Sudden loss of compression

99WR400..After about an hour of hard riding I shut the bike off to talk about the next section of trail. About 10 minutes later I went to start the bike and it had very little compression. Took a quick look at the compression release cable and lever arm at the head, and it moves freely with no exterior sign of sticking. Getting ready to pull the top end to inspect, but I was wondering if anyone else has experienced anything similar. The bike ran strong right up to the point that I shut it down, with no sign of a loss of power in any way.

Sounds like its time to look under the valve cover. I'd check valve clearances 1st, if valve clearances look ok then do a compression check. You didn't mention if /how you got home. Did you push start? Maybe kickstarter failed, I've seen broken kickstarter gears before. Did the bike kick back during starting some time prior to your problem?

I have a 99 WR 400 and the same thing has happened a few times, but usually after sitting overnight. It was caused by a valve sticking from a bit of frost buildup on the stem of the open valve. I've also heard of a light layer of rust forming on the stem from condensation causing the valve to stick open.

Since your bike was running fine before you shut it down it may be a sticky valve, although a 10 minute shutdown doesn't give it much time to rust. :cry:

had a similar thing happen with my 426. Get one of your mates to push start you and you'll probably find it'll 'fix itself'. Ever since I've been using MotorEX Valveguard the problem hasnt occourd again

Take a good look into the carb and see if your slide plate has broken. Better yet, pull the slide out of the carb and look at it. Were your revs high when you shut it down?

My 98 WR did the same thing. Revving high at shut down and no compression when I went to start it up again. Started taking it apart and looked into the carb to find there was part of the slide plate missing. There is only one direction those broken parts go and that is into the engine. Pulled the head and found several pieces of the plate laying on top of the piston. The problem that caused the lack of compression was that one small piece had become stuck between the valve and valve seat causing an open valve all the time. Luckily there was no damage to anything else other than the carb but still very costly at $170 Cdn for the plate and seal.

Check the slide then the head and cylinder.

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