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How do shims/clearances work?

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Not to be too pedantic but the valves don't grow longer as you said. They DO wear into the head as you also said lol.

I have seen "cupped" valves where the head has rolled and the valve is longer.

CoKTM

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Do not run the bike. Check the valves. If they are in spec, then I'd flush the fuel tank, replace the pilot jet, new spark plug. But check the valves first.

How does my bike sound running http://s46.photobucket.com/user/Xin250/media/IMG_0035_zpsjnfdutsr.mp4.html?sort=3&o=4I need to tune some stuff on the carb to make to run correctly I am sure. That was with the choke pulled out because that's the only way it runs. And my bike starts first kick you spray starter fluid into the airbox. I don't know if that helps with my issues.

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How does my bike sound running http://s46.photobucket.com/user/Xin250/media/IMG_0035_zpsjnfdutsr.mp4.html?sort=3&o=4I need to tune some stuff on the carb to make to run correctly I am sure. That was with the choke pulled out because that's the only way it runs. And my bike starts first kick you spray starter fluid into the airbox. I don't know if that helps with my issues.

Don't bother with anything until you do a good cleanup on the bike: change fluids and oil filter, clean the carb and replace the pilot jet, new air filter and check the valves. Running the motor with it it's current condition isn't doing it any favors.

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The valve shims in a 250f are about the size of a Aspirin, and vary in thickness to add valve clearance, or reduce valve clearance.  Valve clearance is the space/gap/clearance in between the valve and camshaft.  Your bike has 2 intake valves, and 2 exhaust valves.  Let's say that you intake valve clearance is supposed to be at .004 thousandth, that's just a little thicker that a sheet of paper.  You're exhaust valve clearance is supposed to be at .010 thousandth, a little thinner than a matchbook cover. 

 

I'm not going to go in great detail on the adjustment procedure, but basically you'll pull your valve cover, roll the engine to TDC (top dead center), and check and adjust the valve clearance as per necessary.  If you've never performed this procedure before, I would definitely get an owner manual or instructions from the innerweeb.  Instructions are fine, but if you're not a very organized, procedural oriented, observant, detailed person, and are a bull in a china shop, I would have someone perform the procedure for me the first time, with you watching.  The 250f is whiney and fragile; and they don't take kindly to improper assembly procedures.

 

Don't mean to be a downer, but I don't won't you to destroy you're bike, or hurt yourself with an engine failure while mid flight.

 

Just a correction it has 3 intake valves not 2. As for shims typically (in my experience) the Yamaha YZ250F's start off with shims in the 1.90-1.80 mm range. If you are shimming below 1.70 mm you are pushing the limits of the valves. Every YZ250F that I have rebuilt that had been shimmed multiple times before dropping a valve or two had shims 1.65 mm or smaller. Keep that in mind. On the 2014 YZ250F they went to the bigger shims like the 405"s have and so far I am seeing 2 mm shims on brand new heads.

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Don't bother with anything until you do a good cleanup on the bike: change fluids and oil filter, clean the carb and replace the pilot jet, new air filter and check the valves. Running the motor with it it's current condition isn't doing it any favors.

Well, I have done all of that except check my valves. All I want to know is how to fix my bike from running so lean as it is right now. I installed a titanium 4 exhaust and a powerbomb header on it so I don't know if that change anything. About to just put it in my front yard with a "free" sign on it. 

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Well, I have done all of that except check my valves. All I want to know is how to fix my bike from running so lean as it is right now. I installed a titanium 4 exhaust and a powerbomb header on it so I don't know if that change anything. About to just put it in my front yard with a "free" sign on it.

I'll take it!

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Oops, someone corrected the 3 intake valves....

 

 

Just watch the youtube videos and go check em lol. If they are not within the spec range, come back here with whatever the gap is currently at. If they need to be re-shimmed, at that point you could get some pointers on that process. I know it seems daunting, but checking them is pretty damn easy.

Edited by J_YZ2fittyF
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Zac and Charlie both called me out.

 

I knew they were 5 valves, just stuck in Honda mode.

 

Principles still the same. 

 

Yea, it was about not confusing him, not correcting you for the sake of correcting. I didn't read past that post to see originally. If he was closer, I would just go help him the first time. He is in MI as well, but he is like 3hrs away....since I had talked to em about this before (the valve clearance checking part).

Edited by J_YZ2fittyF
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Yea, it was about not confusing him, not correcting you for the sake of correcting. I didn't read past that post to see originally.

 

I was just rattling their cage; they meant nothing by it.

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Yea, it was about not confusing him, not correcting you for the sake of correcting. I didn't read past that post to see originally. If he was closer, I would just go help him the first time. He is in MI as well, but he is like 3hrs away....since I had talked to em about this before (the valve clearance checking part).

Question: If my valves are tight, that means that my valves are slightly stuck open still right? which means I will have less compression. So if I open the clearance, how will it close the valve up?

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Not always, ive always assumed tight means that the thinnest allowable gauge for spec wont fit, but others thinner then spec still will. They are on the tight side of spec. When you're valve is so worn that there is no clearance at all where the thinnest gauge you even have wont fit because there is literally zero gap between the bucket and the cam when there should be....that is referred to as "Zeroed"....which means a substantial amount of wear. You're valves may only be a little out of spec and therefore there is still SOME clearance between the valve and the bucket....which means the valve will still close fully, but it is still out of spec, and effects the overall valve operation.

 

The valves always want to close because they have a spring in there and a retainer that pulls the valve "up" (or into its seat) when the pressure of the cam lobe is not forcing it down (or open). So basically, by opening them back up into spec range...you'd be doing the right thing.

 

 

check this link and click on the picture in upper right to expand the engine pic (this is not a yz250f by the way, lol). But it labels the various parts to make it easier to understand.

 

http://www.howacarworks.com/illustrations/engine-with-overhead-camshaft

 

Notice how the one water droplet or tear shape is pointed up, the other is pointing down. The valve below the one pointing up is closed because the "pointy" part of the cam lobe is not pushing down on the bucket. At that point, there should be a gap between the lobe and the bucket on your bike. Notice the spring is not super compressed because the spring has extended once the force on the bucket (from the pointy part of the lobe) has been takin away due to the camshaft spinning (which opens and closes valves as it spins in time with the crankshaft...hence cam timing). That spring is what causes a valve to close, and keeps the bucket in contact with the cam lobe (when it's supposed to be in contact, lol). Now, the valve on the right is currently opened, the point of the cam lobe is pointing down and pushing the bucket down, compressing the spring, and opening the valve.  This happens extremely fast inside of a motor, especially a bike like these which can hit very high RPM....in order for the bike to operate correctly, the valves need to be opening and closing correctly.

Edited by J_YZ2fittyF
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Question: If my valves are tight, that means that my valves are slightly stuck open still right? which means I will have less compression. So if I open the clearance, how will it close the valve up?

 

Not neccesarily stuck open but possible. The way shim and bucket style valvetrains are designed is if your gap between the bucket and cam (at top dead center) is tighter than manufacturers specifications you have to put a smaller shim under the bucket to increase the tolerance to get it back to spec. As the vaves wear into the seats they sit higher up in the head. This reduces the clearance between the bucket and the camshaft. If the gap gets too tight it will make the bike harder to start and can cause some runability problems. With valve wear if the concentricity of the valve surface to the seat gets out of spec you will eventually cause the valve stem to break from flexing. That becomes very expensive. This is why on the 2013 and older YZ250F's I dont reccomend going smaller than a 1.75-1.70 MM shim.

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I have seen "cupped" valves where the head has rolled and the valve is longer.

CoKTM

I've also heard of valves "stretching", looking at the way valves function it seems plausible? But it's all here say though as I've never seen it myself lol.

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