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Compression question

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I've searched and can't find anything useful. I have a compression Guage which I've benched against an air compressor to ensure it's accurate. I tested my buddies kx250,over 100 hours on top end, very hard to start, no power. Got 90 psi, which I figured made sense. Tested my ktm 250 which has unknown time on top end, starts 1st/2nd kick, lots of power, and got 112 psi. Put oil in the spark plug hole, still got 113. Question is, does my bike really need a top end or is there something I'm not realizing? How does it run so excellently on such a low compression? I know the average for a 250 is around 175psi.

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does the kicker have quit a bit of resistance or does it go down real easy ?  if it has quit a bit of resistance and the engine has lots of power like you say then i wouldnt worry much about that gauge.  if you want to know what condition your top end is in then pull it apart and measure piston for any collapsing and check all the important clearances, ring condition and gap, plating condition etc etc. thats really the best way. nobody ever does it that way because they generally dont have the tools or knowledge. hell even a monkey can use a psi gauge and the reason why everyone uses one. but often times it just gives useless numbers

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I would say mine is at least twice as hard to turn over by hand as the kx. I will redo the top end on both bikes this summer anyway, but the100psi thing still doesn't make sense to me. Thanks for the help, by the way

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Im not sure about dirt bikes but i know on my 2 stroke jet ski, 110 psi means its almost time for a rebuild.  It ran completly fine at 110 psi but when it dropped down to 105 it refused to start. Some motors are just finicky about compression i guess.

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I did my last rebuild on my 300 at 125 hours... once you do the top end you will get a good boost in power. . I have found every 100 hours if you trail ride is a good rule of thumb...

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I have 8 years on my 2006 250sx top end. It has 200psi.

I have 3 years on my 2011 250sx and it has 210psi.

I live at 60' elevation and it was 60° when I did my last compression test.

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 took a couple pictures of the piston on the kx. There are no scratches or wear that you can 'feel' inside the cylinder, and the vertical lines on the piston shown in the picture are BARELY feelable at all, if at all. Does this look like a worn down piston to you guys? There's no blowby or discoloration, and I'm not incredibly experienced reading them. Thanks

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Not at all, but you should get all of that water/coolant off of the piston and cylinder before throwing it back in. I would just re-ring it.

Edited by -Biggins-

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I just rebuilt my 2011 ktm 250xc it had 148psi compression and the only reason i rebuilt it was because it seemed really week on power compared to a friends exact same bike...I put a new piston and rigs in it now it has around 190ish and runs totally different. I wouldn't just ring your bike i would put a piston in also just for good measures.

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What to do next? I have a 2005 KTM 300 MXC, bought it a year ago. It lacked power and had low compression so I did a bunch of work on it. New piston, rings, spark plug, reed valves, repacked the silencer, burned out the pipe, honed the cylinder, rebuilt the power valve, disassembled and cleaned the carb, new jets, new float valve, put in thinner base gasket. Got it all put back together and it started on the third kick...rejoice! But it still has low compression, about 90 psi, using new Harbor Freight tester, and it does run well. It sorts feels boggy, and it surges, and the air box sounds different from what I remember 2 strokes to sound like. It's not zingy at all, sorta sounds like a muffled zing. Hard to describe, but sounds like a trumpet with something covering the end, miffled and boggy. I understand I may have to tinker with the jets, but it's the low compression and muffled sound at tbe air box that have me scratching my head, sounds and rides the same as before doing all that work to it. Your help is appreciated.

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