Here is simple howto, to run your lawnmower off of propane gas instead of gasoline.

It was much easier to run off of propane than I thought it would be. First I pulled off the gas tank and carburetor. Next I got a length of hose. I took one end and put it just inside the intake manifold. I then got my propane stove and took of its burner. I attached the other end of the hose to the stove's gas outlet, then used some duct tape to try and hold everything in place.

At this point I basically have an unregulated propane source going straight into the mower's intake manifold. Surprisingly the thing fired up on the first pull. It never starts that easy on gasoline. This is a pretty cheap lawnmower and it is usually a real pain to start.

The engine ran very rough. It seemed fuel starved. The propane stove doesn't put out that much gas. I imaged if I had a larger valve or drilled a slightly larger hole in the stove's outlet, that it would run better. I also didn't regulate the amount of air going in, so the fuel mixture was probably running pretty lean.

But my point was just to see if it would work, and I was surprised that it did. It only took 20 minutes to set everything up. I doubt I will be mowing the lawn using propane power, but it wouldn't take much more work to get the mower running full tilt on propane.