I have a 98 ford conovan, 2 litre petrol and LPG. Idel smooth on LPG but runs rough on petrol. Have replaced spark plug, plug leads and disitrubitor cap. Blew out carbie with air. But still runs rough on petrol. Please help.

 

Don't know if this will be much help , but how about we look at what is used with petrol that is not used with LPG ?
How long has the petrol been setting in the tank? Could it have managed to get some water in the tank? If the car runs alright at higher RPM on petrol , might want to check the petrol filter from the tank to the engine. What do you think?