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When starting a motor the emf from the battery creates spark (ignition system), but it also moves the engine's pistons (starter).
My question(s): 1. Which requires more power - the ignition system, or the starter? 2. If my battery is low (say, 11V) which is more likely to NOT work properly? Is there a general rule, or does it strongly depend on the engine? 3. Is there an easy way of checking if a given system is getting enough power (e.g. measuring voltages on the fuses)?
If your motorcycle uses an electric starter (as opposed to a kick starter), the starter will draw far & away more current than the ignition system - like perhaps fifty times as much. A bike with only a kick starter only needs a tiny battery, while one with only an electric starter requires a big one.
When battery capacity is too low, generally both the starting system and the ignition suffer... and GENERALLY they both fail at about the same level of low capacity - as long as the starter is capable of cranking the engine, typically the ignition system is also capable of delivering a spark sufficient to start the engine.
One very quick check, if you think your battery capacity is dropping off, is to hang a meter ON THE BATTERY POSTS (not the fuses), then try to start the bike. If, while cranking, the battery voltage drops below about 75% of nominal (9V for a nominal 12V battery), then the battery's capacity is too low and it requires charging or replacement.
