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 f@rt 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.

here is your starting point.

It's got to be worth a lash, but again as i wrote, there should be enough airflow at lower revs on petrol considering there is enough to suit LPG at over 4000rpm. At least at the moment I can leave it on LPG and run it around and save me some money.

the carby was never designed to have a gas mixer sitting on top of it.

None of them do,but they still run dont they? If it was that bad they would never be fitted surely?

it's posible that the mixer has upset the tuning of the carby. Not all this stuff is bolt on and forget.

fair call. I will be running it on gas most of the time so it will be fine until I take it to my garage for its greenslip inspection. i was going to get them to check the LPG tune anyway, so maybe they can have an informed fiddle. It would just be nice to know if it's something stupid I could fix.

As an update, some one has suggested that the diapraghm in the mixer (the bit bolted to the carby with the air cleaner on it) could be sucking closed causing the stutter down low and it only comes right once the air flow is up there keeping it open. I removed the afore mentioned diapraghm on the weekend and this indeed seemed to fix it. The only thing I haven't done is actually check the vacuum at the manifold as originally with help from this thread thought that it was screwing up the vac signal to the carby. Anyway is there a way to adjust or hold open the diapraghm to prevent it sucking closed at lower revs? It is an IMPCO 425, and it may be to small for the engine, but it should be fine in the lower rev ranges where it is tipping over at the moment.