a lot for your help and I'll let you know the outcome.
Is it possible that the gear spun on the canshaft ? I have seen this happen before. What cams are in the bike ?
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nickvalve
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« Reply #7 on: April 14, 2012, 07:25:42 AM »

Crank sounds fine. Jumped time with the marks off. Yeah , I have seen gears move on cams [ usually aftermarket] . Pull the ignition cover and flywheel and look at the crank cam chain gear, they have a very small tooth count/ diameter , and they will wear quickly. If that gear is worn you should replace it if you put in a new chain. Also pop the intake cam cap off and look for any siezures on the right cam bearing surface. They don,t jump time for no reason.
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flipside
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« Reply #8 on: April 14, 2012, 12:03:37 PM »

I'll try that flipside. Shoot even if it is fixed I won't have confidence in it....time for a teardown I guess.BTW I put her back together and tried to kick her over..the compression is so great that I had to put a moto boot on. That's not normal because regular shoes have always been fine in the past.Weird! Anyway thanks a lot for your help and I'll let you know the outcome.
Is it possible that the gear spun on the canshaft ? I have seen this happen before. What cams are in the bike ?

Yes but it's usually the intake cam and that wouldn't mess with the compression release.
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langhammx
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« Reply #9 on: April 14, 2012, 02:04:32 PM »

I'll try that flipside. Shoot even if it is fixed I won't have confidence in it....time for a teardown I guess.BTW I put her back together and tried to kick her over..the compression is so great that I had to put a moto boot on. That's not normal because regular shoes have always been fine in the past.Weird! Anyway thanks a lot for your help and I'll let you know the outcome.
Is it possible that the gear spun on the canshaft ? I have seen this happen before. What cams are in the bike ?

Yes but it's usually the intake cam and that wouldn't mess with the compression release.
Copy that... That's why you fix bikes and I run cable !  Razz
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Alex641
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« Reply #10 on: April 14, 2012, 04:31:53 PM »

Well it defiintely jumped time because i can't get the punch marks to line up opposite each other with the case. Don't know yet if the chain slipped on the gear below or the gear on the cam slipped and yes they are aftermarket WEB Cams I believe. This would explain the heavy compression because the decompression is out of sync with TDC. Also tensioner seems fine...but maybe not strong enough or chain too stretched?I'll check back later. Thanks.
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langhammx
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« Reply #11 on: April 15, 2012, 08:03:07 PM »

Well it defiintely jumped time because i can't get the punch marks to line up opposite each other with the case. Don't know yet if the chain slipped on the gear below or the gear on the cam slipped and yes they are aftermarket WEB Cams I believe. This would explain the heavy compression because the decompression is out of sync with TDC. Also tensioner seems fine...but maybe not strong enough or chain too stretched?I'll check back later. Thanks.
I would check to see that the gear did NOT spin on the cams. Not that I don't believe FLIPSIDE, but I've came across this before.
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flipside
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« Reply #12 on: April 15, 2012, 08:12:54 PM »

Well it defiintely jumped time because i can't get the punch marks to line up opposite each other with the case. Don't know yet if the chain slipped on the gear below or the gear on the cam slipped and yes they are aftermarket WEB Cams I believe. This would explain the heavy compression because the decompression is out of sync with TDC. Also tensioner seems fine...but maybe not strong enough or chain too stretched?I'll check back later. Thanks.
I would check to see that the gear did NOT spin on the cams. Not that I don't believe FLIPSIDE, but I've came across this before.

It could happen...but a trained eye can see the position of the cam lobes when the lowerend is TDC. The dots will be close but just off. Kinda like they used to be on the 426's when the timing was good. Just off a little bit. Gave the DIYer's fits on those years.
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nickvalve
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« Reply #13 on: April 16, 2012, 02:24:15 PM »

Well it defiintely jumped time because i can't get the punch marks to line up opposite each other with the case. Don't know yet if the chain slipped on the gear below or the gear on the cam slipped and yes they are aftermarket WEB Cams I believe. This would explain the heavy compression because the decompression is out of sync with TDC. Also tensioner seems fine...but maybe not strong enough or chain too stretched?I'll check back later. Thanks.
I would check to see that the gear did NOT spin on the cams. Not that I don't believe FLIPSIDE, but I've came across this before.

It could happen...but a trained eye can see the position of the cam lobes when the lowerend is TDC. The dots will be close but just off. Kinda like they used to be on the 426's when the timing was good. Just off a little bit. Gave the DIYer's fits on those years.
Yes , I agree , those early yz's had stretched cam chains that were off ' a little bit ' . Very common .
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Alex641
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« Reply #14 on: April 16, 2012, 03:27:59 PM »

We'll know soon enough...I'll keep you guys posted.
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