Hello FA'ers!

This a first-time post so I'm sorry for anything that shouldn't be done here!! Just let me know...

Anyhow, I have a Toyota 42-6FGU25 with the GM Vortec 6cyl (LPG) that is giving me fits. The truck starts and runs fine under NO load. If I put it in gear or use any throttle angle above idle causes it to backfire about once every three seconds--but only loaded. Idles and revs fine and holds full throttle with no issues (while not moving of course). The backfires are coming from the exhaust. There are no trouble codes stored, spark and compression are good. If ANY load is applied (even tilting forks) I get the backfires. Wires and cap look good, etc. I know that nobody has messed with the engine--I guess it just started one day. As a gas engine enthusiast my guts tell me its either a timing or mixture issue tho I can tell by distributor bolt that it has never been moved/re-timed. Might this be advance? And does anyone know what these should be timed at?

Thank you SO much in advance for your help; you folks are so kind and knowledgeable and I am happy to be in your midst!

Peace,

DoctorRon