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yz125 low compression

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Hey guys, just bought my first dirt bike yesterday at a snag of a price but there's probably a reason for it. The owner swore it had less than 20 hours on the top end rebuild (professional racer owned prior to him) and after checking it out I closed the deal. Today, I go to start it and notice the kick start moves with ease. I can literally start it with my hands. So do I need a new top end or does the cylinder need to be honed? I am going to borrow a compression tester tomorrow and get some numbers. 

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Hey guys, just bought my first dirt bike yesterday at a snag of a price but there's probably a reason for it. The owner swore it had less than 20 hours on the top end rebuild (professional racer owned prior to him) and after checking it out I closed the deal. Today, I go to start it and notice the kick start moves with ease. I can literally start it with my hands. So do I need a new top end or does the cylinder need to be honed? I am going to borrow a compression tester tomorrow and get some numbers. 

Check the compression with a gauge and remember throttle wide open, I just rebuilt the top end on my sons cr125 with a new OEM cylinder and i can start it by hand. Im used to kickin' my 250 it just seems easy  :lol:

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Do I just hold the throttle wide open and kick? Also by hand starting I mean like there is less than 6 pounds of pressure needed to push on the kick starter

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Do I just hold the throttle wide open and kick? Also by hand starting I mean like there is less than 6 pounds of pressure needed to push on the kick starter

yes with pressure gauge installed hold the throttle wide open and kick 10 times or so . Pressure should hold above 120 psi

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yes with pressure gauge installed hold the throttle wide open and kick 10 times or so . Pressure should hold above 120 psi

She pushed 125psi. So should I be worried as I hear people getting like 150psi?

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She pushed 125psi. So should I be worried as I hear people getting like 150psi?

I wouldn't sweat it , since you just bought it probably not a bad idea to pull just the head off to take a look at the cylinder walls , you can look through the exhaust too, just easier for me to pull the tank and head.  You also get a better look and most cylinder wear is on the exhaust port bridge unless you don't have one like me :thumbsup: .  At least you will know that soon it will need a cylinder or just a top end kit. or just ride it  :ride:

Edited by Beau 88

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She pushed 125psi. So should I be worried as I hear people getting like 150psi?

125 psi is *way* too low. Is it running well? I am suprised you can start it. Hopefully all you will need is a top end rebuild. If you want to go really cheap you can probably just replace the piston ring for about 10 bucks and nothing else. You should see at least 175 psi. Typically more like 180-190. Taking apart the cylinder in order to get to the ring will give you an oppertunity to inspect all sorts of important stuff, and you will know they true condition of the engine. What you should replace depends on your budget.

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Well since I just dropped 2 grand on the bike I'm not going to cheap out on parts. I just took it around the neighborhood (luckily no cops showed up) and she runs amazingly, second gear wheelies! Only problem is that it bogs right off the start, probably the jetting/fuel mixture? I'm at 32:1 

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Well since I just dropped 2 grand on the bike I'm not going to cheap out on parts. I just took it around the neighborhood (luckily no cops showed up) and she runs amazingly, second gear wheelies! Only problem is that it bogs right off the start, probably the jetting/fuel mixture? I'm at 32:1 

Oh, your first post seemed to indicate you got it for an exceptionaly low price and that perhaps this was because something was wrong with it. Yeah, for that much money definitely do the top end right rather than cheaping out. It will still be less than 100 bucks if you can do it yourself. Of course the cheapest cheap-out of them all is to pretend it is fine as is and doesn't even need a top end. Low compression will only efect ease of starting and low end power, btw. It will still run perfectly at high RPMs. You did the right thing to get a gauge and measure it so now you know for sure the top end needs work. This problem needs to be addressed before thinking about re-jetting.

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Before u buy a piston mic. The cylinder. Make sure it's not out of round. And get someone who's been around bikes to listen to the motor. See if they can here piston slap. Cause if u have piston slap or out of round cylinder. Your going to have low compression again.

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