a serial number (and in this case, the type of engine, if you know it) might help us get to what the possibilities are.
Also when you say "I just did a tune up" can you define how much of a "tune up" you did? Did you do a tune up that included replacing the distributor cap, rotor, spark plug wires, gaping and replacing the spark plugs (and at what gap?) and doing a compression test while the spark plugs were out? (what was each cylinder's compression, "wet" and dry?) what did the old spark plugs look like? What type of LPG system does it have? HAve you checked the pressures in the LPG system?
Actually sounds like spark plug wires or firing order, would be my first W.A.G. but it is a BIG and -really- Wild A. Guess.
like Robby The Robot used to say in the 60s TV series; "Lost in Space";
"NEED MORE INPUT, Will Robinson"