if this is an older (pre 2004) unit, I would take a timing light to check it, and would also want to insure the vac advance is working and not stuck, so not only disconnect and plug the vac. line to the distributor and check where the timing mark is, but to reconnect it, and see what it is doing when you rev the engine, (should advance the timing).
Well I work for myself, and I am extremly new to this type of work. I will pull the plugs, and recheck the gap. They are the 44 plugs. I belive I read somewhere that they should be gapped at.35? Is that right? So you don't think it is a fuel (propane) problem. I know it is hard to say over the net, and not hands on. Just looking for direction after I gap them weather it helps or not.
I would not blame the LPG tank or LPG flow, as this would be worse at higher speed, rather than lower speed, kind of like when you run out of gas.
I would make you pull the plugs and gap them, if you worked for me.
if firing was off you might not notice at higher speeds. same with poor compression on only 1 cylinder. it might feel like a bad spark plug wire would. which has a tendency to "smooth out" when not under heavy load.
Cap,rotor,plugs, wires. I didn't gap the plugs. If firing was off it would have issues at all throtle speeds. Its an older 4.3. I will have to look for the numbers. We have tried a couple different tanks with the same result performance wise.
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"