The 4000 rpm check is two parted.
The first part is to confirm that the engine will run smoothly on the Main Jets and not just at idle.
The second part is to see that the engine will run if either Mag were to fail.
You know from the past just how much the rpm loss is at 4000 RPM and it should be the same each time.
You can run the Mag check at any rpm you wish, what you will not know is how much rpm loss to expect as normal at the different RPMs.
Why does the RPM Drop???
Under normal operation, both Plugs fire at the same time and two separate flame fronts propagate through the cylinder consuming all the fuel.
With only one plug firing, there is only a single flame front propagating through the fuel and it takes slightly longer to consume all the fuel.
This will have a different effect on the power produced depending on the speed of the engine.
In either case, unless a plug is misfiring, all the fuel will be burned and there should not be any unburnt fuel in the exhaust causing any After-Fire events.
The engine is designed to run on either MAG alone at all RPMs, if it wasn't, there would be no sense of having dual ignition in the first place.
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An interesting side note...
The Fuel injected 912iS and 915iS engines use an ECM module that does NOT fire both plugs at the same time.
The Plugs fire alternately on every other cycle.
This allows the ECM to identify which plug in which cylinder might have a problem and remove it from the firing sequence with no power loss and the only indication being a warning light on the panel.
When performing an ignition check on these engines, the ECM fires the same plug on every cycle resulting in no rpm loss.
And because the ECM also enrichens the mixture slightly because it thinks there is an ignition failure, The engine RPM actually INCREASES during the ignition check.
