One thing to try to keep in mind: the spec for charging on the 750 is 13.8 volts max. The "regulator" system is actually a "voltage limiter" system that was designed to keep (1960s-style) batteries from overcharging and then drying out. In the early 750 the battery frequently overcharged and boiled acid out, putting it onto the frame, mufflers, and other parts. This necessitated a voltage regulator type change by the K1 that added a big (usually green) power resistor on the back of the regulator which is the "mid-charge" device: when the voltage gets above 13.2 volts the inner relay contacts pull open, which inserts this resistor in series with the field coil, reducing the charge to 7-8 amps (instead of 16 amps when the relay contacts are relaxed (NC contacts), at low battery voltage). If the voltage reaches 14.2 volts, the regulator-limiter pulls the relay fully closed, shorting out the field coil to itself, to reduce the alternator output to 0.5 amp until the voltage drops down again. This prevents battery overcharge and boiling.
If a "modern" reg-rec is installed because you want to have 1990s-like battery voltages, be sure to get a battery that can support it. The typical Bikemaster or Champion battery (Chinese-made) will not, and they will self-destruct quickly. An AGM battery can handle it, although they are expensive and often don't fit these bikes. I am finding that even the "maintenance-free" ones out there don't like having more than 13.8 volts pumped into them, either, so I have been making sure the regulator-limiters are properly set on the bikes I work with. The YUASA batteries meet the specs for these bikes perfectly.
If you have a Bikemaster or Champion battery, be advised of this: they have VERY CHEAP recycled, impure lead in those [piece of junk, Chinese-built] batteries. They will do some of the strangest things as a result: I have seen them fully charged, then as soon as the electric starter is touched, they drop to 10 volts. If the load is removed from them, the meter slowly climbs back up to 12 volts over about 15 minutes' time (without any charging). This speaks to VERY poor lead-acid absorption, which is caused by lousy lead and poor insulating plates (speaking as someone who made machines for years that build the world's finest batteries - Gates/Hawker/Enersys spiral cells).
