Similar to Pole gaps on any Magneto. Crankshaft end float May affect wear there. Same with the distributors and axial shaft play will increase the gaps there as will arcing over a period of time. Rotor and caps may Leak current when damp. especially. 300 RPM cranking speed is not uniform. An idling engine nearly stops approaching TDC and when starting behaves the same. Don't forget spark Plug are usually gapped at about .032" when supplied.(.8 mm). Leaving them as supplied will certainly affect how the Motor starts max Jab gap is .022", with recommended reduction to.020", if experiencing Hard starting. A dirty Plug Insulator may also cause tracking but that's more Likely under Load. It takes More Volts to fire a Compressed Mixture. Commence cranking with the throttle Fully closed. That gives the starter the easiest life as there is practically No mixture to compress and resist the starter. Older Starters may also need attention. Worn Brushes. Commutator out of round Armature Polling and solenoid Points cooked. Double check all battery connections and engine earthing. There should be no Heating of any terminal and if the Battery is even a BIT SUSS chuck it, and always have it fully charged and big enough to do the job. IF the Battery is a long way away maybe UP the Wire diameter to reduce Voltage drop. Nev