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?

Sounds like unburned fuel vapor is going into the exhaust system after it is shut off and then something hot is igniting it. Not sure what the timing should be set at but most forklift motors are 4-7 degrees BTC at idle. Too advanced timing will make the spark plugs and exhaust too hot. Maybe check timing and **** if needed, use a colder heat range spark plug, and turn the idle speed down if possible.

We are having this happen on 3 trucks which were converted to petrol by a company in Europe. The problem generally occurs when they are hot and switched off after high revs.
Does anyone know what the ignition timing should be for these when running on LPG?
Thanks,

Hi
Had same problem, turned out to be the distributor that was faulty and breaking down,have a look at that first maybe you've got the same problem.

is this "backfire in the exhaust system or in the intake system? does it do it every time?
I think you have a leak in the exhaust system, and maybe a bad valve.