Compression issue (02 426)

If you have by chance seen any of my threads regarding my WR, you'll be familiar with it's recent history.

To make a long story short, some time back we were adjusting the valve clearances and the half-circlet thing from the intake cam fell in the engine (we didn't realize this at the time - bunch of newbies to engine work).

Ran the bike for a race and a few rides and then swapped out the exhaust cam for the 03 yz exhaust cam. One ride later the bike died.

When fixed, it was discovered that the circlet ripped apart the stator and flywheel. Got that fixed, raced it - heard a familiar noise (more pieces of the circlet in the engine still).

Took it back in to the place that had originally cleaned it up (got the circlet out and replaced the flywheel etc) and had them search for more pieces they neglected to get the first time. They found more pieces and did a good job of getting everything else out. Judging by the amount of metal retrieved, we're certain it's been completely eradicated.

But while at the shop, the mechanic said we didn't have enough compression. Was reading at 90lbs, and he said it should be at 140lbs.

First question - is that right? Should we be expecting 140lbs of compression?

Second question - this obviously makes suspect the idea of a top end, but I have a question regarding the 03 yz exhaust cam. It has auto-decompression. When we put that in, we took the old compression cable out and just left the spring and bolt in the engine where the cable used to be. Could that be letting out compression? Is that right - or should we have put something in there to plug it up? Do you think we need a top end? I really can't do a top end at the moment money-wise after all we've been through with this thing recently. I'm hoping there's something we needed to do extra on the 03 yz cam that I wasn't aware of.

First, you can't get an acurate compression reading by just kicking the bike over because the cam does not spin fast enough to pull the pin in that opens the valve. You need to tow it in 2nd or 3rd gear and get a reading that way. Yes. there is a rubber plug that you can get from Yamaha (about $5)that plugs the hole where the decomp lever goes through the head.(03-05WR's). These have been unreliable for some. There are billet aluminum plugs out there that have a new seal and they are retained by the same bolt that retains the decomp lever. I got mine through TT store. That bolt can strip very easily. When removing the seal, I found it easiest (valve cover on) to use an allen wrench to push the seal out from the inside using the bolt retainer hole for access. The best route is to just pull the valve cover. If you don't find a round metal spring in the seal then it shot into the head somewhere. The same thing happened to my friend and he is a mechanical super genious. Same result wasted the stator. I'm not sure what the comperssion reading should be.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now