you need to backtrack the usual…red cable on the starter post …the other being a ground.
whatever system you have….. high current starter loads are normally handled by a relay (selenoid) vs a simple key switch contact whose contact surfaces ar to small for high amperage loads.
you should see 12v on the starter post when activated ….. maybe a few volts less while it spins. make certain the battery is top shape.
3-4 volts at the starter means the inbound voltage is weak due to a resistive path… or the starter is defective and drags the voltage down…..if so I suspect smoke should appear 🙂 anytime. seems you had some
in conclusion backtrace from the +12v starter post …the wires are there just follow them back to the battery which should have clean post contacts … you could also have corrosion inside a crimp somewhere between battery and motor… both gound wire and battery wire play a role so clean everything up… or replace them
if you do see 3-4 V again.. .twist , poke bend all the power wires you can access…. one may have a bad crimp or has oxydized internally at the crimp point.
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