This is definitely a hack and is just barely useable. But, if you want to experiment with propane powered small engines it is a good place to start.
This project was born out of necessity after I put a cut in the fuel tank of my Trimmer Mower. If you search on YouTube there are lots of people that have done similar things.
What you'll need:
1) A Lawn Mower - Preferable Old :)
2) A Propane Torch with a knob, NOT a trigger.
3) A length oh hose
4) A propane tank
5) A hose that fits over the end of the torch after the flame tube is removed snugly.
Step 1: Prepare the Torch by Drilling Out the Jet
First, remove the flame tube (atleast that's what I think it's called.) This is the part of the torch where the air and gas mix.
Next, you should see a Jet, it is a little thing that screws into the torch. While it has a hole in it, the hole is so small that I couldn't see it myself. Go ahead and remove it.
Get the absolute smallest drill bit you have (I used a 1/32nd inch bit, and it was too large.) and drill it out carefully. The surface of mine was paper thin so it was more like pushing through aluminum foil then drilling.
After drilling, re-install the jet.
Step 2: Push Hose Over Torch End and Ziptie Tank to Mower Somewhere.
Step 3: Remove Air Filter and Zip Tie Hose About 1/2" Above Carbuerator.
I had to remove the air filter on mine to get it to work properly. This is very bad for the mower so you probably want to make sure it's an old mower :)
Once the airfilter is removed, place the hose within about 1/4" of the entry of the carbuerator and zip tie it in place.
The reason for this, is that if you place the hose directly into the carbuerator the engine will only pull in propane when you try to start it and not oxygen. Both are required for combustion. If you put it too far away the engine will not pull enough in.
Step 4: Open the Torch Valve a Crack, Start the Mower, and Adjust the Mixture!
1) As lpg (liquid petroleum gas) evaporates it gets REALLY cold. (See Boyle's Law). The torch isn't made to handle this, so ice will build up around the torch valve and so you'll need to repeat this procedure anytime the engine starts to run rough.
2) This method doesn't meter the fuel in proportion to the air flow very well. What this means is that as your mower's governor opens and closes the throttle as the mower comes under and is free from a load (respectively) the propane flow is not regulated. The hose being slightly away from the carbuerator causes just a little metering, similar to the way the Wright brothers' engines worked. Where a vapor fuel source was placed in the air path and as air flow increased it would draw in more fuel vapor (the difference being that an airplane with a fixed pitch prop pretty much has a constant load depending upon rpm and altitlude (and the wright brothers weren't exactly setting altitude records).
Adjusting the Mixture
Adjusting the mixture on pretty much all gasoline engines is the same. Start the engine. Now close the valve until the engine starts to slow down and stall (too lean). Now open it until it starts to slow down and stall (too rich), and set it in the mid point. That's it.
Cut the grass :
Because there is little or no fuel metering if you run over tall grass the motor will probably die. So cut slowly trying to keep a constant load on the mower.
Step 5: What I Would Do Differently Next Time.
This guy: http://www.aircooledpropane.com/ has the best setup I've seen for DIY propane. He has his propane experiments on lawn mower documented so you can see his progression from something similar to this to his current ultra reliable Garretson regulator setup he has today.