A little help with yz400f valve adjustment???


Im in mid process of checking and adjusting the valves on my 98' yz400f. (first time dealing with shim under lifter set up). I bought the bike used, pretty well used it seems. It sounded like I could hear the valves "slapping" a little bit at idol. So I figured one or more of them must be a little bit loose and I'd better get in there and do a valve adjustment. Im not mechanic, but have a clymer manual and few tools.

After rotating the engine to TDC on the compression stroke so that the punches on the cam gears are flush with the top of the cylinder head and the cam lobes are facing opposite of each other.... here are the measurements I recorded.

Clymer manual says intake should be .150 to .200 mm

the right intake is at approx. .115 to .120 mm (pretty tight i think)

the center intake is at .150 (in range, barely)

the left intake is at .140 (a little tight I think)

The manual says the exhaust should be at .250 mm to .300 mm

the right exhaust is at .265 mm (in range)

the left exhaust is at .280 (in range)

So it looks like Im going to leave the exhaust valves alone and just do the intake valves. I started to pull the cam retainers off (after removing the cam tentioner), but could not get a damn socket or wrench onto the bolt that holds the retainer over the center valves retainer. There is just no room between the bold and the retainer to fit anything in there.

So.... do I need some special tool, do they make a super skinny wall socket or something?

Also... should I get shims that would put the valve gaps right in the middle of the adjustment ranges or is it better to go more towards the larger side of things (.200 mm) to get more intake flow?

Oh... and when reassembly the shims, the manual says to us molybdenum disulfide oil? What the heck is that? Have to go to a engine builder to get that?

Sorry for the stupid questions, im just a shade tree mechanic whos a little nervous tearing apart the top end like this. Thanks for any advice :) :) :)

Just an 8mm wrench should fit, I don't see how you could be having problems?

As far as adjustment goes, the tighter they are, the more flow you get, not the other way round. I would just adjust them to mid range.

Moly disulfide oil is just assembly lube.

I don't use that stuff and have never had a problem. I just use motor oil.

Don't be worried about it mate, just don't drop any shims or cam cap retaining pins down the guts of the engine. Stuff a rag in where the cam chain is to stop stuff falling in.

What's the compression like in the old beast?

Moly disulfide oil referenced here is assembly lube. In a brand new assembly using new cams, lifters, etc., it's called for. Here, just paint the parts with clean motor oil.

On your choice of shim, bear in mind that the specified range is a mere .002" from tight to loose, so picking your clearance within that is splitting hairs. For practical purposes, any clearance in the specified range is good. If you do want to split hairs, tighter is better than looser, but stay within specs.

You may find some shims that are marked with sizes that don't conform to the count by 5 increments that the service shims come in. These are OEM assembly shims. If any marks are not readable, mike the shim CAREFULLY, and compare it with your measurement of a shim of a known size. You are working with small differences.

You should be able to use a 1/4" 8mm (or 5/16") socket on the cam caps with no problem. There is a half circle steel guide clip under the cap where it sits over the bearing. Don't drop it. On reassembly, the caps MUST be seated against the head by hand; DO NOT draw them down with the bolts. Torque them evenly in 3-4 small steps.

Recheck your clearances. Be patient. :thumbsup:

Thanks for the advice,

Ill just use some new engine oil on the shims upon reassembly. I have no problems fitting my 1/4" 8mm socket over any of the the cam retainer bolts, EXCEPT for the the one that is towards the center of the head over the center cam retainer.... its just too tight.... going to the parts store today and I'll look for the thinest walled socket I can find, and get this job rolling again. Probably need to get a compression tester as well.... good look'n out Mutu.

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