660 timing marks; crank & cam

[rdjc]

hi,
i'm currently helping a friend get a 'new' 660 motor into his skorpion. i took the head off as the exhaust port was full of swarf from the previous owners 'maintenance'.

now i can't remember which marks we used to line up the cam and crank.
i can find the tdc mark on the crank, but don't know where the three 'dots' on the cam have to be when at tdc.

any help would be appreciated. the yamaha dealer said they don't deal with bikes this old, how very helpful.

Rdjc

ex skorpion sport owner (R695JGK)

[Bill Jurgenson]

hi,

The two screws on the cam gear are vertical, i.e. aligned with the cylinder centerline and with the cam lobs down, i.e. away from the rockerarms.
The small markings on the flywheel rim are obvious; when the flywheel has been pulled, the slot for the woodruff key is also "TDC" so to speak, i.e. up in a straight line thru the cylinder center with the two screws of the cam gear. The crank timing gear has a centerpunching iline with one of the teeth to line up with the key slot. You never have to remove thse cog, however, as long as you don't want to replace the left main bearing. You can't pull it without a special wheel puller anyway.
If you take off the timing gear, be careful when tightening the screws. Yamaha gives 20Nm =14footpounds. Those ears on the cam break off easily. Use Loctite.



This is an entirely different timing chain setup (Slipstream Rollerchain conversion), but the pics show the orientation.

[rdjc]

the bike is now up and running again,
thanks for the help.

rdjc

[Skorpion660]

Hi Guys

Re Bill's explanation below re cam timing

The two screws on the cam gear are vertical, i.e. aligned with the cylinder centerline and with the cam lobs down, i.e. away from the rockerarms.

With everything at TDC the next step is to fit cam chain tensioner (cct) but when you fit cct and insert spring and chain is tensioned it moves the cam off of TDC, this must affect timing or am I missing something?

Terry

[Bill Jurgenson]

using stock items, there is no way to set the cam timing beyond having it in the correct position. Otherwise it will be one or more teeth off and that is immediately visually apparent. The chain tensioner does move the cam slightly, but not that much.

To do it right, you would have to find lob center:
http://www.muzzys.com/articles/lobe_centers.html
and to do that, you would need a chainwheel with slots like in my picture.
The stock cog of the XTZ engine is too weaken if you enlarge the holes to slots. There is an OEM cog which can be used, but which model has slipped my mind. It, however, does no have the provison for the decompression thingy.

bottom line: line up crank and cam as explained. If you want desperately to check, insert a screwdriver into the hole for the tensioner to push the chainguide forward while holding down the cam in its seats. You can see how much (or little) the cam moves due to the tensioner.