Recently replace the rear plugs on my verada and ended up with a miss.
I took the old plugs and put some brand new copper plugs (better for mixer LPG system I've been told)
Cleaned the manifold with Nulon Intake cleaner also.

Put the car back together and got a miss. Thinking it may have been the leads, i took the manifold off again and put new leads and new intake manifold gasket. Still missing.

I changed the rear plugs back to what was there and also changed the distributor cap. Still Missing.

I've checked the resistance on all the leads, they range from around 7-9kohms

The car runs with a miss on petrol, but backfires and conks out when on LPG.

I took the plugs out from the back again to check and noticed unburnt petrol on the rear far right and middle plugs.

just as a guess from a unqualified guesstimator but perhaps it may be a dizzy cap?? check that as well but im sure the others have come across this and be able to give you a more qualified opinion

Hey tania, yeah already tried changing the dizzy cap, no luck :(

Silly question, but were the leads were properly seated on the plugs (eg. clicked on, and not loose), and the plugs were gapped correctly?

are they in the right firing order seems weird unless there is a air leak in the intake manifold system some where

Silly question, but were the leads were properly seated on the plugs (eg. clicked on, and not loose), and the plugs were gapped correctly?

Yep, double checked that. I did notice one or two plugs sit a couple mm higher on not flush with the rocker cover, but they wouldnt go in any further with normal force

are they in the right firing order seems weird unless there is a air leak in the intake manifold system some where

Ive got the firing order correct, only replaced one plug at a time so i dont mess things up, as for a leak, im not too sure where from, i checked all breathers and connectors, all seem fine

Does the beast miss on both petrol and gas? If so, you have either a duff ignition system or you have an air leak in the intake - probably the intake manifold gasket. If it only misses on gas, check the gas mixtures. You can check the ignition by running the beast in total darkness, and look to see if it is arcing to earth or to another plug lead.

Incidentally, copper plugs will probably wear out a lot faster than the platinum plugs, and you will need to replace them more often. Getting to the rear plugs is a pain...

Misses on both, just stalls on Gas. Intake manifold gasket has been replaced and bolts were properly torqued. Copper will wear out faster true, but I think out of the Platinum, Iridium and Copper, Copper is the best for LPG (Stronger spark).
Should I try replace the entire distributor?
Im running out of ideas of what it might be

The strength of a spark really depends on other factors. The main one is the electrode tip and the probe. The smaller the tip, the better. Ideally, you should have a point because the spark jumps best from a point. This is impractical because the tip would wear out rapidly. If you look at NGK Platinum, the tip is 0.6 mm dia. NGK Iridium tips are 0.4 mm dia and that it thin indeed. I used Iridium in both my Pajero and my wife's Magna (both runnu=ing on gas as well as petrol). Definite improvement in performance and slight improvement in economy with the Iridium plugs. My main concern was tip erosion, but I checked the Pajero plugs at about 30000 km and the gaps were exactly as I had installed them, so the plug wear is fine. The Pajero fires each plug every revolution (it has 3 coils), the Magna fires every second turn, so the Magna shouldlast a lot longer again.

Champion recommend that you close the gap down by about 0.1 to 0.2 mm when you use gas. This is because the gas mixture has a higher resistance and therefore needs a slightly smaller gap.

Back to your problem, I would say that you have an air leak somewhere, which is leaning out the mixtures and therefore the beast is failing. As I said, check the plug leads in total darkness and see if you can see any trackingof the leads. That is a start.

What brand leads are you using?

The strength of a spark really depends on other factors. The main one is the electrode tip and the probe. The smaller the tip, the better. Ideally, you should have a point because the spark jumps best from a point. This is impractical because the tip would wear out rapidly. If you look at NGK Platinum, the tip is 0.6 mm dia. NGK Iridium tips are 0.4 mm dia and that it thin indeed. I used Iridium in both my Pajero and my wife's Magna (both runnu=ing on gas as well as petrol). Definite improvement in performance and slight improvement in economy with the Iridium plugs. My main concern was tip erosion, but I checked the Pajero plugs at about 30000 km and the gaps were exactly as I had installed them, so the plug wear is fine. The Pajero fires each plug every revolution (it has 3 coils), the Magna fires every second turn, so the Magna shouldlast a lot longer again.

Champion recommend that you close the gap down by about 0.1 to 0.2 mm when you use gas. This is because the gas mixture has a higher resistance and therefore needs a slightly smaller gap.

Back to your problem, I would say that you have an air leak somewhere, which is leaning out the mixtures and therefore the beast is failing. As I said, check the plug leads in total darkness and see if you can see any trackingof the leads. That is a start.

Checked the engine in complete darkness no arc or sparks leaving the bay. Plugs are gapped at 0.9mm at the moment, so 0.2mm below the normal. Its also a colder plug.

As i have the 6G74, would I also have 3 coils ? Maybe one of the coils are dead. I think Ill try a drizzy out of my other Verada

What brand leads are you using?

Currently back on standard Mitsubishi leads, took them out and put some NGK leads, then replaced them when i noticed the engine missing. Put the old leads back

Okay so after changing the distributor in the melting heat, no result. About to lose my s*** with this car.
Any other options guys? I dont suspect a intake leak as I smoothed the faces applied sealant and put a new intake gasket. Didnt remove the throttle body, so didnt change that gasket.

Have you plugged in a spark plug to the lead and checked it's firing? I notice you said that the plugs had unburnt fuel on them. That just means they're not firing. Plug a lead into the appropriate place on the dizzy cap, plug a spark plug in the other end and check that it's sparking while the engine cranks. Be sure to hold the lead by the rubber boot though, and hold the plug against the intake manifold.

Silly question, but have you checked all your vacuum lines etc, and have you checked everything else is plugged in correctly; distributor plug, MAF plug etc. The line between the rocker cover and the inlet... things like that.

Is the miss getting worse as the revs increase? At any point in the rev range does the miss get better or worse? Is it a miss with a flat/dead spot or just a consistent thing?

I've done leads and stuffed it up once, but you knew it was misplaced leads. The only time I've had a "miss" is when I've left either the MAF unplugged or left a vacuum line disconnected.

Mind you, I'm no expert AND I don't have a clue about LPG systems. Just start simple, back trace everything you've done, check and double check the little things before you get fixated on leads/plugs.

I have a Verada running on LPG and i highly recommend Iridium plugs - the finer tip gives a more intense spark and Iridium can withstand much higher temps so will last much longer than copper or Platinum. If you've checked the plugs, leads and leaks, i think swapping out the distributor would be my next step. My car was idling and running rough - i replaced the distributor and it runs great now.

How does it run with the original leads again?

Do you have the leads in their correct postion? The position of No 1 lead in the dist cap is not as you would think. The engine will still run, but like sh1t if you have stuffed up the leads. Check the manual. It isn't easy to check because you cannot really see which lead goes from where. Also, are the plug caps pushed right home? If one is not fully seated (in the distributor cap or at the plug), you could get intermittent missing. You have to thump the leads hard home on the plugs.

All plugs were intact, I also changed out the MAF from my other Verada that runs perfect and no result. But anyways... PROBLEM SOLVED.
Stupid me had leads 3/5 in the wrong place *face palm so hard I give myself a concussion*
Took it to relative whos been a mechanic for 20 years, picked up the problem in under a minute. Sigh. Guess it goes to show how something so obvious and small can cause a damn
nightmare.

@Erad, I wish I saw you comment before I went through more hell and tried more fixes haha. You were right on the money.

But on a good note, after cleaning the TB and putting new leads and plugs the cars running like a gem. Cheers fellaz !!

@Peteraaa I heard that copper has better electricity conductivity than iridium or platinum, hence why its better for the denser LPG compound.
The electrode on the iridiums are smaller with more direct spark and I do agree with handling higher temperatures though.