I set ignition base timing to 10* on LPG powered forklifts and set plug gap to.027" with standard ignitions. K25 has coil-on-plug which will allow for a wider plug gap,.035 IIRC. Set intake and exhaust valve clearances, base timing, and set up fuel system. Verify primary pressure at vaporizer test port at 1.5-2LBS with engine running, pressure must hold this value with engine shut off. Check operation of slow lock valve on VFF30 unit by unplugging vacuum line with engine running, engine must shut off after all fuel in transfer line to vaporizer has been depleted. Set idle air bypass screw on mixer for highest idle, set WOT valve on vapor inlet fitting to minimum rich setting which will hold full load high idle speed. Something in the exhaust stream is getting hot enough to ignite stray hydrocarbons so you need to address the temperature of the exhaust gases and the presence of unburnt hydrocarbons in the exhaust. Does this truck have a catalytic muffler with a working pyrometer gauge?