Yes, but 4.2 may be a little high. Did you see the Bubba post? And remember, voltage out is going to depend on voltage in, so it is going to be different running vs non-running and at different rpms running.
Another older post:
What you are looking for is ~ 3.5A through the points. This is calculated by using Current (A) = Voltage (V) divided by the Resistance (R) in the circuit, A = V/R. Measure the coil resistance (I will use 1.0 ohm for an example) and add it to the ballast resister value (say 1.4 ohm), so total resistance would be 2.4 ohms. Battery voltage should be ~6.4V, it will be less when cranking the engine and higher when being charged by the generator, ~7V.
For this example, A = 6.4V/2.4 ohms or 2.67A, that is too low which indicates too much resistance in the circuit. At 7V the current would be 2.92A, so still on the low side. If you use the same coil the resistance would remain at 1.0 ohm (again, this is just an example you would need to measure your coil), then you need to change the ballast resister.
If you used a 1.0 ohm ballast resistor the current would be A = 6.4V/2.0 ohms or 3.2A and running with the generator charging A = 7.0/2.0 or 3.5A.
These are just example numbers the higher the system current the hotter the spark but the quicker the points burn and the capacitors go. Like everything it is a compromise, 3.5 to 4.0A is a good range to shoot for.
