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How do shims/clearances work?

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I am new to 4 stroke dirt bikes, and was just wondering how exactly shims work on bikes. I am interested to know what they do and how they work because I will be re shimming my dirt bike soon. Also I am wondering on what the clearances have to do with the bikes performance and how it starts. Thanks

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To over simplify: one side of the shim rides on the end of the valve stem, and the other side rides inside the "bucket". This allows you to adjust the overall length of the valve/shim/bucket assembly to precise tolerances. Your cam love rides on the outside of the "bucket" and causes the valve to open as it rotates. If your shims are too thick (longer valve assembly) the valve may not completely close and the valve will leak, burn valves, be hard to start, or not run at all. If you shim is too thin (short valve assembly) the valve does not fully open or maintain proper duration and you loose power, get more valve noise, and risk breaking top end parts. As you ride your bike your valves tend to get longer over time (the valve cups, and the valve mating surface with the seat wears. Therefore when you're valve clearance goes out of tollerance you generally have to put in thinner shims to account for the longer valves.

CoKTM

Edited by coktm
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To over simplify: one side of the shim rides on the end of the valve stem, and the other side rides inside the "bucket". This allows you to adjust the overall length of the valve/shim/bucket assembly to precise tolerances. Your cam love rides on the outside of the "bucket" and causes the valve to open as it rotates. If your shims are too thick (longer valve assembly) the valve may not complete close and the bike will leak, burn valves, be hard to start, or not run at all. If you shim is too thin (short valve assembly) the valve does not fully open or maintain proper duration and you loose power, get more valve noise, and risk breaking top end parts. As you ride your bike your valves tend to get longer over time (the valve cups, and the valve mating surface with the seat wears. Therefore when you're valve clearance goes out of tollerance you generally have to put in thinner shims to account for the longer valves.

CoKTM

Thank you so much! That was probably one of the best explanations I have heard of something .... it played out in my mind.  :thumbsup:

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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.

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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.

Thank you for the tips! I probably plan on having someone help me the first time doing it so I am not so worried about messing my bike up! Also, I believe my bike is a 5 valve :p 3 intake, 2 exhaust

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To over simplify: one side of the shim rides on the end of the valve stem, and the other side rides inside the "bucket". This allows you to adjust the overall length of the valve/shim/bucket assembly to precise tolerances. Your cam love rides on the outside of the "bucket" and causes the valve to open as it rotates. If your shims are too thick (longer valve assembly) the valve may not completely close and the valve will leak, burn valves, be hard to start, or not run at all. If you shim is too thin (short valve assembly) the valve does not fully open or maintain proper duration and you loose power, get more valve noise, and risk breaking top end parts. As you ride your bike your valves tend to get longer over time (the valve cups, and the valve mating surface with the seat wears. Therefore when you're valve clearance goes out of tollerance you generally have to put in thinner shims to account for the longer valves.

CoKTM

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.

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Thank you for the tips! I probably plan on having someone help me the first time doing it so I am not so worried about messing my bike up! Also, I believe my bike is a 5 valve :p 3 intake, 2 exhaust

 

Sorry, still stuck in Honda state of mind, plus, I have a 13 and 14 KTM 250sx.

 

What's sad, I looked right at your avatar and seen the Yamaha; but never switched out of Honda mode.

 

I rode Honda for several years.

Edited by WALKINGWOUNDED

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Sorry, still stuck in Honda state of mind, plus, I have a 13 and 14 KTM 250sx.

 

What's sad, I looked right at your avatar and seen the Yamaha; but never switched out of Honda mode.

 

I rode Honda for several years.

do you have the sx or sxf?  the sx doesn't have valves, the sxf does.

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Sorry, still stuck in Honda state of mind, plus, I have a 13 and 14 KTM 250sx.

 

What's sad, I looked right at your avatar and seen the Yamaha; but never switched out of Honda mode.

 

I rode Honda for several years.

Good ol' honda! I still love my cr 125 

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Never owned a Yamaha, but were they notorious about "valves tightening" as Honda?

 

I'm guessing with 3 intake valves they were lighter, and had softer springs, which may have reduced valve seat destruction.

 

Sounds good on paper anyway.

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Zack, you have a used bike with unknown history. What is known is very questionable. You do not want to shim the valves at this point. You need to check them and if they are in fact out, you need to accept the reality and redo them.

 

For those curious, he got a bike used that was missing the air filter that is very hard starting.

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Good ol' honda! I still love my cr 125 

 

In 2013 I decide I wanted to stay with 2-strokes, and a new bike every year, so I went with KTM, since Honda bailed on 2-strokes in 07.

 

I have an 03 CR134R, an 06 CR250, and an 07CRF450R.  They're all going on the auction block very soon.

 

I'm going to buy a 15 250SX at the first of the year, but also plan on buying a 150SX too.  I absolutely love torturing a small bore around the track.

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Zack, you have a used bike with unknown history. What is known is very questionable. You do not want to shim the valves at this point. You need to check them and if they are in fact out, you need to accept the reality and redo them.

 

For those curious, he got a bike used that was missing the air filter that is very hard starting.

 

Missing air filter, and hard starting.....hmmmmm :thinking:

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SX 2-stroke

right. sorry. i thought the OP was stating he had the sx which made me wonder why he was asking about valves.  too much screen time lately. starting to blur my cognitive function

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Zack, you have a used bike with unknown history. What is known is very questionable. You do not want to shim the valves at this point. You need to check them and if they are in fact out, you need to accept the reality and redo them.

 

For those curious, he got a bike used that was missing the air filter that is very hard starting.

Well I guess I left out the part that the guy told me that he never road on it without the air filter off of it(He could be lying but for my sake I will trust him sense he is my brothers friend). The bike has been sitting for over 2 years also.

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Also, the bike actually starts up by kick if you spray a little starter fluid into the air box. I got it to start first kick when I did that, but I would rather not take off the seat and spray it in every time I go to start the bike. I don't know if that is a sign of something else. Also, here is a video of my bike running http://s46.photobucket.com/user/Xin250/media/IMG_0035_zpsjnfdutsr.mp4.html?sort=3&o=4

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do you have the sx or sxf?  the sx doesn't have valves, the sxf does.

 

I'm sure I structured a sentence incorrectly somewhere; hope I didn't confuse anyone.

 

Had a customer bring by a huge tin can of dark chocolate cookies.

 

I'm all swolled up like a poisoned pup.

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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.

Yeah, I don't plan on running it again until the bikes valves are checked. 

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