I noticed something just now, when I increase the throttle the tachometer doesn't rise until both cylinders fire. From what I read in the workshop manual, the tachometer is linked to the ignition system on the top (rear) cylinder somehow. I assume I can concentrate on this cylinder now - does anyone know how the tachometer gauges rpm?

Coincidentally, the rear cylinder doesn't fire until the RC valves move to their first position at 2000 rpm....
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'87 MC16