Valve clearance adjustment and cam chain tension on tuned YZ450

After finding to my delight that the 350SXF valves are in tolerance I turned my attention to an overdue check on the tuned 04 YZ450F.

 

First gotcha was getting the motor to TDC as this tuned engine has a non-standard timing side crankcase cover. No inspection ports for the timing mark or for turning the engine over. You can see what I mean in the photos. It does, however, have a slotted, threaded, inspection cap on the valve cover (non standard AFAIK) between the cams. A tentative prod with a little finger suggested the cam chain was possibly loose. You can also see it has a manual tensioner.

 

No problem to begin with, back to how I used to do the DOHC Gilera thumper. Plug out, valve cover off and in to top gear and use the back wheel to rotate the engine until the inlet valves had closed and TDC was reached. With care I could see the punch marks on the cam wheels end up where the should be. The bad news, at least a couple of clearances are significantly wider than spec. The possibly worse news. The cam chain hangs between the sprockets with lots of slack. I know there is a guide built in to the cover but should it be this loose at TDC with it at TDC and the cover off? Remember this has a manually adjusted tensioner so presumably it will be no tighter when running?

 

Without a crankcase locking bolt KTM style I'm going to have to drain the oil (well that was going to be a given before long anyway) to remove the timing side crankcase cover so that I can  check the crank doesn't move when it comes time to replace the cams after shimming.

 

Before I get that far though, the manual refers to removing the carb to do the valves. Checking them seems possible without (but after withdrawing  the hot start plunger to give better access), but is it essential if shimming is required? I've not had to shim valves before as the Gilera Bialbero though DOHC uses followers with screw and locknut adjusters. So I'm being very cautious.

 

DSC01273.JPG

 

DSC01277.JPG

 

The chain should be nowhere near that loose.  One of the things wrong with manual tensioners is that they require nearly constant check/adjustments to be correctly set.  The addition of the inspection plug at the cam cover was likely to allow adjustment with the engine hot, at which time the chain should be at about zero true slack to slightly taut. 

 

As to the draining of oil, not so if the engine can be run prior to removal of the left cover.  Run the cold engine for about one minute to scavenge the sump, then shut it down to cool for 30 minutes.  Some oil will spill as the cover is removed, but the amount won't be significant.

Many thanks for that. Willpower have just confirmed it should be about 1mm when cold as a starting point. The cold running tip will be very useful next season if the clearances need adjusting more often. It only had a total of about an hour and half running this season but to be fair my lad has hit the rev limiter more than a couple of times. Remembering my old 510 Husky, I'm hoping the chain hasn't worn too much and can be adjusted OK.

 

I'm used to a rubber belt with idiot proof pull spring tensioner (so long as you remember to check it) . The Bialbero belt is really quite easy to change (has to be on age no matter how many hours use) anyway. 

 

Now I need to study the manual again carefully before going any further.

With a single cylinder engine, you can actually feel TDC by rotating the crank left/right within the 4-5 degrees on either side of it.  Try it, you'll see what I mean. For the purposes of valve adjustment, and even cam timing by the marks, this is generally accurate enough to work just fine.  Only in the case that one was trying to degree out the cam timing to analyze or adjust the lobe centering, etc. would it be necessary to be more precise. 

 

Remember that you need 22.5 degrees of crank rotation to move a single tooth on the cam gear.  I don't think you'd be likely to make that big an error as to where TDC is.

I see what you mean. I guess a long rubber toothed belt with quite a distance  between the cam wheels is actually easier to get one tooth out  (and people have) than a chain and I never got that wrong in many belt changes and several head gasket jobs. At least the access on the YZ  is less restricted than on the orange bike.

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