F350 Starting Issue

Ford F350 / 1989 / 180000 / 5.7

Propane truck with a manlift on back. Truck ran fine but started to run/start rough for about 20 minutes before dying altogether. Inspection revealed damaged coolant hose at vaporiser which was replaced. Engine now cranks very well and always sounds like it is engaging with the RPMs increasing or decreasing as engine is throttled but never catches. Solid spark off coil, both propane regulator and vaporiser stripped and cleaned. Controls on manlift bucket that shutoff/start engine checked and bypassed. Hoses checked for vacuum leaks, replaced ignition module, all new spark plugs, distributor cap and rotor fine, checked all fuses and fuse able links.

Any thoughts?