Hi John,
I have quite a fleet of belt-drive motorcycles, but not a four horse.
The first thing you need to know is the width of the belt: C section is 7/8" and D section is 1". Chain-cum-belt bikes like the big Douglas often use/need a 1" belt. If the belt is too narrow the risk is that the belt will pull to the bottom of the pulley, often after a little wear, at which point there is no forward progress.
Next step is to figure out if an endless belt can be used. Not sure if you can undo the rear seat stays to get the belt in? If so, there is nothing quite like an endless belt. Smooth and no trouble, ever. The only problem is getting the length right. A really good stockist might have belts in 1" increments, sometimes even 1/2", so if a C79 is a bit loose, a C78 might be perfect. They are cheap, too. Fiddly to fit because you need to slightly disassemble the frame, but you should only need to do it once.
Good quality re-enforced, cut to length, v-belt is hard to find, but works well with a proper, articulated belt joiner - not the rubbish flat plates that are sometimes supplied. The belt needs some canvas/fabric layers to make it strong enough so that the joiner screws don't pull out. Often endless v-belts have a cord of some kind at their core, and don't respond well to cutting then drilling for joiner screws.
Some people swear by multi-link belts, and if you can get one that isn't bright orange it would probably be fine.
Can I recommend "The Vintage Motor Cyclist's Workshop" by Radco. He devotes a couple of pages to belts, not to mention everything else to do with real vintage bikes like the 4 horse.
Cheers
Leon
