Bent Decomp exhaust valve.. again


13 replies to this topic
  • whittim

Posted May 19, 2011 - 01:34 PM

#1

Here a few weeks ago i was out on my 2001 yz 426 when i lost compression coming across a field i figured i must of pulled the decomp lever. Well today after a rebuild and replacing the exhaust valve and guide the same thing happened... i used a weisco titanium valve and kibble white guide. made about one lap around the yard a wam hit the same valve and i know i didn't touch the decomp what the hell is going on??

  • grayracer513

Posted May 19, 2011 - 01:39 PM

#2

More info:

What piston?

What cams?

Valve springs?

Is the return spring engaged on the decompression shaft correctly?

  • whittim

Posted May 19, 2011 - 02:48 PM

#3

all stock and the decomp return works great

  • yz450fcranker

Posted May 19, 2011 - 03:04 PM

#4

time to go try oem

  • grayracer513

Posted May 19, 2011 - 04:00 PM

#5

all stock and the decomp return works great

When were the springs last changed?

Something is bouncing the exhaust valve off the piston, and the compression release won't realistically do that. After all, you can kick it all the way through with the release pulled, right?

The only things that come to mind are cam timing wrong, valve float due to weak springs, and (don't laugh) the piston installed backward.

  • whittim

Posted May 19, 2011 - 04:04 PM

#6

Well i figure the valve spring is wore out and is hanging the valve open for an extended period of time, anyone know if a stock ss valve replacement would be a good option? I want to do all of them before i have this problem again i just put out 250$ for this valve, guide and everything and now its bent again...

  • whittim

Posted May 19, 2011 - 04:06 PM

#7

When were the springs last changed?

Something is bouncing the exhaust valve off the piston, and the compression release won't realistically do that. After all, you can kick it all the way through with the release pulled, right?

The only things that come to mind are cam timing wrong, valve float due to weak springs, and (don't laugh) the piston installed backward.


Well the piston installed backwards has come to mind.. i had a guy who races the MX's and builds them put it together and that has been sitting at the back of my head so ill have to check it out.

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  • grayracer513

Posted May 19, 2011 - 06:18 PM

#8

Well i figure the valve spring is wore out and is hanging the valve open for an extended period of time, anyone know if a stock ss valve replacement would be a good option? I want to do all of them before i have this problem again i just put out 250$ for this valve, guide and everything and now its bent again...

If it is a titanium valve, the OEM springs for the titanium valved '01-'02 model should be adequate. If you are in doubt as to whether that's heavy enough, the OEM 2000 model springs are heavier. A query to Wiseco about springs might be useful, but don't be surprised when they try to sell you theirs.

  • whittim

Posted May 20, 2011 - 04:53 AM

#9

right on thanks, i plan on using the bike as a trail bike and i don't drive it extremely hard, i don't need the extra revs on top end so i think the ss valves will be the way to go thaks for your input!

  • Waylon43

Posted January 31, 2012 - 01:01 AM

#10

I know this is an older topic but it fits what I'm looking at. I have a '99 YZ400, Iv'e had it since '02, it has always started easy, ran great, no major problems at all with it when I rode it a lot. the last few years I've only rode it a couple times a year.

Early this year I went to go riding and the float was stuck in the carb, no big deal, fixed that and I couldn't get it started, so a guy pulled me with his 4-wheeler to try pull starting it. Afterwards I noticed it didn't have much compression, it has some but I can push the crank by hand. I had just put a new decompression lever on it so I thought maybe it was holding it tight or something, but it isn't. now I'm down to thinking I bent the exhaust vavle while I was being pulled, I did use the decompression while being pulled a couple times.

Hate to say it, but I've never opened up this engine before.

What's the easiest way to tell if the valve is bent? I got the vavle cover loose before I came to work tonight, tomorrow I'm going ot take the motor mount out of the way so I can pull the cover the rest of the way off, will I be able to tell at that point or will I have to pull the head off?

Thanks for any help!

Waylon

  • grayracer513

Posted January 31, 2012 - 07:53 AM

#11

You can't bend the valve with the decomp lever. Think about it. If the lever can be pulled and the engine turned over with the kick starter, doesn't that tell you that the valve clears the piston?

What can happen if the lever is pulled while the engine is turning at a higher speed is that the lifter can become damaged, and may possibly cause the valve to be unable to close all the way.

To check for a bent valve, or one that is stuck, check your clearances. If one has more than the specified clearance, it's questionable. With the engine in the timing position (TDC on compression, lobes pointed up away from the lifters), tap the lifter gently a few times with a punch to bounce it open and shut. The trouble may be no more than a bit of carbon on a valve stem. If so, clearance will drop back to something tighter. If it's bent, the clearance won't change.

If that doesn't restore clearance to a normal range and compression doesn't improve, have a leak down test done to see where the air escapes.

  • Waylon43

Posted January 31, 2012 - 06:56 PM

#12

I think I have found my problem, at first I was using the 2 marks on the intake cam sprocket to get it at TDC. when it was lined up the lobes didn't look right to me, and nothing seemed like it matched up like it should. So I then removed the timing mark plug and lined the "I" up for TDC and put a little screwdriver in the spark plug hole to ensure it was at TDC.
These pics are when it is at true TDC.
Posted Image
Posted Image

I guess the intake slipped a few teeth?
I'm going to inspect the sprockets better when I get hom in the morning to see if they are worn, if they are and I replace them can I replace them with a newer style cam (auto decompression) without much trouble? Is that the only difference between non auto and auto decompression, the exhaust cam?

Thanks!
Waylon

  • RocketRoss57

Posted January 31, 2012 - 07:43 PM

#13

check this

http://www.thumperta...078#post3706078

  • grayracer513

Posted February 01, 2012 - 08:23 AM

#14

check this

http://www.thumperta...078#post3706078


Those instructions apply only if using a YZ450 cam in a 426. Doing it that way is no longer the most practical approach, since aftermarket auto-decompression cams are widely available, and OEM cam prices are significantly higher than they were. Use a cam made for a 426 and the timing is simplified, also. Goes together just like stock.

In any case where you nave a cam jump time, you need to check carefully the following three things:
  • Timing chain, for stiff, binding links or wear (replace it regardless of what you find)
  • The bearing surfaces in the head and cam caps (a partial seizure is often the cause)
  • The tensioner (anything loose, sticky, or suspicious about the way it works)
Cam sprockets are not replaceable without replacing the entire cam. Depending on what you find, the exhaust may be the only thing you need to buy to convert to AD.





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