Shimming Woes

[dingleberry]

I'm having trouble getting my head around this. Are the rear valves open and closed the same duration as the front?

[fret not]

Yes, same cams.

[Rikugun]

dingleberry - it's not the lift or duration that's in question but rather the rear cylinder valve timing. POD is suggesting that valve opening and closing events (expressed in crankshaft rotation degrees) of the rear cylinder is different than that of the front. There is a formula using those numbers that gives a "lobe center" value that is typically within a range. The closer the lobe center value is to either extreme of the range gives certain engine performance characteristics as outlined earlier in a table compiled by POD. Lobe center formula shown 2/3 down this page: http://www.muzzys.com/articles/lobe_centers.html

POD recognized Yamaha went to the trouble of marking the cam sprockets for front and rear, not just intake and exhaust. Why would this be if the cams are the same front and rear? He also noticed that when installed, the locking pin orientation  - which coincides with the lobe - was different between the front and rear cylinders relative to the valve cover gasket mating surface. He then hypothesized that the front and rear cylinders cam timing were not the same. Moreover, they could be easily changed by simply reversing the cam sprockets. Since there are separate intake and exhaust cams, the lobe center value could easily be altered. Contrast to a single cam with both intake and exhaust lobes which would have to be replaced (with different lobe-center grind) to accomplish the same thing.

[dingleberry]

Ok, getting it now. That link was quite helpful, thanks.

[Rikugun]

No references I could find in the archive.  the cams are well documented in the service guides, except for timing, which is understandable as it's fixed by the pin.  Precise timing can be easily worked out with a degree wheel (I have instructions on how to make one on your printer somewhere.

POD, have you given the degree wheel idea any further thought? I guess the proper way to do it would be with the stator cover removed using the rotor bolt but what a PITA....  Had you thought of another method?

[The Prophet of Doom]

Using the rotor bolt to mount the degree wheel is the usual way, with a piston stop down the spark plug hole to measure TDC, and a dial test indicator on the shim bucket to measure valve actuation.
 
If you were to trust the etched TDC markings (misplaced on the majority of bikes) then backlash is not such a big hairy deal, you could I suppose use a degreeing wheel on the cams or rear wheel (in gear) or even on the socket you turn the crank with.  Set to zero as you pick up the backlash and only turn one way.

Me, I'd just do it the "proper" way  Cam timing is too critical to risk getting it wrong.  It's not such a pain in the botty, but there's the issue of a $20 gasket and an oil change I suppose.

[pinholenz]

Hi there POD. Is that pesky 400 running now or did you end up buying a Corolla?

[The Prophet of Doom]

It's sprung an oil leak in the valve cover I'll take a look at tomorrow.
I couldn't bring myself to get a Corolla, so I got one of these

[pinholenz]

Love your work!!

[dingleberry]

Whoa, look at all the smoke coming out the back. Either that citreon is rooted or it's the getaway car from the Rainbow Warrior.

[The Prophet of Doom]

Love your work!!

Its a funny old world.  Some people get to make a great living driving stupidly fast cars, others hardly get by giving hand jobs to Ukrainian sailors.  Somehow it doesn't seem fair.

Whoa, look at all the smoke coming out the back. Either that citreon is rooted or it's the getaway car from the Rainbow Warrior.

Mine smokes like that.  I thought it was normal for French cars.

[Re-Vision]

Renault's don't smoke, don't start either.      BDC

[Rikugun]

If you were to trust the etched TDC markings (misplaced on the majority of bikes) then backlash is not such a big hairy deal, you could I suppose use a degreeing wheel on the cams or rear wheel (in gear) or even on the socket you turn the crank with.  Set to zero as you pick up the backlash and only turn one way.

I have considered the degree wheel on a deep socket but wondered about repeatability and indexing it each time it was removed. Do you happen to know how many teeth are on the cam sprockets?

[Re-Vision]

Do you happen to know how many teeth are on the cam sprockets?

Mine has twenty-eight.    BDC

[The Prophet of Doom]

I have considered the degree wheel on a deep socket but wondered about repeatability and indexing it each time it was removed. Do you happen to know how many teeth are on the cam sprockets?

I tried that, but there's a lot of slop in the bolt - socket.

28T on the XZ400 also, 42T down below.

[Rikugun]

OK, thanks to both of you.

[dingleberry]

I tried that, but there's a lot of slop in the bolt - socket.

Man, I have a lot of trouble with that too!

[dingleberry]

Ukrainian sailors have 42 teeth? Crikey

[Walt_M.]

POD, are you saying the cam drive sprocket is not twice the tooth count of the driven sprocket? Or is there another gear in there I don't know about?

[dingleberry]

The bottom gear (42T) has an inner gear that's part of it (21T) that drives the cam chain to the cam sprockets (28T). The bottom gear (42T) is driven from the crank shaft gear (28T). I haven't done the math, but I guess the Yammy guys did to arrive at a 1:2 ratio.