crsmith

I have recently rebuilt a 400 for my 72 galaxie LTD. it has a 780 holley, holley streetmaster lowrise single plane manifold, a 268DEH cam, TMI 9.5 to 1 pistons and 302 closed chamber heads with boss valve springs and 4v valves.
I initially did the aggressive break in on petrol, but after a trip to Newcastle and back where it gobbled up 20 litres of petrol (gave the missus a fright as it ran out 10 meters from the home town servo!!) I decided to refit the LPG mixer/tophat so I could afford to run it. it is a 425 impco and mixer. I now come to my problem. I managed to start it on LPG straight away with no adjustment to the timing and proceded to take it out for a fang. It still starts really wel.l
To my delight it went just as well on gas as it did on petrol. But now when I switch it back to petrol it runs like a hairy goat. you can nurse it up to revs,but give it a hit on the loud pedal and it misses and farts until the load goes off it. I have even tried disconnecting the fuel shut off and rehosing it straight in case there was a restriction there,but to no avail. There seems to be enough airflow as it goes well on the gas, but I don't understand how when it is switched over to petrol it is now so poor.
could it be the top hat mixer assembly after all and how do i fix it? i could just disconnect the LPG again and refit the aircleaner assembly and try it to prove a point, but again I don't get how it can have such adequate air flow on LPG and be poor on petrol. Again I have not adjusted the timing in any way. Help.