Thermosyphon cooling doesn't work too well in Florida. I have had two 4/4s Climax and Standard engined (still have it). I also have a Matchless Trike.
I believe the temp of the air and that of rad is too similar to get good movement and hence the lack of cooling. Only when everything is perfect, will it work, and that situation is fleeting. Live with it or install an inline water pump (and most likely, an alternator.)
Cheers,
