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'07 WR450F decompression issue

By sledhead999,
in WR400F/426F/450F

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sledhead999

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Hey guys,

Just recently did a top end job on my WR450. When I put it away for winter, it was running fine no issue. It had been awhile since it had been opened up to check the valves. I figured it was time to put piston, rings, and timing chain in it.

 

After I got it all back together it wouldn't start. No compression..... Zero. So I checked the valve clearance. The intake valve on the exhaust size had no gap. I take it apart clean it, put it all back together, didn't change ANYTHING, now it has .005" gap. Perfect, right in the middle tolerance! Retest compression 120lbs with the head cover off. Great i'm thinking, except now something isn't right with the decompression system. It will stall the starter with a brand new battery. Try and kickstart it with your foot and it'll about break it. Had to drag start it to get it running. Idles great no issues. Engine rolls over freely with no bindage. Rode it two miles, no issues. Tried to restart it with E-start hot, still stalls the starter. I played with the cam on the decompression system. It seems to return to home position with the spring and no issues. It even moves the little "button" that pushes on the valve bucket.

 

Any ideas of what I might be missing???

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grayracer513

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He said in his post that it doesn't stick:

 

 

It even moves the little "button" that pushes on the valve bucket.

 

A couple of things come to mind.  The first is that your lack of clearance/compression was originally brought about by a tilted lash pad (shim) on the valve in question, and you corrected the problem without realizing it.

 

Regarding the compression, or lack of decompression, I think we can rule out an incorrect cam (some aftermarket cams or those originally intended for YZ's don't interact well with the WR e-start) since you had it running OK before.  So that leaves incorrect timing (one tooth advanced would double the length of the compression stroke in crank degrees), or, this other little quirk the AD cams have.

 

The auto decomp system is set up to retract at around 700 RPM crank speed.  When single cylinder engines are shut down, or even more particularly, when they are stalled, they have a tendency to stop when the hit the compression stroke.  "Most of the time", at least when shut down normally, they'll bounce back from that a little bit, but not always, especially when stalling.  If the engine stops on compression that way and does so before the RPM's drop below 700, the decompression pin (the "button" you mentioned) will be trapped in the retracted position between the exhaust cam and the lifter, and the flyweight spring isn't strong enough to force the lifter off it's seat.

 

The good thing about this last possibility is that it's very simple to both determine whether that's the problem, and to work around it at the same time.  Next time you get stuck against big compression like that, put the bike in gear, engine off, let the clutch out, and pull it backward just a little to back the engine away from the compression stroke.  If the pin was trapped, it will now be released and operate normally.

 

The pair of cams in the picture is not yours, is that correct?

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sledhead999

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He said in his post that it doesn't stick:

 

 

A couple of things come to mind.  The first is that your lack of clearance/compression was originally brought about by a tilted lash pad (shim) on the valve in question, and you corrected the problem without realizing it.

 

Regarding the compression, or lack of decompression, I think we can rule out an incorrect cam (some aftermarket cams or those originally intended for YZ's don't interact well with the WR e-start) since you had it running OK before.  So that leaves incorrect timing (one tooth advanced would double the length of the compression stroke in crank degrees), or, this other little quirk the AD cams have.

 

The auto decomp system is set up to retract at around 700 RPM crank speed.  When single cylinder engines are shut down, or even more particularly, when they are stalled, they have a tendency to stop when the hit the compression stroke.  "Most of the time", at least when shut down normally, they'll bounce back from that a little bit, but not always, especially when stalling.  If the engine stops on compression that way and does so before the RPM's drop below 700, the decompression pin (the "button" you mentioned) will be trapped in the retracted position between the exhaust cam and the lifter, and the flyweight spring isn't strong enough to force the lifter off it's seat.

 

The good thing about this last possibility is that it's very simple to both determine whether that's the problem, and to work around it at the same time.  Next time you get stuck against big compression like that, put the bike in gear, engine off, let the clutch out, and pull it backward just a little to back the engine away from the compression stroke.  If the pin was trapped, it will now be released and operate normally.

 

The pair of cams in the picture is not yours, is that correct?

 

Yes, your correct, not my cams. I simply Googled WR450 decompression pin to make sure we were using the same terminology, etc.

 

You could be onto something here. I replaced the timing chain simply because I felt it was "time" and had been in there a while. Comparing lengths to the old chain, I'll bet it didn't even stretch .125". Anyway at true TDC both of my indicator dots sit slightly below the horizontal head surface. But if I jump up a tooth (clock wise), it would be quite above the head surface. To make matters even more confusing the "illustration" in the manual shows a different clocking to the decomp cam that what mine looks like. The "flat" part of the decomp cam is more horizontal than angled in the manual. Where its not a photo its hard to tell if its correct or not...

 

I take a pic of mine and let you guys decide!

Cam_Timing.JPG

Edited by sledhead999
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Grand Master

grayracer513

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The same illustration of the cam sprockets has been used in all YZF manuals for a few years, and isn't correct in every detail for every bike.  For example, it shows an incorrect pin count between 12:00 o'clock marks for late model YZ450's.  But that pin count isn't how cams are timed, and the focus of the picture is to show the correct orientation of the timing marks.

 

The marks themselves will very rarely ever line up perfectly, and what's necessary is to visualize whether moving the cam an entire tooth either way will make it closer to aligning or that much worse, as you did.

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sledhead999

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Ok, I'm gonna ask a newbie question. When the motor is rolled over CCW to align the TDC marks, there is the first vertical line on the flywheel that's aligned with the case. A second vertical line is also present with a arced horizontal line connecting the two vertical lines. 

 

Is this showing the dwell time at TDC?

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grayracer513

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No, neither of those is TDC.   There are 3, not 2, radial marks on the flywheel; the first two that appear, rolling CCW, or forward, are connected by a horizontal, as you said.  These two are the normal range for idle spark timing, and can be used to check that with a strobe (it's messy, BTW).  The third one is TDC.  The array looks a lot like "HI".

 

An aside: There is essentially no true "dwell" time at TDC.  If you use a dial indicator on the top of the piston, you can see that.  There is a certain amount of time spent within .020" (or any given distance) of TDC in terms of crankshaft degrees, but virtually none right at it.  The dwell is a function of the ratio of the rod length to the stroke. 

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sledhead999

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Hmm, now I'm gonna have to check the timing again. I would have been setting it at the first vertical (I) part of HI. That could explain why both of my dots were slightly below the head surface.

 

Thanks for the feedback!!

 

This makes me wanna build a thread adapter for my dial indicator for peace of mind...

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grayracer513

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You don't need a dial to time the cams.  A screwdriver or any other such probe is accurate enough for the job because you can only either be right, or 22.5 degrees off one way or the other.  It's not trying for 5 degree accuracy or anything.

 

But, if you used the first radial mark, running left to right, and then had the exhaust cam timing mark below the edge of the head, then I think that accounts for the problem.  If you turn the engine forward to the last "I", that would put the mark a full tooth below, meaning that it is, among other things, letting the exhaust valve close too soon, which runs up the cranking compression.  Check it out.

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sledhead999

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You don't need a dial to time the cams.  A screwdriver or any other such probe is accurate enough for the job because you can only either be right, or 22.5 degrees off one way or the other.  It's not trying for 5 degree accuracy or anything.

 

But, if you used the first radial mark, running left to right, and then had the exhaust cam timing mark below the edge of the head, then I think that accounts for the problem.  If you turn the engine forward to the last "I", that would put the mark a full tooth below, meaning that it is, among other things, letting the exhaust valve close too soon, which runs up the cranking compression.  Check it out.

 

I think your spot on this one. I haven't opened it up yet, but I think your right on the mark! Thx!!

 

If the spark plug is in the engine, and I have a T-handle wrench on the crank, I can get to within 2 degrees of TDC just by feel.  So can you.

 

I'm a machinist by trade, the the dial indicator comment was mostly coming from there...lol

 

Now that I think about it, I don't think I could align the long travel indicator into the spark plug hole with the frame in the way anyway.....oh well its all good ?

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