Valve clearence

Hej.

I´ve just checked the valves on my YZ250F -01 and all are in spec except one intake valve. The one on the kickstarter side has no clearence at all. I havent noticed any problems with the engine except that the idle speed can go up and down sometimes. Can I just re-shim it and think of it no more or shoud I replace that valve?

//Jarmo

Yes, just re-shim it and be done with it for awhile. While you have the cam caps off, remove all of your shim pad adjusters and write the sizes down somewhere in your manual where you won't forget them. Then sometime down the road when it's time to re-shim again, you'll already have the current shim sizes written down and you'll be able to order up any needed new shims all in one fell swoop without having to take apart your engine twice. Hope this helps. Maniac :cry:

Thanx, I wanted to hear that and I also wanted to be shure that the valve has not taken any serious damage by running the engine with no valveclearence.

Tried with a 1.85mm shim and meashured the clearence to less than 0.10mm but more than 0.05mm so would´nt a 1.80mm shim do the trick?

I have to order that one and wait for a couple of days. :cry:

Well, yes, you should. I've ridden them a shade too tight for lack of shims before, but nobody could really recommend it.

ok, so I "should" replace it (the valve)? But what can happend if i don´t? Except for some power loss could the valve break so it causes a major engine damage ?

I friend of mine has equipment to test the valves for leakage but that could take weeks. I really don´t want to wait that long. :-/ My suspension is at Factory Connection and im really eager to try them out.

When I did my valves last spring, my middle intake had no clearance whatsoever. I think stock shim was like 180 or something like that and ended up with like a 155 or maybe a 145--I wrote them down. I did not pull the head off to check and I had stock shims. I did write them all down. Most dealers will swap shims for you--they want the odd-ball size ones that are only availble from the factory in the bike. They will swap with the nearest 5 shim thickness thikness (what is the measurement--5 100's of a mm?). Anyway, I ended up with e valves just a tad on the loose side and the i valves all nice in spec. I was told to check them again after no more than about 4 easy hours on the engine. If the center intake tightened up, then I better change it out. It didn't. I checked again after a couple hundred miles and it's still good--no changes. I am planning to tear it all down again soon and maybe have a good valve job done.

BTW--my hanging idle and starting problems disappeared. Any idle problems are easily cured with a tweak of the Zip Ty fuel screw.

Rudy, I'm a little worried about your valve that had to take a 155 or 145 shim to get it in spec. If you started with a 180 shim then you have changed it 14% or 20% depending on if you went with a 155 or 145. According to the 10% rule, you're in danger of breaking that valve. :cry:

Rudy, I'm a little worried about your valve that had to take a 155 or 145 shim to get it in spec. If you started with a 180 shim then you have changed it 14% or 20% depending on if you went with a 155 or 145. According to the 10% rule, you're in danger of breaking that valve. :cry:

you dont want to be the first on your block to digest a valve. valves are plenty cheap compared to total top ends.

Oops,

Didn't make myself clear. Vortex, I meant just reshim it. What may be frustrating is that if you drop to the next lower size, you could, as happened to me, find that you need one size smaller still.

You don't have to seriously consider replacing a valve that has not reduced its shim size by about .20mm (.0076")or so (judgement call). This indicates about .12mm (.005") wear at the valve seat and the face of the valve. If it's a race bike, you may want to be more critical than that.

The surging idle is one of the things that tight intakes can cause.

BTW, I have determined that it is possible to get a fairly accurate ballpark reading on shim size without bolting the cam in place. What needs to be done is to line up the key groove in the ball bearing with the "shadow" left on the cylinder head and tap the bearing down into the bearing pocket until it is seated using a plastic mallet. Measure the shims only on a "go, no-go" basis. If you think you have the right size, torque up the cam caps and double check. Never depend on the readings you get with the cam just laying in the head for your final numbers, but if you are careful enough, you'll find it helpful in situations like yours where you have nil clearance, and no way to know how far to go to bring it into spec.

You don't have to seriously consider replacing a valve that has not reduced its shim size by about .20mm (.0076")or so (judgement call). This indicates about .12mm (.005") wear at the valve seat and the face of the valve. If it's a race bike, you may want to be more critical than that.

I guess I don't understand the geometry of the valve lifter. Could you make more clear what you mean by a .2 mm shim change being equivalent to .12mm of wear at the seat/face? I thought this would be a linear (1:1) measurement. :cry: :cry:

It isn't linear because the valve wears against a seat placed at 45 degrees to the stem. Since the valve has to move toward the seat at that angle, rather than straight into it, if the valve/seat wear a given amount, the resulting movement up into the head would be roughly 1.4 times the wear (for 45 degrees)

Oh, I see what you're saying. You're accounting for the valves having a 45 degree contact angle with the seat.

I was talking about Rich Rohrich's guideline of 10% shim wear being an indication of impending doom. Since a shim change is much easier to measure than seat wear, that would be an easier benchmark to go by. The only problem I see with it is it doesn't differentiate between valve stretch and seat/valve face wear.

A .20 mm change in shim might be a good benchmark for all I know. Rich has a lot more experience than me, so I consider his input as good advice. What is your experience with four stroke valve failures? How did you get that number?

Ten percent of a 185 (1.85) shim is .185

That's pretty close to .2, no?

As far as differentiating between wear and stretch or tuliping, there's no way to know which one is responsible for the loss in clearance as far as I know.

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