engine swap nightmare continues help please

Im going to start from the begining again on this one and see if anyone can help me further.

Firstly i have a 1997 p reg 4.6 hse thats been lpg converted ive replaced my perfect running for example didnt miss etc but overheated engne for one from my local scrappy that was from a 96 4.6 that had also been converted for lpg.

after alot of stressful sundays i got the new engine in but it missed as i gave it to a friend to finish off.
he found the source of the miss was the complete o/s bank was down so mhe replaced all the plugs and leads with no avail.
then it was fond to be a multiplug that was slightly undone that caused the misfire.
once connected it ticked over fine for 10 min my friend reved it up and all was god he though he had it fixed.

then he gave it a rev and it backed fired stopped

now it starts fine ticks over great on all 8 cylinders but as soon as you go to rev it it goes to cut out and their is a distinc noise of air being drawn in from somewhere. we cant find if its a breather or anything we havent got a clue. its strange becaus eit ticks over great just doesnt want to rev? it sometimes when reved trys to go but misses and splutters and backfires slightly into the airbox as though its getting to much air?Also if you take the air intake pipe of from the intake it cuts out completely?

Its so thrustating as its so close yet so far? does anyone know of what the sucking noise could be that came after the pop? which is now their everytime you try and rev it? anyone know why it wont rev?

any ideas advice things to try are all gratefully appreciated

they often do that, mine does when i stall it whilst adjusting lpg system, i dont think that is your cause.

so, you have spark to all plugs?

i ticks over fine?

you introduce fuel and it cuts?

you could do with a obd lead and lappy on it. it may give you an idea as to whats happening.

ideally you need to establish i theres enough fuel going to the engine.

have you checked the air gap on the crank sensor?

the cam sensor?

all leads are connected around the top of the engine?

all plugs in tight and correct on the throttle housing?

air intake manifold and plenum sealed, no leaks or breather tubes off?

check the length of the air intake hose, no splits/holes? is there the plasic horn bit on the bottom about half way down?

beneath that theres a plug connected to what looks like a vacuun pipe, is that on and the correct plug?

without bieng there its not easy to suggest stuff.

maybe if you can take a few detailed piccys of the top of the engine/any angles that show stuff that may be relevant???

thanks gav when you say air gap what do you mean? it has a spark to all the cylinders and it ticks over fine but its a soon as you try and rev it of tickover it wont rev.

i think the air gap should be fine.

what happens if you floor the throttle?

or

gently press the throttle?

check the connections on the throttle housing.