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Tagosa

decompression valve sticking

9 posts in this topic

I have a 01 426F and have been having a problem where the decompression valve will stick open every now and then. Which then causes the bike to not have any compression and it won't start off course. I have been able to get around the situation by "flicking" the decompression lever or by pushing the bike in first gear until it comes unstuck. After it comes unstuck the bike starts and runs fine and will restart with no problems. Has anyone else had this problem? Any suggestions?

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Your bike does not have a decompression "valve."

It has a decompression mechanism, that indirectly operates an exhaust valve, and this mechanism is apparently sticking.

The cable at the other end of the decompression lever terminates at the decompression arm down on the front right portion of the head. There is a spring there that should be returning the mechanism to the "off" position when you release the lever.

So, we can isolate the problem to three areas, the lever on the bars (unlikely), the cable (most likely), and the arm (could be the culprit) by testing each part for some kind of snag.

My guess is your cable is getting in some kind of a bind that prevents the spring loaded arm from returning to its "home" position. I would disconnect the cable from the lever so I could check each part.

To detach the cable, remove the lever bracket, this should allow you to create enough slack in the cable to remove it from the lever (just like you would a clutch cable). This in turn should give you enough slack at the arm to take the cable off from there as well. Now see if you can manually move the cable, arm, and lever, then you will find which is sticking.

The only thing I can think of that would cause the arm to bind is it has somehow worked it's way loose, and either pulled it's spring out of position or is binding on the small bolt that secures the arm in the head.

If the cable is worn, very dirty or routed improperly it may bind.

If the lever bracket has been squashed the lever may stick.

Hope this helps.

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Thanks for your reply.... I knew I should have explain what I've done so far. I've done everything except remove the actuator arm that opens the exhaust valve. The spring actuator arm moves freely returns to its "home" position properly. I've even removed the cable and operated the lever by hand and it moves freely. This is what makes me think that the exhaust valve itself is hanging up. Otherwise the actuator wouldn't move freely. On a properly functioning actuator you can feel the arm make contact with the valve and feel the tension from the valve spring. When mine sticks there is no resistance. I plan to remove the actuator arm and check that out but time hasn't been on my side. I'm sure most of you can relate....

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Just make sure the cable is not binding or tightning up when the bars are put to the locks right to left.

I found my 00 was actually opening the decomp when I turn hard to the right. The cable was not routed correctly...

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Originally posted by Tagosa:

Thanks for your reply.... I knew I should have explain what I've done so far. I've done everything except remove the actuator arm that opens the exhaust valve. The spring actuator arm moves freely returns to its "home" position properly. I've even removed the cable and operated the lever by hand and it moves freely. This is what makes me think that the exhaust valve itself is hanging up. Otherwise the actuator wouldn't move freely.

Hmmm. Then maybe your exhaust valve IS sticking. That is unusual, but not unheard of. How many hours are on the motor? You may consider removing the head cover and having a look see (and checking the shims and clearance while you have your head in there).

Be careful with the small bolt that keeps the compression arm’s shaft in the head, it only catches on a few threads and so it is easy to pull them out. I have also found that, for whatever reason, it is also easy to damage that particular oil seal.

Hope this helps.

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I had the same problem with my 98 yz400. I would have no compression ocassionally when trying to start the bike. Then the next minute it would be back. I just got done taking the cylinder head off last week. One exhaust valve was stuck open an 1/8 inch because of a stuck lifter. I found that the decompression acuator arm had made a small nick on the top edge of the exhaust valve lifter. This in turn caused the lifter to nick the lifter bore when the cam opened the exhaust valve. The valve and spring were in spec. Luckily we have a machine shop at work and they were able to blend out the nick in the lifter bore. I have a new lifter on order and should have everything back together soon. I think the damaged was done by using the decompression lever to bump start the engine or the lever getting hit during a fall with the engine running.

:)

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The damage was probably caused by the decomp shaft "riding" or touching the valve bucket.

During normal operation the buckets rotate to keep the wear even. If you don't have enough slack in the decomp cable, or it has a kink in it, or something else is causing the decomp shaft to hold the bucket in place then it will eventually tear up the bucket.

As long as the shaft does not make constant contact during operation using the mechanism to bump start, kick start, or even engine brake, is perfectly harmless.

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You know what i had this as well on my 400. Turn your handlebars back and fourth and watch the pivot on the cylinder. I bet it is opening and closing like that if so re adjust your cable.

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I've had a similar problem with my 00. I'd have to say it's the small bolt holding in the acutuator arm. This bolt is so sensitive compressing the arm and it causes the stick your talking about. It took me several tries, heli coli, and red loc tight to solve the problem. As stated before be extemely careful tightening that bolt because it can cause serious grief.

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