Unusable on petrol, fine on LPG

Hi guys,
Please give me some suggestions on this.
1995 4.6 with single point LPG system. Attempts so far to fix have been new spark plugs and a serving of redex!
When starting up on petrol it's very lumpy and the engine will run so rich it chokes itself to death and brings a tear to the eye of anyone unfortunate enough to have got a whiff of the fumes! If you try and drive it off on petrol it dies. If you start it and switch over to LPG it runs sweet as a nut! Also to confuse things further if it's warmed up having been on LPG you can then switch back to petrol and apart from running slightly rich it is ok?
Any ideas would be most appreciated!
Thanks
Andy

sounds like you have either injector problems OR **** fuel how long has the fuel been sitting in the tank?

Hi mate,
I run half a tank of redex injector cleaner mixed with oetrol through over a period of a couple of months because it's not drivable on petrol so it was just going through when starting her up! Then I filled it back up with half a tank of normal petrol about 3 weeks ago and all is still not well

forget spark plugs, leads, etc ,if it will run fine with lpg but not petrol then your problem is most likely with the temperature sender for the petrol ecu.