3.5 v8 run on gas brilliant, won't run on petrol at all?

Just bought a 3.5 efi v8 as a half runner, will run lovely on gas (once you can get it to switch over) will not fire on petrol and will cut out as soon I switch it back when it's running on gas, think its electrical but I'm not very electrically minded, any ideas before I call an auto spark? thanks!

Have u checked that petrol is reaching the cylinders?

intank fuel pump dead

Check fuel pressure, if OK look at injectors. Dual fuel cars can have problems with poor quality petrol deposits clogging injectors.

My last car (dual fuel) HAD to use BP or Shell or would run like a pig

Found it! Broken earth wire to in tank pump thanks guys!