No compression on start-up?

I have a 99WR bought in Nov 01, yesterday could not get compression before starting, we checked the decompression cable/lever seemed ok. After starting the bike normal starting rest of the day. Is something sticking open? one of the valves or decomp, or piston rings. Hope to avoid serious damage the bike runs great,maintained well, after 12yrs on two strokes I like four more.

This is not the fist time this has happened. I would be surprised if there is anyone that it has not happened to. You did not say if you had gotton it started. Most of us usually kick it a bit and it will start. Once started the problem disappears. There has been No resolution of this issue from Yamaha or anyone else thus far. It happens to me mabe once or twice a season, mostly when it's cold outside. Many of us have avoided reoccurances by ensuring that the bike is put away with the piston at TDC....That seems to help.

Bonzai :)

I have the same problem on my 98 YZ. I found the same topic on the YZ BB and sent a message and got the below reply. Hope this helps...


I rode it that way for several months. It would simply kick through with "no compression". I would still be able to get it started, and once it warmed up, I wouldn't have that problem for the rest of the day. I would be able to start it up going through the normal starting procedures, and it would have plenty of compression from that point on, until it sat for awhile (overnight). After sitting, it would seem as though it didn't have any compression, and I would start over.

When I finally got around to tearing in to it, I found the exhuast valve on the right side (the one that the decompression arm pushes) was actually hanging open. When I tore the head apart, all the components looked good (valve guide, valve spring, valve, bucket). Somehow the spring would get a little cocked and missalign the bucket. Since all this stuff resides in a "hole" in the head with pretty close tolerances (you'll see once you take the valve cover off), any missalignment would cause the bucket to rub against the side of hole. There was actually a burr that the bucket would hang up on as the valve returned to the closed postition. When it was warmed up, it was fine. But if the engine was cold, the first time you kicked it over the valve would open, the bucket would hang up on the burr, and the valve would remain open until it actually started and warmed up. I simply removed the burr, and replaced all the valve compenents in that hole for good measure, even thou!

gh I could find nothing wrong with them. I haven't had any problems since.

Hope this helps out, let me know if I wasn't too clear....


Thanks for the great advice, I will look closely at the valves at my next clearence check, the no compression happened for the first time last November in cold weather riding in Massachusettes the second time was last week, all the time in between the bike ran good, my last valve check was in March but did not look for other problems other than clearence measurements. I have read hundreds of posts on this site, the help here is as invaluable to me as the manual, sometimes I feel I should be mailing checks out to people for the things I learn, hope to return the favor some day, thanks again everyone.

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