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Bike suddenly lost all compression

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I was out trail riding my 2002 yz 125 when I heard a rattle.. I stopped to make sure nothing was loose and when I went to start it there was veer little compression but it ran. Then at the next stop I went to start it again and there no compression at all. 5 rides ago (all woods with lots of single track) I put in a wiesco prolite and a replated cylinder. I haven't had the time to go pull it apart yet but I was wondering if this could be caused by a worn bottom end?

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Thanks for such a fast reply, Would a worn bottom end cause that the ring to wear out quickly? I run the bike on premium gas at 36 to1 with the Lucas semi synthetic

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I dont think the bottom end has to do anything with compression.

The rings or ring gives you compression. But if you are a hard rider and ride high in the rpms then only running one ring, then thats going to wear out faster than two rings. So if the ring isnt broken maybe start cheap and replace that ring.:lol::thumbsup:

Just my two cents

Thanks and god bless!!!!

Alex

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Yea man is this your first top end or first time you've experienced this? I'm new to it as well just got back my old 98 cr250 but my extremely knowledgeable buddy said he's never had one gradually lose compression. He says it's like a light switch. One minute you got it and then in a instant it goes south, so don't feel like your bike is doing anything un ordinary, just letting you know it's time for another one. Mine just did the same thing basically throttle stuck wfo on me at the track and when I got it back up the kickstart felt like it lost a ton of compression. Still runs good, sounds good, but isn't near 100%

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Pulled it apart and found that it was running lean could worn reeds cause this couldnt find any air leaks.

If it was running lean maybe go up one on the main jet. How did your plug look? Also, if you check your reeds, make sure there is no gap in between them and the rubber....If there is. replace them.

How'd the top end look?

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Slight chip In the piston dead center facing forward no cylinder damage lots of white buildup on the plug and piston. It has never ran lean till know jetting was spot on for the past two years. There is a slight gap in the reeds could that cause a lean condition?

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How about the fact that it was 35-40 deg and clear dry air no humidity that day, I had to turn my air screw in a turn and a quarter that morning. I was on the YZ with the two KTMs

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How about the fact that it was 35-40 deg and clear dry air no humidity that day, I had to turn my air screw in a turn and a quarter that morning. I was on the YZ with the two KTMs

Is jetting really that sensitive? On your yz what are jetting specs for Washington winter?

Thanks

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Really weird that your jetting has always been the same and or good and now all the sudden it went lean. Are you sure there's nothing that you could have done/changed...New expansion chamber or? How did you test for air leaks?

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Last time it ran lean was when there was a crack in the carb joint so I just looked for cracks in the rubber, blown gaskets, or anything like that and couldn't find a thing. Is there a better way too test?? It's already apart I can post some pics tomorrow thanks again for all the help everyone

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Last time it ran lean was when there was a crack in the carb joint so I just looked for cracks in the rubber, blown gaskets, or anything like that and couldn't find a thing. Is there a better way too test?? It's already apart I can post some pics tomorrow thanks again for all the help everyone

You can always run a leak down test to see if you have any leaks. But if there is a space in your reeds then i would replace them.

As for jetting two strokes can be very finicky when weather changes. My buddy's 11 ktm250xc is bogging all over the place now in PA since its starting to get cold. So hes going to have to make carb changes hahaha

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Yea the carb was cleaned about a month ago. So if there is small hole in the reeds could that cause it to be lean?

Could it cause it to be leaner than it was, Possibly. Would it cause it to lean out to the point of failure, No.

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If your bike is jetted perfectly for say, 70F, then you stand a very strong chance of melt down at 40F, especially depending on the riding conditions (running lean in deep sand on fast trails, melt down, running lean putting around at idle, might be a bit boggy but not deadly).

Jetting is sensitive. Some guys jet for 10F temp changes. Some guys never touch it and just run pig rich when it's hot out. I typically look at it in about 20 degree intervals. Typically by 30F change I can change a main jet. What I run when it's 90F out would melt my motor down at 50F if pushed.

Sprung/leaking reeds typically run rich. It lets the positive pulse from the crankcase travel back into the intake and pick up fuel from the carb (carb doesn't care which way the air is flowing, it just sees a pressure drop and reacts). Then on the next intake cycle the air is pulled back in, with the normal fuel charge, plus the extra fuel that was pit out a split second earlier. Typically this shows itself more at low rpm with hard starting, blubbery rich running, poor throttle response, etc. Now, if the reed block is leaking to atmosphere, then it could be lean.

Now white build up (ash deposits) on the plug usually shows something is being burnt, transmission fluid, weird fuel additives, etc. My truck does it horribly on cylinder's 6 & 8 because of a spectacularly small intake manifold leak. Amazing how much ash can build even though it only pulls about 1 qt every 1800 miles. One day I'll get around to fixing it.

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