The Kubota, John Deere and Yanmar permanent magnet units are all similiar, perhaps identical, and supplied by a Japanese crowd called Kokusan-Denki. Here are the specks for the Kubota unit.

They state the following:

Voltage 12

Normal Output 150watt

Max current 14amp

Rotation Clockwise

Output rpm 4250rpm

Charge starting 1800rpm

Drive pulley 100mm

Alt pulley 58.5mm

Ratio 1:1.7

Salient points are that at the alternator pulley we seem to need 1800rpm to turn off the charge light (ie 12 volt or more) and that anything over 4250rpm seems unnecessary. I can vouch with smoke that 8225rpm is definitately not OK!! Have to keep in mind that the regulators for these units do not work by varying field strength in the alternator. The latter just keeps on going full bore and the regulator works by dumping any excess load to heat via the heat sink or when it reaches a limit there the alternator coils themselves get hotter prior to meltdown.