Valve Cover gasket keeps blowing

Hey guys, I have a very frustrating problem.

My valve cover gasket keeps blowing on my 07 wr450f.   it blew a few weeks ago so I replaced it and adjusted my valve clearance while I was in there. I put it all back together and it blew again after about 2 hours of riding.  This time when it blew, it blew hard. Both the half-moons came completely out and there was about a 5 foot cloud of grey smoke that shot out from the half-moons.   I was able to put the valve cover gasket back in over lunch.  I removed the breather hose and checked it for clogs, nothing.   Just as a test i left the breather hose disconnected to ensure it was not clogged.  2 hours later it blew. 

Im pretty frustrated.  This is my first bike ever and i don't know how to fix it. 


Do you guys have any suggestions?




More than likely the vent hose coming from the valve cover has been pinched off at the frame/subframe junction

Krannie,  thanks for the suggestion but the last time it blew I purposely left the vent hose disconnected to eliminate this possibility.  

any other suggestions?

You have another vent at the starter

Have you changed any gaskets recently?

You think the starter vent hose could be clogged?   I don't see how that would cause the valve cover gasket to blow. Im a noob to this stuff though so maybe theres something Im not aware of.  

For what its worth: my starter doesn't turn the engine over if it is at TDC.  could these be related?  

The only gasket I changed was the previous valve cover gasket.   Oil changes and oil filters.  


Again, thank you for any advice and insight you can provide. 

If the piston stops on the upstroke, but past where the auto decompressor has closed then the starter motor isn't man enough to turn the eninge over

Rock the bike backwards, in gear, to move the piston back down the bore and so activate the decompressor

This is not related to your issue, as this is a common 450 issue


You have a blockage, which his why you valve gasket blew out

Might be blocked in the metal pipe out of the valve cover, the one which you pulled the pipe off

With engine running you should get pulses of air out the valve cover pipe and the starter cover pipe - check this is the case

GuyGraham thanks for your input,

when I had the valve cover off to check the valve clearances, I checked this metal 90 degree pipe and it was clear.   I could blow through it no problem.  After I started the bike up, I could feel that pulsing air you speak of.  I am absolutely positive that it was clear when I installed it. 

Something must be causing it to clog over time. (any ideas?)


could my valve seals be worn out in combination with leaky valves? is valve blow-by a thing?

Im starting to think it has nothing to do with a breather hose being clogged as I have checked that about 60 times. 

Def nothing to valve seals or leaky valves-its to do with the air displaced by the underside of piston as it moves up and down bore plus the extra pressurizaton from gas blow by past the rings and the heating effect from the engine on the ir in the crankcade which is causing a rise in pressure in the crankcase which cant vent itself and releases by blowing the half moons out

Have you checked the pipes all the way back to the air box?

1 from the valve cover to y piece

1 from starter motor to y piece

1 from y piece to air box

The y piece itself

The air pipe entrance to the air box

Edited by GuyGraham

Ill check everything tonight when I get home.  Im determined to get to the bottom of this. 

-Checked the breather from the starter motor I even pulled off the cover to the starter motor. It's clean in there.

- checked the breather from the valve cover, clean

- checked the y connector , clean

- the only hose connected to my air box goes to the AIS,

The last hose connected to my y connector goes out the bottom of the bike checked it, clean

I blew through everything and everything was open

Any other ideas? Soooo. Frustrating

For what it's worth my oil dipstick showed about 1cm below the fill line

You're not using silicone , are you ? Silicone is slippery and sometimes just tightening things down can squeeze out gaskets. 

For what it's worth my oil dipstick showed about 1cm below the fill line


Are you sure you are not over filling the oil?

You are emptying out of (2) drains, correct?

You should drain the oil and measure it. I'll bet you have like 1.4 in there...

Completely overfilled or venting is blocked or hoses routed wrong.

Well I think I found the issue.  After I checked every breather hose about 20 times twice, I decided to look under the hood. At first I noticed the inside of the valve cover was scraped in a circular pattern. Then I noticed the end of the hot start mechanism was worn a little.  This must mean the cam slid out of its position and into the valve cover. At this point I pulled the entire exhaust cam out. 


After further inspection under the camshaft i found a few gouges.   The little dowel the comes out of the cam for the hot start has gouged some of the metal next to the front left exhaust valve.  Do you think me AND the previous owner installed this incorrectly?  

I checked the valve bucket and it still moves up and down smoothly.  I don't want o have to buy a new cylinder head. I am wondering if you guys thing I need to get this fixed or if it will be okay if I just make sure the cam is installed correctly.  I just have to make sure that circlip is in the correct groove. 

far away scrape vc.JPG

close vc scrape.JPG

case scrape.JPG


hot start wear2.JPG

exhuast cylinder .JPG

close up gouge.jpg

far pin wear.PNG

worn pin.jpg

Edited by pbcrazy9898

Thats the auyo decompressor not the hot start

If the circlip was fitted then the cam would be located in the correct position and not touching the head or valve cover so this is from a previous time unless you are say there was not circlip fitted to the bearing

Was the circlip fitted when you took the cam out?

If the bucket rotates smoothly then then you are ok-trim any metal that is rubbing from where the decomp pin has toyched the head

Edited by GuyGraham

Thats the thing that boggles my mind, the circlip was installed both times it blew.  Do these get worn out? 

The other option suggests it was not seated in the groove on the bearing but actually seated next to the cam bearing.  Over time it slowly slides out of its spot and touches the valve cover.  (Please excuse my terminology i don't know what the exact names of these parts are. )  Ill post a picture because I could see that is was worn a little on the right side of the bearing.   

circlip locates in the groove on the bearing and the groove in the head but doesnt wear as such

Might be poss that bearing has moved perhaps and so the cam itself has shifted over. Dont know if there is room for the bearing to move towards the cam lobes which would cause the cam sprocket to move closer to the valve cover

Are the cam sprockets in line when the ex cam is fitted to the head?

Edited by GuyGraham

Im not exactly sure what you are saying but I think the circlip does seat in the bearing.  I looked at the bearing and there is no way the bearing could move as it is held in place by the design of the can shaft. 



In the second picture where the circlip is at the side of the bearing,is it possible to fit the cam like this ie with the circlip located in the head correctly but mislocated on the bearing as this would result in the cam sprocket being too close to the valve cover

Edited by GuyGraham

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