Excuse the lack of paragraphs here. Haven't worked out how to transfer what's in the box format to published so look like a teenager typing on a mobile phone. Bear with me!
So... I'm running OMVL/ Millenium single point on my '95 4.6. Vapouriser, stepper and lambda sensors (sensor) are new.
I know that these setups aren't optimum for extracting the last few HP from the motor, due to the limitations of single lambda and single point injection of gas into the inlet. The system is set up to pretty much optimum, using the setting the OMVL mixture up at 50% restricted, 2500 rpm static method, tweaking with the software then locking down the limits on the stepper motor to +- 30 idle and +- 40 out of idle. It runs and idles fine BUT I'm getting bored with having to use the petrol switch as a turbo button when I occasionally feel like caning it. There's a huge difference on full throttle max power demand between LPG and petrol...
I'm thinking it's running weak on LPG at the top end, and my tortuous logic is thinking that it's the locked down stepper motor (ok actuator) that's causing this, by, in effect, acting as a second "main jet" and only partially opening to its pre-set limit.
I've found by trial and error that the actuator limits need to be locked at at the low end, or it's a bitch to start (actuator closed= no gas), but any reason why I shouldn't set the actuator limits to fully open at the out of idle top end? That would just leave the vapouriser to take care of the demand.
I don't have access to a 4wd rolling road (that'd be too easy) so my next proposed move is to get a very long lead for the laptop so I can monitor the lambda, find a long hill where I can floor it for 10 seconds or so in top, remove the actuator limit at the top and monitor the lambda- tweaking the vapouriser to suit.
Anyone (Richard?!) got any views/ thoughts on this or know of an easier approach? I'm applying petrol logic to a gas problem, so I might be chasing my own arse on this, but if it is running weak on full demand, I need to sort it before it goes pop.

Yes, you are chasing your own arse. The amount of gas going in is dictated by the setting of the vaporiser, not by the stepper, the stepper is purely there to fine tune the mixture when needed. If the system has been set up correctly the stepper should hardly move whether at idle or at full bore. The amount of fuel required is directly proportional to the amount of air so as you open the throttle, more air goes in so more fuel should go in too. If it doesn't it's because the vaporiser isn't capable of supplying enough. Since my new engine now has 20,000 miles on it and is nicely run in, I can hit the sport button, floor the throttle, watch the rev counter hit over 5,000 rpm before the box changes up and still wonder how the hell you can get something weighing 2.5 tonnes with the aerodynamics of a small bungalow accelerate that fast. If there is a noticeable difference between running on LPG and running on petrol, it's wrong and needs tweaking.

So here is the complete, from the top, published on about 3 website forums and now about to appear on a 4th, how to set up an OMVL R90E vaporiser on a Leonardo/Millennium single point LPG system wot I rote a while ago.

Setting up a Leonardo

Software is simple enough to get things going. Plug it in and start her up. On the main screen you'll see things like the lambda output and actuator opening. To start from scratch you ideally need a manual valve to fit in place of the actuator. If you don't have one, I've made one using a plastic water pipe Tee piece and a seat belt mounting bolt but you can use just a length of hose and squeeze it with Mole grips if you have nothing suitable.

You can use the manual valve to get things close in case there has been any random twiddling on the vaporiser. Take the stepper actuator out of the hose and fit the manual valve but leave the stepper attached to it's cables so you can see what it is doing. Set the manual valve about half open. Go into the software and under the Actuator tab (under Optional Configurations), set the upper and lower limits to 255 for idle and out of idle. If you make any changes in the software you must hit Return after typing the new value in or it doesn't save the setting and reverts back to what it was. Also make sure the actuator default lock box isn't ticked. Screw the idle bleed screw, the top one or smaller one if it's an older R90E, all the way in. With the computer connected so you can see what is happening, start the engine and get it running at around 2,500 rpm (screw the throttle cable out on it's adjuster if you are on your own). Adjust the manual valve for highest revs (and adjust the throttle to keep it the same) and check the opening on the stepper. It wants to correspond with what you have the manual valve set at and, looking at the software display (hit F12 to display what is going on), you'll see the lambda sitting either somewhere in the middle of the scale or flipping between extremes. You want the actuator and the manual valve to be somewhere in the middle, around 80 - 150 on the software display. If it is right at the top (fully open), you need to unscrew the main bias screw on the vaporiser half a turn (the lower one with the spring under it) at a time until it is in the middle somewhere. Too low (fully closed), screw it in. Re-tweak the manual valve to get the revs back up. Keep doing this until it is sitting somewhere in the middle. You can check that the actuator is working by watching what does when you adjust the manual valve. Screw the manual valve in a little to make the mixture weak and you will see the lambda go green and the stepper open up to try to compensate. Screw it out to make it rich and the lambda will go red and it should start to close.

Now remove the manual valve and throw it somewhere, you may need it one day but hopefully not for quite a while, and fit the stepper in its place. Start the engine and hold the revs up to confirm that it is still in the wandering around the middle and the lambda output is flipping from one extreme to the other. Let it drop to idle and see what the stepper sits at then. It wants to be roughly the same at idle as at 2,500 rpm. If it closes down, screw the main bias screw in a touch to bring it back up. Rev it again and check that the stepper is still in roughly the same area. You'll need to try this a bit to get the idle and out of idle opening the same. You won't be able to get it spot on and a slightly higher opening at idle is fine. If you can't get them to match and the idle opening is always lower than the out of idle, then that is a sign the R90E is showing signs of age. Going rich at idle, allowing more gas through than is needed, is the first sign. If it opens up at idle, then try screwing the main bias screw out a touch. If you find you can never get the out of idle opening within a sensible range and the idle is still opening up, then open the idle bleed screw a touch. I do mean a touch, it is very sensitive, especially on the newer ones with two 6 mm adjusters.

Finally, when you are happy that it is as good as you are going to get, close the actuator limits down to something sensible, say +-30 at idle and +- 40 at out of idle. If you are lucky, you should be able to get it looking like this https://www.youtube.com/watch?v=b6sGp2I9lts which is my 4.0 litre on an identical system.

If the lambda sensor is dead the Millennium will be adjusting the mixture using incorrect information so getting it wrong. The 5-0V Titania sensors used in a GEMS P38 give 5V for a lean mixture and 0V for rich. So if the lambda sensor has died it is giving 0V, the Millennium sees this as a rich mixture so will close the stepper down to make it leaner. That will cause you a problem..... In this case, using your manual valve, get the vaporiser set as above and note the actuator number. Tick the Default Lock box and type that number into the box that appears next to it. Then set the idle and out of idle steps to +-5. That way you will have locked the actuator opening to that fixed opening where the mixture is as close as it is going to be for most of the time.

Backfires are caused by one of two things, either an iffy spark due to a dodgy plug or HT lead or a lean mixture. With your experience with older stuff you must have seen engines spitting back through the carb when the mixture is lean. It's no different except you've got an inlet manifold and plenum chamber full of a combustible fuel/air mixture, hence the big bang as the whole lot ignites. Under the Vehicle Configuration menu you'll see the settings for your car. If you want you can set the Changeover to Alternative Fuel start and then you don't need to go through the switch flipping routine to start on gas. Standard settings are changeover on deceleration at something like 1,200 rpm. That way you just leave the switch in the gas position, start the engine (which will be on petrol), blip the throttle and as the revs drop, it'll changeover. You may find it dies, or tries to die, as it changes over. In which case, increase the Fuel Overlap setting. This increases the delay between turning the gas on and the petrol off (I think mine is set to 1 second). As cold running enrichment is not needed on gas, there is no need to warm up on petrol first, just start and drive (although to prevent the chances of the vaporiser icing up in the first few seconds in the depths of winter, you ideally need the vaporiser plumbed in series with the heater not parallel as many are). I know a 110 Defender owner who's petrol pump died about 4 years ago and he just set it to start on gas and never bothered fitting a new fuel pump. He tells me he'll get round to it one day......
.

LOL- that was the (much appreciated) HOWTO you sent to me :-)
Which, of course, I followed religiously

If it's running out of steam at full bore, you didn't get it right. DO IT AGAIN and get it right this time......

Sitting the laptop on the passenger seat or in the passenger footwell and flooring the throttle to see what the stepper does will tell you what is happening but it is almost certain it will try to open up to get more gas in but the vaporiser can't supply enough. It might be full of heavy ends or you might have a restriction somewhere. Squashed pipe or clagged up filter maybe? I assume you do have an OMVL R90E vaporiser and not some crappy Italian thing rated at 190 Italian horsepower so only good for about 130 on a good day? That's what was fitted to my Classic when I first got it and that went all sick and flat at anything over 1/3rd throttle, swapping for an R90E cured it completely.

Yep- nice shiny new R90E. I've followed your instructions religiously (each time I pray it's going to be sorted) 4 times now. Come back to pretty much the same base setting on the R90 give or take 30 degrees of turn on the main screw every time.
I was thinking along the lines of blocked filters/ kinks in the lines too. I'm under the thing replacing all the UJs and fluids at the moment so will follow the line from tanks to vapouriser and see how many filters I can find to clean.
For such a simple system it's a pain that I can't get it totally right. I thought Lucas Mk2 injection was fiddly, but this takes several biscuits!

Died on gas completely yesterday, so I guess now is the time to dig deep into the system. Workshop full of other stuff though. Why do these things always pick the perfect moment?!