Too bad you did not need a flywheel while I had a new style I was trying to fit on an early engine.
No one wanted it and I sold it cheap on eBay.
Water under the bridge...
If swapping the coils made no difference and each cylinder has good spark then I and the compression is equal and good on all cylinders I would pull the carb adapter and take a good look at the reeds.
“And the plot thickens”. Found something interesting during the seafoam run. The boat we put this motor on had a temp gauge so I put a sender on the motor, there were other concerns at the time so I kinda forgot about testing/calibrating the temp gauge. Unfortunately this lead to ignoring it completely. Someone else babysat the motor for the seafoam run for me. On the last cycle I checked on it and notice standing by the motor the temp gauge was somewhere less than 100 degrees, probably 75. The hose was supplying water at I would guess 60 degees. So I put my hand on the power head just behind the thermo housing with it running and the head was cold. The motor was finishing the last run cycle and burning the remaining gas so it had been running over 20 minutes. My first thought, there was no thermostat. There was a thermostat but it had pinch in its seal small shreds of rubber, looked almost like drill fillings. I could blow thro the thermostat. Cleaned and tested with hot water and it did open fine. Also tested the boat temp gauge the same way and it work. Reinstalled and test ran, the water temp never came up. Thermo is 130 degrees. Any idea what’s keeping the motor from warming up if the thermo is working?
I am ordering a new thermostat.
Also I did try one of the 2 tachs that did not work on the regulator output on the AC output and it did work.
Still need to inspect the reeds, waiting for the snake cam.
There should be a spring under the thermostat housing that holds the thermostat against the horse shoe shaped spacer and rubber thermostat seal. If any water is leaking past the thermostat seal, you may get a low reading.
If everything looks OK, I'm not sure I'd worry too much about the temperature at this time of year. Considering the incoming water temp was low, what you're seeing might be normal. Since the cooling system is not a sealed pressurized system like in a car, it's not going to behave exactly like what you see in a temp gauge in a car.
The most important thing is that the motor is not over-heating.
As far as the tach issue...did you check it right at the new regulator, or were you using any of the existing wiring harness.
When ever I install the regulator that has the two red wires, and the wiring harness only has a single connection, I always tie the two red wires from the regulator together. Don't know if that makes any difference. I would think that they should be the same.
As far as the charging voltage, you won't see any increase in voltage until the motor gets up to about 1500 or 2500 RPM for about 5 to 10 seconds. Then you should see anywhere from 13.5 to 14.5 volts DC at the battery. If you are seeing just fully charged battery voltage 12.5-12.7 volts DC while at 1500 to 2500 RPM's, then I would suspect the charging system is not working. Either the charging voltage from the stator to the regulator is not good, or the regulator is bad.
There is a spring and the rubber seal looks fine and for sure the motor is not overheating.
The wiring in the boat had previously been replaced. Before and after replacing the regulator a second tach was installed directly to the regulator and did not respond. When the second tach was wired to the AC it did work. I need to try the tach in the dash connected to the AC. I will check the wiring also.
OK I understand the charging situation and haven't monitored it at a condition as you said where it should be charging, the battery is up and in good shape.
If the reed valves all look good I guess I'm back to the flywheel.