I have one, that I should have seen coming: With the jackshaft kit, you can't use momentum to start the gas engine, you have to kick start with the pedals while rolling, or pedal and start while somehow gaining speed. I have an electric hub motor on the front end, and used that to start the gas engine, but that is still a momentum start, and I couldn't do it with the jackshaft kit. You HAVE to kickstart it with the pedals, which is a pain. And coaster brakes are out, too. At least I saw that coming.
The third time I rode with the jackshaft, I failed to get the engine started the first try, and lost too much speed, so I ended up pedaling the bike, with all the weight, in addition to trying to kickstart the engine, and it broke the sprag clutch, and ended the project completely.
The bike will roll even with the clutch engaged, and the engine off. In that state, only the pedals are connected to the engine, and that's the only way to start the engine. The only reason to disengage the clutch is to pedal without the engine, so you aren't pedaling the engine, but it doesn't effect the bike's ability to roll because of the freewheel, and you can't turn the engine with the wheel to use momentum to turn the engine. Hard one to explain. But this was a big deal for me, weather you realize it or not, you mostly use momentum to start the engine. You don't try to start the engine whilst pedaling away from a dead stop, do you? No, you get moving first. With the kit, you have to either do the same thing as starting while pedaling, or get more speed going then coast faster than you have to pump the pedals to kickstart the engine. It's a pain, and the first time I accidentally tried pedaling and starting at the same time, it broke, that's a huge load.
Just to be fair, here, I bought the cheap, Chinese kit, not the real SBP kit, which is a lot better quality. But I'm glad I did, I just didn't like it, since it made my bike entirely too hard to start, where it was effortless before, I wasted less money this way.
