have a 1993 4.6, which is nice to drive, expensive but nice. i have an 'Impco' gas system fitted, which always worked well, until a head gasket change, now it won't idle, or run very well. Had new plugs with the new heads, just wondering if that may be the problem, and if changing leads and regapping the plugs might work?
found this from a gas fuel pseudo expert. Sound right or not?

Items such as Distributor caps, HT Leads, Rotor arms etc. can appear to be serviceable but cause problems when on LPG. This is because LPG / Air mixture is much harder to ignite than Petrol / Air. The voltages generated are up to twice as high (expect to see spark voltages of 10-15Kv's on petrol, whereas on LPG they can reach 25 -30Kv's). Higher spark voltages are likely to break down the insulation of any component, causing 'tracking', misfiring etc. which results in backfiring. Thus, faults that are NOT evident when on Petrol can show up on LPG. Although all engines can suffer misfiring caused by old parts, in 6, 8, 10 and 12 cylinder ignition systems the spacing between the 'poles' on the distributor cap can be quite small. It is easier for a spark to 'jump' to the wrong pole as a result. It's crucial this and other components don't cause this problem when running on LPG. As a result, the voltages within HT components can become far higher when running on gas as opposed on petrol, typically 12-15 Kv's on petrol and up to 30 - 35 Kv's on gas. Meaning the insulation of HT components is at least twice as likely to break down running on gas, often resulting in tracking, misfiring etc. The point where we hear the statement 'It doesn't misfire/backfire/ stall on petrol'. In the light of the above, it wouldn't. To help reduce the problems when running on gas, set plug gaps to .030" (.75mm). See what a reduction in plug gaps does on a 'scope. Too wide = High voltage. Reduce gap = Reduced voltage. Reducing the plug gap won't dramatically remedy backfiring, but it will certainly reduce the risk. Another factor often missed is that once HT leads etc, have been exposed to higher than normal voltages their insulations are damaged to the extent that a later reduction in plug gaps cannot stop the leakage. Misfiring and the ensuing backfire will still occur. You'd expect something to be damaged by this! Such damage makes just the place where damp likes to dwell, making things even worse. If that is the case, only replacement of the damaged components will cure the problem if the backfiring is really due to ignition faults in the first instance.

I've not heard of Impco, but I'm guessing it's a single point system?
Are you sure you've not got an air-lock in the reducer? If it's freezing up or not heating sufficiently it won't work properly. You've had the heads off, so drained/ refilled cooling system so that'd be a good place to start
BTW, what year is your P38?

Impco is an American system designed for big V8 engines with a hoofing great 4 barrel carb sitting on top of them. Like everything American it is absolutely wonderful and far superior to anything designed anywhere else in the world, to Americans, but to the rest of the civilised world it's old technology, very expensive to buy spares and not particularly good at that.

There is some truth in the stuff you've found in as much as LPG does need a healthy ignition system and any weaknesses will show up on LPG before they show up on petrol. However, most of what he is talking about relates to a distributor system such as on a RR Classic and not the system we've got on a P38. Which do you have? However, even if the ignition system is on it's last legs and is arcing and sparking in all the wrong places, if it will run on petrol it will still run, albeit worse, on LPG. Ignition spares, particularly rotor arms, for the Classic distributor are notoriously poor quality these days(since Lucas became part of one of the other spares companies) with even brand new LR sources ones failing in a matter of weeks. They break down so the spark doesn't go from the centre to the tab on the end and on to the correct HT lead but track down the outside of the rotor arm body to the distributor spindle. Making sure the rotor arm is spotlessly clean is one short term fix but often an old well worn rotor arm will work better than a brand new one.

As OB says, an air lock in the vaporiser will cause a problem but I don't think the Impco system is even sophisticated enough to use a temperature sensor so it wouldn't stop it from starting, particularly not on petrol.

thanks for the replies.
Big Girl is a 93 plate with 203000 on the clock, but with a rebuilt engine at 190000.
it ran very well before hand, would even start on gas in the summer, but not in winter. Have learnt a lot more about gas systems just reading this site, thanks to Chris who did the head gaskets for me and pointed me to this place of learning! He's a really nice geezer, and seems to know his way around these motors, which i don't.
the Impco is a bloody great thing sitting to one side of the engine, and plugged in by a huge pipe straight from the airbox. Haven't had it serviced, as no one in Ipswich wants to touch them it seems? all bar one that is, but I ain't paying £700 for a gas service!
Had a couple of back fires, one totaling the airbox in quite an interesting and exciting way, the other just scaring the straights across the road. other than that she has been good to run for the past three years. looked at the rotor arm, looks to be in good condition, but happy to replace it, if any one knows of a good make to buy? leads though, seems i would need another mortgage to get them!

For £700 you could replace the Impco with a singlepoint system the same as I run on mine, twice......

93 means it is a Classic so would have originally had a 3.9 or 4.2 (if LWB) but I know a few people have fitted 4.6 motors and used the front cover from the original engine to retain the distributor. HT leads aren't that expensive, you could try a set of these http://www.island-4x4.co.uk/lead-high-performance-9498-hls101xp-p-4106.html and while you are at it, treat it to one of these http://www.island-4x4.co.uk/distributor-rotor-facet-stc1857f-p-4107.html.

It’s not a 93, it’s a 94 4.6 p38, how did mot go ? I’ll give you a bell tomorrow, most probably something silly as was switching over no problem..

But he said he's looked at the rotor arm??? If it's a P38, it ain't got one (and the HT leads I linked to above would be the wrong ones).

yep probably a 94, but as i said, i am no mechanic. i know you put fuel in one end, and water in the other, and if you fuck that up, it won't go!
not too sure what i looked at then? should be the bit where all the leads end up? There's a 'thingy' in there, (nuff technical talk from me!) but it looked ok to me.
But then i repair squeaks and bangs by turning the radio up!
Hi Chris, will look forward to the chat, i am as you know, clueless! You get to chat about that big annoying piece of spectacular American metal engineering that was always in the way again.