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CRF250R losing shims?

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I'm working on a 2005 CRF250R.  I bought this as a basket case project and am trying to figure out what I've got going on here.  When I first got it, both valve keepers on the left intake valve had stripped and the valve dropped a little bit.  Luckily it didn't hit anything, but it was enough that it had no compression when turning it over.  Replaced all the keepers on the head and everything seemed to be great.  Filled the ports with WD-40 and everything seemed to be sealing properly.  Put the cam in the head on the bench to check all the valves and everything checked out ok.  I was at .005" on the intake side, and .012" on the exhaust side.  Pulled the cam and installed the head.  Double checked everything as best as I could and put the rest of the bike together.  Pulled it outside and fired 2nd kick.  Rode it down the street a couple blocks and let it sit.  Fired it back up and rode it down to the car wash with no issues on the way there or on the way home.  Had a radiator leaking and a gas leak so pulled the bike apart to fix those.  Let it sit for about a week waiting on parts.  Got everything back together and went to start it, with no luck.  Kicked for about 15 minutes before it lit, and noticed it seemed extremely hard to kick.  If it was warm you could forget about starting it.  Tonight I went out there, choke on and about 3 twists of the throttle and it fired after about 10 kicks.  I knew it still wasn't right so I pulled the valve cover back off.  Was double checking the valve clearances and noticed that the exhaust rocker wasn't even touching the decompression as it was turning.  Pulled the rocker off and both shims were missing from the top of the exhaust valves.  Luckily I found both of them before they did any damage but I'm puzzled as to what caused this.  I'm wondering if I didn't knock them off before the cam was ever installed (bumped the exhaust rocker and they stuck to it or something) and it was just sheer luck that it fired when it did?  When I put the shims back in tonight the valves were spot on what they were before.  Just curious to hear if there's anything else I need to check, or if I just pulled a stupid?  Thanks. 

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When I rebuild a motor, I recheck the valve clearance a second time just before putting the valve cover back on, with timing chain in, cam bolts tightened, etc.

 

So if this was my motor, it wouldn't be possible for this to be an assembly mistake, so I would automatically look for the problem elsewhere.

 

However in your case - who knows? What process did you go through when putting this back together? 

 

Did you check the valve clearance when you put it back together?

 

Did you install the shims with the head sitting on the bench, and then tilt the head while trying to get it back in the frame?

 

It is possible for the shims to fall out if the clearance is way way off, but i've never seen that happen.

 

If the exhaust valve is stainless and the springs are for TI valves, in theory it could float at very high RPM providing enough clearance to lose the shim, but then it probably would have done a lot more damage. It's a theory but highly unlikely.

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That is exactly what I'm thinking happened.  I assembled it on the bench, and didn't recheck it after I got it in the frame and everything assembled.  I think it was just a fluke that it happened to start good the first couple times I tried it.  Since everything is in spec I didn't see how it was possible for them to come out while running so I figured I'd try to get as much info as possible on what I could have done so it doesn't happen again. 

 

As far as the springs, I have no idea what's in it.  The kid I got it from didn't have a clue, and I personally didn't know how to tell if they were Stainless or Titanium.  I never had it past half throttle on my street in the couple times I rode it so I wouldn't think valve float would come into play yet, but thanks for the info there. 

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Maybe your valves are binding in the guides from being bent. The resulting valvetrain slop could liberate a shim. Just a thought. If you bench assembled the cam it would be close enough in the motor to not lose a shim...

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I could be totally wrong on my thought process here, but as fast as everything is moving in there if a valve stuck long enough to allow a shim to come out wouldn't the piston smack it?

 

I bench assembled the head to check valve clearances before I assembled everything, but I then removed the cam before I put the head in the frame.  I'm hoping that I either tilted it too much, or I bumped the exhaust rocker and let them fall unknowingly while I was attempting to get it on the engine.  I'm going to put it back together and see what happens.  If it does it again then I'll be digging a little deeper into the bent valve theory.  I'm just trying to cover all my avenues before I put it together.  Thanks for the info guys.

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Aaah, I bet you did drop the shims if the cam was removed prior to dripping the head onto the cylinder. You can leave all that stuff on there until the head is on, then remove the towers & cam to install the head bolts. Just be careful as you can still dislodge the exhaust shims while flopping the exhaust rocker around.

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