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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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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Chevy troubleshooting

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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99 Mazda Premacy automatic with rough idle. The car developed this problem shortly after having the radiator and head gasket replaced. Now idles extremely rough at a max of 500/600rpm but runs perfectly...

Sorry to say I would tend to agree with the mechanic at this point. Too bad all of that didn't get sorted out before spending big bucks on the head gasket repair. The cylinder with 90 psi is probably the reason for the rough running-makes me wonder if the mechanic used due diligence. Was the head checked for straightness of the head to block mating surface? It doesn't take heat very long to warp an aluminum head, and subsequent head gasket failure. Or it could be an exhaust valve not seating well, or a worn cam exhaust lobe on that cylinder. The bottom end has not "now failed", probably pretty worn piston rings; it is just questionable that the low cylinder is from the bottom end or the top end. A leakdown test of that cylinder would pinpoint the problem-I think I would suggest you get that test done-it is more precise than a compression test. It may help you decide-the leakdown test definitely will find the reason for the low compression in that cylinder.
Even so, if you got that one cylinder back to snuff, you would still have an engine with 130 psi as top compression-that is a bit on the low end, probably below the minimum compression that Mazda would recommend. It would still run good, but you might notice a loss of power, compared to an engine with 150 psi or more.

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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If cylinder compression is below 120 psi as an average, you should be looking into a rebuild event-rings and valves need attention. If any cylinder is below 100 psi, it may not even fire, certainly will produce no power, and could be a head gasket failure, very worn rings, or valve problems.
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and i have 106 psi and the servicebook says it should be around 140-190.. thx for quick answers!If your checking the compression right I suggest pulling your top end , checck your rings maybe replace them, also put a micrometer to your cylinder refer to your service manual for the specs. If all that checks out order a high compression top end gasket kit,I use a kit made by Moose Racing, you don't neeed to modify your engine with this kit. Iput one one my 2000 KX250 2stroker, and could notice a better pull in the engine.,,,

Nov 10, 2008 | 2004 KTM EXC 125

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