As the engine increases in RPM the spark timing needs to become more advanced. On old bikes you had to set the timing manually (usually by turning the left grip sort of like the throttle). Then they came out with mechanical timing advance where spinning weights would move the points system and advance the spark. Now they use an ECM where a magnet on the crank shaft sends a signal to that box. The box calculates how fast the engine is spinning and sends juice to the coil when it thinks the spark should fire. With an ECM there are a lot fewer moving parts than with a points system and mechanical advance so they tend to be more reliable. I usually unplug the ECM before welding on the bike so I don't fry it.I look forward to your build. Last winter I built a Japanese tracker with scrambler style pipes:I've gotten lots of good feedback about the pipes. I'd like to do something similar on a magneto ironhead soon.