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jmkybf

Low Compression?

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I rode my '98 WR 400 last Thurs., and I went out to the garage today to try and start it up. I could kick through all 4 strokes now, and before I had to use the decompression lever to get past top dead center. It would only sputter for maybe 1/2 a second and wouldn't fire. Could the valves be out of adjustment or could it be something worse? I'm wondering why this happened when it performed flawlessly on Thurs.

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I would check for a sticky decompression cable. Maybe it's holding the valve slightly open, robbing compression.....

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The compression loss could be due to water or raw fuel in the cylinder, it can wash the oil off the rings and make them stop sealing. Try giving the bike a good kick, this can sometimes be enough to splash some oil up from the crankcase and return compression. If that doesn't work pull the spark plug and put a teaspoon or so of oil into the cylinder, this will work. The first time my bike did it I freaked out, only to find it's just another quirk of these high performance 4-strokes.

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The compression loss could be due to water or raw fuel in the cylinder, it can wash the oil off the rings and make them stop sealing. Try giving the bike a good kick, this can sometimes be enough to splash some oil up from the crankcase and return compression. If that doesn't work pull the spark plug and put a teaspoon or so of oil into the cylinder, this will work. The first time my bike did it I freaked out, only to find it's just another quirk of these high performance 4-strokes.

Good thought. Last fall, I parked my WR400 and left the petcock on. Of course, the float needle had some crap on it and didn't close, and in addition to making a mess on the garage floor, a considerable amount of gas made its' way into the crankcase most likely by flowing into the combustion chamber and leaking past. Anyway, when I kicked the engine over, there was very little compression. After flushing the engine with fresh oil and refilling, the compression came back with a few kicks. I have to think that the gas did indeed wash the oil off the rings, piston and cylinder wall causing the problem.

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Thanks for the information. That is what it was. I found out the hard way that my petcock doesn't work. I turned off the fuel and trailered the bike back to home after riding, but the fuel kept on flowing. All it needed was to set for a little bit and a couple of good kicks. Thanks again.

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