2008 YZ450 - locked her up

Just wanted to say thanks to gray and others that helped answer my questions. I got it all buttoned up last weekend and have put an hour's worth of heat cycles on it so I think it's all good. Taking it to the trails this weekend for a final break-in. I'm thrilled to have it running again.

Edited by nucular

If you've ridden it for an hour, break-in's over.  Go ride it.

I'm having a slight issue with the bike now and I would appreciate any thoughts you might have.  I took it out and put about 2.5 hours on it the other weekend.  Maybe an hour in, I decided to check the oil level and found that it was barely registering on the stick.  I thought maybe I messed up a ring or something so I added some to bring it up.  Then I was afraid that I was burning oil excessively so I checked it a couple more times over the day and had to add a little more oil each time to get it up to the full mark (maybe 50 ml each time).  I was going to pull the cylinder and check the rings but I first drained out the oil only to find that it had 1400 ml of oil in it.  I guesstimate that I added between a quarter and a half quart that day so that adds up.  The question is, why was it not registering?  I searched several threads and found issues about the return pump not working, the external line being plugged, and even a couple about a hole in the case near the balancer shaft.


So far to troubleshoot, I have cranked the engine with the flywheel cover off and oil is definitely coming from the pump.  I have also removed the external line to make sure it is clear.  I can blow through it into the tank easily with the dipstick out but i can not blow through with the dipstick in.


I do not see a hole in the case but I am not exactly sure where this has occurred for others.  I put in a brand new oil pump as part of the rebuild.  I did not open it to ensure there is a pin on the internal gear but I would imagine there is.  My next step was going to be to remove the pump and take a look.  Tonight I am going to pour 30 ounces of oil directly into the tank via the vent and see if it holds and for how long.  It did not do this prior to the rebuild so I think something is different.  I wondered if my yamabonding of the case halves wasn't the greatest and the seam that seals the tank was poor allowing the oil to quickly run back into the crankcase.


If it is an issue of it draining out of the tank too fast, it seems odd (to me) that filling it up to 1400 ml eventually made it show on the dipstick.  That fact makes me think it's more of a return issue.

Well, now I am thinking I may just be crazy.  I poured about 25 ounces of oil directly into the tank and it held a full reading for a good 10 minutes at which point I stopped checking.  I filled it up to total 950 ml through the filler hole and ran it for three minutes.  During that time I checked the oil check screw which seeped properly.  I checked the oil level after three minutes and it was right at the full mark.  I did notice that it ran down a little quicker after only a couple minutes - maybe because the oil was thinner?


Now I am wondering if the oil was so thin that day that when I pulled out the dipstick, I couldn't get a good reading on it or maybe when the engine is hot from lots of riding that it is so thin it drains back into the case quicker?  I run Mobil 1 Racing 4T.


I haven't yet checked the oil pump - I thought I might try another experiment of riding it for 30 minutes or more to see what happens.  I may try a different oil too.  One other thing I wondered - while I can't blow air into the tank through the vent hose, I can blow air easily into the clutch side case.  Is that normal?

There's one more possibility I haven't seen anyone bring up: A leak between the case and the low pressure check valve.  That would allow oil leaking past the pump rotors to drain into the crankcase, and yes, more so when hot, to be sure.  I run the same oil, BTW, somewhat interchangeably with Amsoil MCF.


That's the oil seal with the steel ball behind it.  If that's damaged, set too deep, if it was reused, just this kind of thing might happen.  You should also loose some oil pressure, though, too, so it's a worry.  If the check valve spring, ball, or seat is damaged, you'll get rapid oil migration after shut down, but no loss of pressure.  Could just have a piece of gasket  or some such caught in it.


The oil tank volume in the crankcases should behave like yours does.  The tank is not open to the atmosphere anywhere, and only way pressure blown into the tank through the venyt hose can get out with the dip stick in the bore is past the oil pump one way or other.  Going the other way, through the hose into the crankcase should be easy because the crankcase is vented to the outside air.  The purpose of that hose is to connect the tank with the atmosphere via the crankcase so that the air pressure always matches up. 

Well that is definitely a possibility then.  I did re-use that seal but I never removed it.  I just cleaned up the cover and put it back on after the rebuild.  Maybe I should just pull that and replace it and see what happens. 


How would I know if I am losing pressure? Check the pressure screw when the engine is hot?  It seems odd that it only did it after an hour or so of riding and not right after the oil change.


What is the purpose of the seal/circlip that sits in the crankcase cover just above the check valve?  I removed/reinstalled that one for sure. (edited to add - appears the idler shaft might rest in there? if so, probably not a problem.  I thought maybe there was an oil path under there)

Edited by nucular

Just to close the loop on this - it was the low pressure check valve. I ran it pretty hard this past weekend and when I checked the oil, it was right at the full line. It must have also been doing it to some degree even with cold oil. Looking back at my service history, I was routinely adding just over 1 quart when doing an oil change and this time it took exactly 1 quart.


What is the purpose of the seal/circlip that sits in the crankcase cover just above the check valve?  I removed/reinstalled that one for sure. (edited to add - appears the idler shaft might rest in there? if so, probably not a problem.  I thought maybe there was an oil path under there)


That's the seal that transfers oil from the oil filter in the case cover to the crankshaft.  Very important.  It also has to be installed the right direction (numbers facing out at the installer), which in that case means that the lips of the seal face out toward the crank.  This seems backwards if you think about it too much, because it means that the high pressure oil will cause the seal lips to "blow down" and it will leak.  That's true, but it's also what the engineers want to have happen.  The crank spins at 11,000+ RPM, which would put a lot of pressure and friction down on the crank nose, and quickly wear into it.  With the seal turned around, the oil forces past it, creating a no-contact condition between the seal and crank.  Because the circlip contacts the seal lips as they give, the whole assembly becomes a kind of high pressure limiter, "regulating" the oil pressure as well.

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