Compression check ?

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#1
This may be the most elementary question asked on this board today but I still have to ask it.

When checking compression do you check it with the engine cold or at operating temperature?
 
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#4
I've checked mine (YZ250) at cold only once before and it was at 235psi. What would it be if the engine was warmed up. Higher....lower ? How much?
 

darringer

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#6
I have checked mine both hot and cold, within a matter of 10 minutes, and found the difference to be about 5 psi. My compression is around 210-215 on my 250.
 

yz250flash

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#7
I checked the compression on my yz125 this weekend. Cold 180 psi, Hot 175 psi. Seems to run strong with about 5 hours on the top end.
 
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#8
YZ250flash, I'm guessing you have owned or still currently own a YZ250. What years are your 250 and yz125. The thing is...is that I have a '01 YZ250 and am wanting a 2003 YZ125. Whats the acceleration difference btwn the two. I wont be bored riding the 125 after riding the 250 for 6 months--will I?
 

yz250flash

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#9
I have a 2000 YZ250 and an '01 yz125. The power band on the 125 is much more noticable than the 250. Keep it in the higher rpms and it screams. In tight situations and coming out of corners you have to keep the 125 revved up and slip the clutch to keep it in the power band and maintain speed. As you know with the 250 you can just throttle out of a corner.

Although there is only about 22 pounds difference between the two the 125 feels like you can throw it around. It feels a lot more nimble than the 250. I actually feel a little more confident on the 125 because it feels more forgiving. If I twist the throttle too much at the wrong time I don't feel like it is going to come out from under me. To tell the truth I bought the 125 for my girlfriend to ride but I like riding it so much that a lot of times I will take it out instead of the 250.

Just this past weekend I had a friend that races a CR250 break his clutch cable just minutes before the race. Since he races in an open class we snatched the numbers off his bike and put them on my 125. After the race he wanted to buy the bike. He said he had never had such fun in a race. The bike handled great and he didn't seem as tired afterward. We compared his practice lap times on his 250 with the lap times on the 125 and they were about the same.

Bottom line is that a 125 is a little more work to ride than a 250 in the sense that you have to constantly change gears to keep it in the power (my '01 125 has a 6 speed gear box, the '03 125 has a 5 speed gearbox). But in my opinion the 125 makes up the difference with its agility and pep.

I don't think you will be bored with the 125, in fact I think you will have a blast. The only thing you may miss is the low end power of the 250, especially if you ever ride tight woods.
 

bedell99

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#10
When taking compression in an engine temperature has negligible effects. Let me explain. As long of the temperature of the air inside your cylinder stays almost constant. ( I know it doesn't) It doesn't matter if it the engine is cold or hot. So the air might rise a couple of degrees as it is being pushed in a hot engine, but probably no more than probably 10 degrees. What is even more cool is that it remains a linear relationship. Remember that fellow Charles well he had this law. It is called the Ideal Gas law. PV=nRT. So if you want and compare the differences take them at both and average the results. What is weird is the YZ250flash got the opposite of what law's of thermo has to say. He got a lower reading for a warm temp, which is quite odd. Enough of all this science stuff, I knew I would use that engineering degreee for something. I personally takes it when it is cold.

Erik
 

RM_guy

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#11
I take it when it's cold too. I only use the compression reading as an indicator of wear anyway so as long as you are consistent with when you do it, you'll be OK.

I understand what Bedell99 is talking about but I find the compression is lower when the motor is hot. It depends on how much oil is in the cylinder and how viscous it get at room (or lower) temperature. The thicker oil seals better.
 

fuzzy

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#12
Yeah, in all actuality I always check cold, then hot. A worn top-end will show a big loss between cold/hot....
 

bedell99

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#13
You know what, Now that I think of it. The oil must play a role with the sealing capability of a piston with the cylinder, but now the question is does it seal better when the oil is hot or cold. I would think it would seal better when it is warm for 2 reasons. 1) The piston and ring expand a little providing a better seal and 2) The gas/air mixture evorapates more and leaves a bit more oil onto the cylinder instead of getting apush out of the exhaust port, which than provides a better seal.

Erik
 
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#14
Thanks yz250flash, that was probably the most understandable & unbiased "synopsis" I've ever heard. At other forums, people don't give me real answers that I'm looking for. You've told me everything I need to know(and were dealing w/ the exact same bikes practically--'01 YZ125/250).

Oh, and one more thing. I havn't noticed the 250 being costly in repairs at all! If I were, for example, to buy a 2003 YZ125 upcoming spring...would I notice having to replace the ring and piston a little more often. Or alot more often?

And you said(I've heard this before a million times) you clutch more and revv while clutching(kinda abusive to ride effectivly) alot more often than a 250. Will that mean that I have to replace the clutch components more often?
Tell me if I'm wrong>>>> A 125 will be a "little" more expensive maintenance wise---right?
 

yz250flash

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#15
rairden - You are correct in the assumption that the 125 will be a little more costly to maintain than the 250. You are also correct in the fact that that the main components are the top end and clutch. Both of these items will wear more on the 125 than the 250. How much more depends on the type of riding you do. But both of these items are also a little cheaper for the 125 and fairly simple to change. It does seem that I tinker with the 125 quite a bit more in order to keep it running its best. With the 125 jetting is more important. If you loose a little power to incorrect jetting on the 250 it still has plenty of power (for most of us riders anyway) however incorrect jetting on a 125 can really effect the power, especially down low.