03 YZ 450 compression issues

[Joel_D1124]

First off I wanna say the thumpertalk forums are the best and I've used them for my own bikes and also for helping friends. Thanks for the helpful info.

Here's the story. I have an 03 yz 450 that I bought in 06 and the bike has been nothing, but great. I've had no major issues but I don't ride the bike very much. In the past 5 to 6 years I've maybe put 60 hours on the bike. I don't get a chance to ride much.

Last year when I took my bike out to the Arizona desert and it was noticeably much easier to kick start. The bike however ran great with the only exception being in 4th gear the bike seemed to be missing with a jumpy sensation like it wasn't firing right and not being near the rev-limiter.

I checked my valve clearances this morning before work and the intake valves and exhaust valves were all .05 mm tighter below spec for each. I was surprised to that all the valves were .05 tighter with no variance and was wondering if this was normal? From the posts I've read its seems people only have one or two valves that are off spec or valves with different variances? The cams looked good and the dots lined up with the TDC and the top of the head and the cam chain seems to be tight? I plan on putting all the valves back within spec near the tight end, but was wondering if this could be a cause of compression loss as the valves should be shut tighter if I understand correctly? Everything I've read regarding the issue seems to talk about valves being loose and open causing loss of compression during kick start. I've maintained the basic maintenance of changing the oil, air filter, etc, but this is the first time I've opened the engine up.

Time for a possible top end rebuild? I unfortunately do not own a leak down tester, but may be investing in one shortly. Any suggestions?

Edited March 7, 2012 by Joel_D1124

[grayracer513]

Leak down tests are only useful in a comparative way, in that there is no fixed standard for how the tester should be made. What that means is that unlike a compression gauge (which, incidentally, is useless for these bikes) that will show X amount of compression wherever it comes from, a random leak down tester will not produce a predictable result in terms of percentage leaking on any given engine. You need to either check yours when you're sure it's healthy, or check several similar bikes with the same tester to get a baseline. If you have a compression loss, they're extremely useful in identifying the source(s), though.

Regarding the valves, that does strike me as odd that they are all so uniform. That's generally what you see in factory builds; all the valves right at the low limit. It could be that you are measuring them incorrectly using too tight a "feel" on the gauge. A clue to whether they have ever been adjusted is to pull the lifters out and check the shim sizes. Keep the lifters organized so you can replace them in the same position they're in now. Service shims are available only in .05mm increments, while factory shims will normally be several odd sizes in .01mm increments, 187's and 178's for example.

About the "miss", you probably have a different problem than you think. If the bike jerks like that only in one gear, and does this only when pushed harder than a certain load level (that is you can make it do it by opening the throttle beyond a certain point and make it stop by backing off), it more than likely the transmission. The trans works by having one gear on splines slide over against the gear next to it and lock it to the shaft by means of squarish lugs, or dogs, on the gear sides. These are slightly undercut so the gears bind to each other under a load, but as the lugs round over and wear, the opposite starts to happen, and the gears push apart under a load instead. Since the trans is shifted by a drum cam, the shift fork can't be pushed out of position like it can in a car, so instead of jumping out clean, the fork bends under the force until the lugs separate, then slams the gears back into engagement again. The good thing is it's much cheaper to fix than it is to do a valve job.

[Joel_D1124]

Thanks for the info. I checked and re-checked the valve clearances cuz I thought the same that maybe I forcing the feeler gauge. I even readjusted TDC a couple times and got the same readings. The bike is still apart down in the garage and I may recheck my work before reshimming. I guess my question would still be could having all tight valves cause any loss in compression from the valves and seats not lining up properly from being too tight?

As for the compression the reason I think it's possibly low was I distinctly remember that this bike was not easy to kick over when I first owned it. Now it kicks over easy with much less force.

I was hoping the 4th gear issue was simply a carb problem or something of that nature but after reading your explanation it makes sense and sounds like my issue. I've contemplated doing the wr trans swap being that I mainly ride in the desert and have read and researched many old posts on the subject in the past. This might give me a good excuse to do it having the cases open if I can find a decent used one. It's do a complete tear down and rebuild the entire trans and engine as its probably now needed on a 10 year old bike or put that money towards a newer bike?

[Joel_D1124]

One last thing. Dude how do you know so much about yamahas and dirtbikes? I know you're a moderator and what not, but your wealth of knowledge is quite impressive. You know more about yamahas then yamaha knows about yamahas. I'm definitely a casual user of thumpertalk and its usually when I have a issue with a bike and google it and the thumpertalk forums always come to the rescue and having your knowledge is an awesome inexpensive resource. Just wanna say thanks as I'm sure many already have.

[grayracer513]

It's not unusual for an older bike to have less cranking compression than a fresh one, of course, but the situation with all 5 valve sinking the same .05 mm is just odd. Normally, it's much more random, but it could be seat wear, too.

If you need the head reworked, it's going to be expensive, and as cool a bike as the '03 was, the next generation ('06-'09) was way cooler. If it were me, I'd move toward getting a newer one, with as low as sale prices have gone. I'd probably fix the trans and sell yours otherwise as is.