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yz250 on shifter kart - possible piston ring replacement

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hey all

got a yz250 on a shifter kart. having trouble starting it. the engine year is 94. 

ive have a feeling its low compression. only issue is that i cant really kick it over because it hits the exaust and rear axle if i take exhaust off. so ive got to use my hand and push it over.

first thing i noticed was the ability to actually use my hand to kick it over. i tried a handful of times, got up to 70psi. i know that isnt 100% because it wasnt kicks to spin the motor quicker.

also, the bore on at the top of the jug is roughly 67.67mm with a set of dial calipers.  i want to try and just replace the ring before doing a complete top end. what size rings do i need? or do i have to also measure the piston?

 

Also forgot to mention it's got a long rod kit. Anyone know what the displacement is? I can measure the spacer.

thanks

 

Edited by Chootem

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I think they were mostly pushed or towed to get running. There's a 125 on here that has the kick starter removed and welding over. Put the tester in it and push it around, see what it pumps up to.
I have no idea with the ring size is but I believe it's 285 cc with the long rod.

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I've driven 125 shifter carts, and a kart with two 100cc motors.  I can't imagine a 250cc shifter kart, let alone a big bore.  Holy crap!

 

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I think they were mostly pushed or towed to get running. There's a 125 on here that has the kick starter removed and welding over. Put the tester in it and push it around, see what it pumps up to.
I have no idea with the ring size is but I believe it's 285 cc with the long rod.


Yeah I think that's going to be my best option to get the most accurate reading.

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If it were me I would replace the entire piston and ring rather than just a ring. You can inspect the engine while your in there.

You could send the piston off to Wiseco and they can make you whatever custom piston you need.  Custom compression height or custom bore diameter should be no problem.

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I have a long rod kit on my 94 yz250 and it does NOT change the engine displacement. The rod is 5mm longer, that's it in no way does it change the stroke of the crank. Going from memory the stock bore is 68mm, and your compression should be 180+. I usually see 200 on a fresh engine. Time for a rebuild. 

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