Bikes and Beards makes them run on butane and propane—sort of.
While gasoline is cheap now, it's only a matter of time until prices skyrocket again. It's one form of the butterfly effect. If a butterfly flaps its wings in the Pacific Ocean, the price of gas goes up. Or something. Anyway, when that time comes and modern civilization turns into a Mad Max apocalyptic hellscape, can you run your motorcycle on other fuels? Bikes And Beards says yes.
In this test, they disconnect the stock fuel systems of several different bikes and instead try various forms of liquid petroleum gas, specifically butane and propane. They're still fossil fuels. They still combust. What will it take to make an ordinary motorcycle engine run on these extraordinary fuels?
First, they try butane, the same thing my camp stove runs on. They create a highly sophisticated fuel delivery system, engineered to exacting specifications, out of a Gatorade bottle, some IV tubing, and a syringe. (I was kidding about the "sophisticated" part.) Their first victim is an excellent choice, the Kawasaki KLR 650. Not that I'm biased or anything.
Bias aside, the KLR is about as simple as a motorcycle engine gets, with one carburetor and one cylinder. The stock exhaust note sounds like a lawnmower engine because that's essentially what it is. They disconnect the stock fuel system, run the engine completely out of gas, then stab the syringe into the intake boot ahead of the carburetor and let her rip. Amazingly, the KLR starts and runs with no additional modifications—at least, until it doesn't. After sitting a bit it will start again, but not for very long. They theorize that perhaps the 650cc engine is too big for their jury-rigged fuel system.
They try again with a 140cc pit bike. The results are the same, including not running after a while. They downsize again to the cheapest scooter on Amazon at only 50cc. It starts and runs for quite a while. In fact, it runs even better on butane than on gasoline. Again, though, it works until it doesn't.
Next, they switch to propane. This is a little more difficult and requires rather precise metering of the propane into the fuel system. Although they are able to get the engine to run, they can't regulate the flow while in motion, making this system impractical. They tried to cannibalize a propane regulator off a generator, but they didn't make this work either.
They leave us with the question of why won't this work for more than a minute? My theory is that the carburetor itself is still jetted for gasoline. These other fuels have different combustion characteristics and require different jetting to run correctly. Engines already exist that run on propane, so what they are trying to do is definitely possible. It will just take a little bit of rocket surgery with jetting to make it work correctly.
Of course, since I'm not a rocket surgeon, I could be completely wrong. What do you think the problem is?