Hi Mark
Yeah the solenoid cuts of the petrol when it is running on LPG so it doesn't use both petrol and LPG although I don't think the engine would run when being supplied by both petrol and LPG.
On mind the solenoid overide tap means that petrol will always be supplied go the carb no matter what the display/control in the cab says.
My test would be switch the can into petrol modding. Turn it off turn the overide tap on the solenoid, then turn the van back on and see if it runs on petrol without cutting out, if it does chances are it will mean the fault is with the solenoid. On mine the solenoid would cut the petrol off fine but then wouldn't reconnect the petrol supply.
The solenoid on mine is on the petrol fuel line after the fuel pump and before the carb.
http://www.bertram-hill.co.uk/classic-car-security.html on this page there is a fuel cut off switch this is what your looking for and you can see the little plastic tap overide on it.
Sorry for such a long reply hopefully it will help I am no expert just starting to learn a little bit so sorry if anything is wrong or doesn't apply to your van. So somebody else might have a better solution or better way of explaining. Another thing to check would be the fuel filter to make sure it is clear of any dirty etc,
1.9 DG petrol and open loop LPG Autosleeper Hightop. 1988
