are you using aviation gas?
its ok but you should understand that an airplane and a car work in different enviornments,
yes av gas will work in just about any internal combustion engine,but an airplanes takes off and climbs to its altitude and the general out side temperature is at or below zero F,
even in the summertime it may reach 20 or 30 tops above zero f
, and an airplane does not turn a high rpm.a typical prop job loafs along at 2 or 3 thousand rpms,
of course there are some high performance airplanes that are exceptions but normally the engine just kinda drones along,
not really what car does, so the fuel your are using is not really designed for automotive use
and modern gas builds up more varnish and deposits much faster , you might consider a lead additive or using actual "off road " race fuel, i mix it with pump gas in the summer and it helps with vapor lock,and does contain some lead hence the "off road use only",i dont need the lead as i have newer valves not needing lead,but for you if it is an older engine you might see if you can get some race gas it might run a little cleaner
sorry for the lecture i just have some literature about fuel and thought i would pass it on