I seem to recall reading about my T250II that if the charging system and battery weren't REAL good, you'd get enough voltage drop for the left side points that it just had a weak to no spark. Something about the way things are wired up stock or whatever. I had the same problem on the left side and cleaned all contacts and new battery and all I know is that problem went away. I think they are magneto's to boot so not sure why the battery would make a difference, but did what I had read and it worked. The key thing that I read was the left side suffered first, odd.
2006 HD Sporty 1200C, 2006 Triumph Scrambler, 2001 Ducati M900, 2001 Honda XR650L, 1994 HD Heritage, 1978 Honda CB750K / sidecar, 1977 Moto Guzzi 850 LeMans, 1976 Honda CB750K, 1965 Honda 305 Dream, 1973 Norton 850 Commando, 1971 Triumph Trophy 650, 1970 Honda Trail 90, 1970 Triumph Tiger 650, 1973 Honda Z50, 1984 Yamaha Virago 1000, 1981 Honda Passport 70, 1970 Suzuki T250, 1971 Yamaha RT1 360