2005 Yamaha F90TR 90HP Outboards Factory Service Manual, Page 83Get this manual
Power unit NOTE: Use deep socket for this procedure Do not turn the camshaft when loosening the drive sprocket nut
Removing the timing belt and sprockets
CAUTION:
Do not turn the drive sprocket counterclockwise, otherwise the valve system may be damaged Do not turn the drive sprocket or the driven sprockets when the timing belt is not installedOtherwise the piston and valves will interfere with each other and be damaged1Turn the drive sprocket clockwise and align the hole on the retaining plate with the projection on the cylinder block, and check that" marks and on the driven sprockets are aligned