hi can anyone help my p38 will only start on gas and runs sick on petrol and kicks out black smoke when reved.thanks

Its probably because your sh--ty gas system has upset the long term fuel trims of the petrol ecu,because its not fuelling the engine correctly itself.
Other than that the oxygen sensors are failing to switch down,so the ecu will widen the injector pulse as far rich as it can go.If the sensors wont switch down to 0v,(rich) you end up with a VERY rich mixture.
What sort of gas system do you have ?

hi it is a romano multi point about 6 years old

If its that old when was it last serviced ? The vaporiser will need a kit,the filter will want changing and I expect all the injector solenoids will be shot.
If its more than 18 months since it was done then it needs sorting.
After the gas is serviced the adaptive values will want a reset on the petrol ecu,the oxygen sensors should be checked on live data or better still an oscilliscope.Lastly the gas ecu should be adjusted to ensure it isnt operating too far away from where it would be running on petrol.
When thats done you have a fair chance of it running properly.

great advice i removed the plenum to find a turret loose changed that and it was still the same i took the oxy sensors from my other 4.0 let it idle for a while and hey presto my computer has gone from 9mpg round town too 14.7 and runs lovely now im going to sell my thirsty 2.5 dse.thanks for the advice