Question about 2003 KTM SX 125

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No top-end power.

I checked the compression of the cylinder and I only got 39 psi. I know you're supposed to check it while the engine is warm, but I didn't have that option, so I checked it cold. That seems kinda low. Does anybody know what the stock compression should be? I looked but I couldn't find it in any of my manuals.

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  • Dick
    Dick May 11, 2010

    If you don't have at least 150 psi you need a rebuild.

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  • 4 Answers

Not for sure ,,but I would think around 100 pounds or more,,,sounds like rings or reed valve

Posted on Apr 13, 2009

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Spark, gas, and compression. And check timing. You've done the major work. Now you have to make sure you have done it right. By getting it running, you know. I'm pretty sure you can rent a fuel pressure tester and a cylinder compression tester at some national parts stores, Autozone or O'reillys, maybe others.
I don't understand your expression "...a little miss from bottom end" -an ignition miss, or a bottom skip or noise?

Start with checking ignition-a blue snapping spark-what you want to see at all cylinders. Then check that gas is getting into the cylinders. Fuel pressure and injector solenoid pulse. Then all you need after that is compression -good even compression in all cylinders-. Timing is right, spark timing is right, it should crank, er start.
Just one thing about compression, on a cold engine you're just looking for gross irregularities, you can't depend on the readings to be very accurate. A good compression test should be done on an engine at normal operating temperature. It gives a more accurate picture of your engine condition. As long as compression is over 100 psi per cylinder on a cold engine might be a general guideline, I suppose, but take that as only opinion, not fact..

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If it runs that well at higher revs, you could just drive it as is-what harm could it do? If the engine craps out, it was just meant to be! It's your call to have this engine rebuilt (again), swap in a better engine, or just run it into the ground till the engine finally gives up. But I would do the leak down test just to know what is up in that cylinder.
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