TW........sparkies come in 2-variety's HOT and COLD. Too HOT and the gas will self-ignite (ping) before its supposed to, lack power and melt the top of yer piston. Too COLD and yer plugs won't burn off the natural carbon by-products of combustion and FOUL out and quit sparkling.
The 1939 low compression N-Engine was designed for un-leaded 70-octane gasoline and cool Champion H-10's. Modern no-lead gasoline has additional additives that condense on the plug business end and FOUL the plug so a HOTTER (temp range) is recommended, such as Champion H-12.
Many find enny Champion to be un-satisfactory in use because of early fouling and short life. And many Champions have built-in anti-radio static resistors which cut-down the available sparkie volts to jump the gap. NOT GOOD with marginal ignition systems.
Howsomevers; we recommend the HOTTER AutoLite 437's (gap = 0.025") with extended tip for self-cleaning and less fouling. HOTTER sparkies such as the 437's are also recommended for oil-burning worn engines too. Always use old fashioned COPPER-CORE sparkie wire 'stedda' modern carbon-core string sparkie wire .......Dell, yer self-appointed sparkie-meister