I sussed it - coolant level !, yes coolant level.
I got hold of the technical info on the SGI system and could see that it is pretty simple, so I concentrated on the ignition.
Having checked and cleaned all the ignition bits I started the engine and just left it on tickover until it switched to LPG. Everything was fine to begin with but after about 5 minutes I noted frost forming on the Vapouriser ! That's not right I thought. I know all about adiabatic expansion and all that stuff but the vapouriser has a water jacket to prevent that sort of thing happening.
On checking of course I found that the water hoses to the vapouriser were empty - and so was the expansion tank to the rad. (in fact so was the top half of the rad itself).
Filled up with water, bled the heater hoses (because the vapouriser is at the top of the circuit) and all is well now.
Unfortunately, I now have to sort the coolant leak and I suspect a blown head gasket as there is a lot of steam coming out of the exhaust even after the engine and exhaust has warmed up. No water in oil though.
Guff
PS still have the misfire on the overun but that may be down to known problems with LPG conversions, for which RPI Engines has the solution at http://www.v8engines.com/
