Belt pulleys are all over the spectrum due to the differences in the diameter of the pulley, and the application specifications. There are ratings in the manuals for each at the pulley. The RPM at the PTO is again a ball park figure in practice, but specific when rating a tractor's HP at its rated PTO speed wide open throttle. The PTO speed in practice is probably between 600 and 450 depending on the tractor, load, and throttle setting. Nobody cares in practice with these older units in the field it is pull the throttle and make hay, or throttle back, and do it "gently" all an operator's call. To your question, three answers. If you want it exact, use a strobe tach (buy, borrow, or rent one) and be sure by flashing it on the front pulley till it is stopped in its tracks, read the LCD numbers. You can set both High and Low idle that way. The second is to use a shaft tach on the PTO to set low Idle and High idle. The ratio is the ratio between the rpm on the Serial number plate, 1600 for instance, and 540 rpm This is the ratio in the drive for the PTO. 540/1600. So at low idle figure the ratio, and make it work. However!! at the slow speed the shaft tach might not be very accurate. A digital shaft tach will be better, but it should be in the middle 50% of its range to be considered a critical number. A tach dwell meter used on the ignition (distributor wire usually) will also give a reasonable number, but not great. The last answer is to set the idle down as low as it will go and not die/stumble/cough/or run rough with out possible smoothness, then take it up a bit and call it good. High Idle is found when the throttle lever pulls the governor lever all the way to the stop, and the carb is opening all the way when the governor demand it. (unless it is broken, or someone has tweaked it, it will be just fine. If it has stock sized tires, an M will go between 16 and 18 MPH in road gear wide open. JimN
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