Ford 4.2L industrial LP Backfiring S&S Tug?

Two trucks, both fueled by LPG. Backfiring when accelerating heavly. Manual says either burning rich or Ignition issue. New plugs and wires. No vae leaks. computer controlled, no way to adjust fuel air mixture or adj timing. Had factory service tech hook up laptop, stumped him allso...Just a shot in the dark, any one ever run accross this problem. Will be highly impressed If I acually get an intelgant answer.