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Two Stroke Compression

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When is it time for a piston kit for my son's YZ125? There seems to be very little compression when you kick it over. In fact, I can push the kick start easily with my hand. The bike still seems to have plenty of power. Your thoughts would be appreciated! :cry:

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If power feels ok I would not worry about it (that much).

Have you done a compression test?

Do you hear the piston making noise? (piston slap)

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Unless your Superman, you should not be able to crank a 125 by hand. Get an inexpensive compression tester at Autozone, and see how much compression the cylinder has and post back the numbers.

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how many hours are on this top end?

how easy does it start?

powerwise, how does it run?

does it smoke a lot?

if you can push it over with your hand, then i'd say its time for a fresh top end.

It starts with three or four kicks and seems to have good power. Bought the bike used and we probably have 50 hours on the engine ourselves. It does smoke a lot but I think I am running it rich.

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If power feels ok I would not worry about it (that much).

Have you done a compression test?

Do you hear the piston making noise? (piston slap)

Haven't done a compreesion test yet and no, I do not hear piston slap.

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Pull the carb and reed cage out. you can then look at the skirt on the piston and see how scuffed it is and sometimes starting to crack at the bottom and work it's way up. You can also see in the cylinder and see if there is any gouging or cross-hatching left.

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I would change that piston asap. You shouldn't be able to push it through the stroke that easily. I remember a similar situation I had with a used CR 250. The bike felt pretty strong, but I just knew it had alot of hours on it when I bought it. When I pulled the piston there was a 2-inch hairline crack running right up the middle of the piston skirt. A couple more rides, and I would have chunked the motor.

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I can start my 125 with my HAND when it's warm - new top end :cry: You have to remember, it's a two stroke AND it's only a 125... Run it until something goes wrong! I guess you could always just do a rebuild for the heck of it...

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Unless your Superman, you should not be able to crank a 125 by hand. Get an inexpensive compression tester at Autozone, and see how much compression the cylinder has and post back the numbers.

Well, its compression is 60 lbs. consistently. I think it is time for new rings!!

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The proper way to figure out if it's time for a new topend is to measure the piston. It's the only way.

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