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i've seen them plumb a line into the exhaust from a small propane bottle and have a sparkplug behind it. then in the cab you push a button to release the propane and ignite it with the sparkplug.
when i was spraying my wet shot i shot flames iv seen someone put a spark plug in the tip and had it hooked up some how to a coil...anyways he was running pretty damn rich and it worked lol...