250F Valve clearance problem?

AKmotorider

AKmotorider

I checked my valve clearances. YZ250F manual said between .10 - .15 for intake, .17 - .22 for exhaust. Intake was less than .002 in (.002 wouldnt fit in there), so its either touching or its .001in. Exhaust is .006in. But heres the tricky part: I checked my intake shims and they are all 1.35mm. After doing the calculations, which I am sure I did correctly, the intake shims come out to be needing a 1.20mm if the intake is 0.001. According to the calculations, that should bring the clearance back to 0.006in. That will do. However, 1.20 mm is the smallest size shim they make it seems. What happens if I need something less than that? Dont valves tighten over time?
 
timoyz

timoyz

You aren't going to like my advice... It's time to pull the head off and rebuild it. When you are that far down in shim sizes, your valves have hammered themselves into thier seats, and you're riding a ticking time bomb. :noidea:
 
AKmotorider

AKmotorider

timoyz:
You aren't going to like my advice... It's time to pull the head off and rebuild it. When you are that far down in shim sizes, your valves have hammered themselves into thier seats, and you're riding a ticking time bomb. :noidea:
Click to expand...
Since, according to calculations, a 1.20mm shim will bring it back in spec, can I still ride it a awhile until it goes out of spec again and then replace the valves and seats? Its the end of the season and I want to get a few more rides in before winter comes.
 
James

James

Staff member
AKmotorider:
I checked my valve clearances. YZ250F manual said between .10 - .15 for intake, .17 - .22 for exhaust. Intake was less than .002 in (.002 wouldnt fit in there), so its either touching or its .001in.
Click to expand...
Not sure if you realized it but you're using both inches and mm in your measurements, it's best to pick one and stick with it.

Recommended Valve Clearance:
  • Intake - .004 to .006 inches or .1 to .15mm
  • Exhaust - .007 to .009 inches or .17 to .22mm
AKmotorider:
Dont valves tighten over time?
Click to expand...
:thumb: They do, which means you have to put in smaller shims.

Your intake has a clearance of .025 mm (.001 inches) and it needs to be .13 mm, so using the following formula:
A = (B - C) + D

[A] Replacement shim thickness (The shim size you need to re-install to get the correct valve clearance)
[B} Measured valve clearance (The measurement you got in Part 1 with your feeler gauges)
[C] Recommended valve clearance (What your clearance is supposed to be)
[D] Current shim thickness (The shim you just removed from motor)

A=(.025 - .13) + 1.35

A=1.245mm

So you need a 1.25mm shim.

Here's an Adjusting Valve Clearances How-To I wrote up some time ago. https://www.dirtbikeaddicts.com/threads/adjusting-valve-clearances-2005-kx250f-rmz250.4214/

With that said, I'd have to agree with Tim in that it's probably about time to get new valves/seats cut.
 
M

MBevans

Pull the head, no biggie and check it out. Most likely the valves are gone, seats pounded..... I'd say pull the head or risk droping a worn-out valve and ruining the piston cylinder and possible rod/crank....
 
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