Advice please!

[Pinger]

Default has risen slightly to 171.
Unblocked 3 more mixer holes and on the driveway it felt hesitant on opening the throttle and as I raised the rpm the actuator went fully open. Conclusion is then when I try to up the airflow the single reducer is lacking.
Re-blocked 2 holes so only 1 uncovered from yesterday and that seemed OK so will drive tomorrow and see what it's like and that fuel is keeping up with it.

Wrong conclusion!
It's the mixer. That one extra (small) hole unblocked has ruined it. It is chronically lean at even second box TPS with actuator opening fully.
The small holes don't communicate 'signal'. The reverse - they rob the central part of the mixer of air flow.
The centre part is plastic and held in with a snap ring. If I thought there was an annular space around that part's periphery (or I could safely create one without structurally weakening it) I'd take the mixer apart and drill communication holes from the centre out to each hole. The mixer you see in the photo is now discontinued and was the last from TT's stock. Unless I can access as spare - I'm not for taking it apart.
Time for a rethink.....

[Gilbertd]

PM me your address and I'll bung the 36mm one I have here in the post. There's no bypass holes like there is on that one and may require a bit of bodgery to fit it, but I reckon it will work a lot better as all the airflow goes through the venturi and the hole in the centre is a lot larger.

[Pinger]

PM me your address and I'll bung the 36mm one I have here in the post. There's no bypass holes like there is on that one and may require a bit of bodgery to fit it, but I reckon it will work a lot better as all the airflow goes through the venturi and the hole in the centre is a lot larger.

Very generous, thank you - and I may take you up on it. There's something else I want to explore first though and I'm going to spend this afternoon crunching numbers on it. More later - if it looks viable.

[Pinger]

Quick questions. Is my AEB ECU capable of running two actuators simultaneously? And if so, where might I obtain the connecter plug/harness?

(If this pivots on total electrical load, I have reduced the number of solenoids from 3 to 2 and can add a relay if necessary).

[Gilbertd]

I wouldn't have thought so. It gets feedback from the actuator so it knows where it is and I suspect getting two feedback signals would completely confuse it. If you were to go down the dual vaporiser route, you'd combine the outputs before the actuator. But, as you've proved the one is capable of providing sufficient gas for clean running up to the red line, I can't see any advantage, just more complication.

[Pinger]

I wouldn't have thought so. It gets feedback from the actuator so it knows where it is and I suspect getting two feedback signals would completely confuse it.

But if they were paralleled would they not be in the same position at all times - and thus present no problem to the ECU? That, BTW is asked by someone with zero comprehension of how electronic feedback, stepper motors, etc work. Only an understanding that two components wired in parallel display a different resistance to a individual one.

If you were to go down the dual vaporiser route, you'd combine the outputs before the actuator. But, as you've proved the one is capable of providing sufficient gas for clean running up to the red line, I can't see any advantage, just more complication.

Still trying (desperately!) to avoid dual reducers - but wanting to increase the airflow but need to increase (or rather maintain) the signal the reducer sees.

[Gilbertd]

They are stepper motors so if one missed a step for whatever reason, they would be out of sync and would never get themselves back in sync. I've seen a dual vaporiser/actuator setup once (on a Rolls) but that used two AEB175 controllers so two independent systems, one for each bank.

I don't think you need two vaporisers. You've proved that one can provide sufficient fuel now you've got rid of the restriction in the supply but I still don't like the look of that mixer. With the holes open there's insufficient airflow through the venturi so insufficient signal and with them blanked off you'll increase the signal but you've then got a very restricted intake for the size of engine.

[Pinger]

They are stepper motors so if one missed a step for whatever reason, they would be out of sync and would never get themselves back in sync. I've seen a dual vaporiser/actuator setup once (on a Rolls) but that used two AEB175 controllers so two independent systems, one for each bank.

It was just an option I was exploring but as per above potential problem, eliminated from consideration.

I don't think you need two vaporisers. You've proved that one can provide sufficient fuel now you've got rid of the restriction in the supply but I still don't like the look of that mixer. With the holes open there's insufficient airflow through the venturi so insufficient signal and with them blanked off you'll increase the signal but you've then got a very restricted intake for the size of engine.

Agreed, neither (in all probability) need or want two reducers. But definitely do want more signal.
Your analysis of the current mixer is exactly as I see it. The previous mixer didn't provide sufficient signal either - as shown by disparity in actuator positions in and out of idle and a tendency to go lean on throttle opening (while well within any fuel restriction limitations). Both mixers in series though should provide sufficient signal without strangling the motor.
More calcs required but so far it looks like I can have the equivalent of a 40.8mm ID mixer (possibly 44.3mm but I'm being conservative here) compared to the equivalent 35.6mm ID currently. Success will depend on arranging the two mixers such that the air has time/space to 'recover' from passing through the first mixer before entering the second.
I'll feed them both through one actuator (though in theory feeding one with a power valve and the other with an actuator is possible - but likely inferior).
I still want to know the effects of two (larger) venturis in series versus one smaller one. If the first one is efficient (it will be extensively modified to eliminate turbulence) I think (hope!) that two will present less restriction than one smaller one.

[LPGC]

I'm not sure the ECUs do get feedback on stepper position, iirc the stepper is closed fully when the system is switched back to petrol, the ECU then knows the position of the stepper purely by keeping a running total of open/close steps assuming that the initial condition was fully closed. In the past I've momentarily disconnected the stepper with the system in operation, reconnecting the stepper sees the ECU assume that the actual position agrees with the calculated running total. But I still think there'd be a problem trying to run 2 steppers from one AEB175 type ECU and not just because of the potential for the steppers to get out of sync.

If you need to fit a much smaller venturi size to that expected, instead of the problem being the mixer and signal it produces it could be the sensitivity of the reducer to signal. Some relatively recently produced versions of a well known reducer are not as signal sensitive as older versions of the same reducer.

Another thing is, if intake air is currently strangled by too much restriction from a mixer the engine will produce more power when that restriction is lifted... Fix the problem with restriction and it still remains to be seen if the reducer will be capable of flowing enough gas (assuming response to signal and mixture are correct) for the higher bhp.

[Pinger]

I'm not sure the ECUs do get feedback on stepper position, iirc the stepper is closed fully when the system is switched back to petrol, the ECU then knows the position of the stepper purely by keeping a running total of open/close steps assuming that the initial condition was fully closed. In the past I've momentarily disconnected the stepper with the system in operation, reconnecting the stepper sees the ECU assume that the actual position agrees with the calculated running total. But I still think there'd be a problem trying to run 2 steppers from one AEB175 type ECU and not just because of the potential for the steppers to get out of sync.

I've abandoned the idea of two actuators. It was only really considered as it would ease the plumbing - one in each line to each mixer from each of the single reducer's outputs. I'll double Y them and use a single actuator.

If you need to fit a much smaller venturi size to that expected, instead of the problem being the mixer and signal it produces it could be the sensitivity of the reducer to signal. Some relatively recently produced versions of a well known reducer are not as signal sensitive as older versions of the same reducer.

It's the new OMVL 90E I have and if I open the bias screw out any further it'll likely fall out! As it is, I've had to arrange a lock on it to prevent that. The bleed screw is still fully home but if I open that I'll further increase the disparity between in and out of idle actuator positions. (It currently idles very slightly rich 100 steps below default on gas pressure alone).

Another thing is, if intake air is currently strangled by too much restriction from a mixer the engine will produce more power when that restriction is lifted... Fix the problem with restriction and it still remains to be seen if the reducer will be capable of flowing enough gas (assuming response to signal and mixture are correct) for the higher bhp.

True, and I'll only know that when I up the air flow (without losing signal). I don't think I'm very far from where I need/want to be. It's strong up to around 3500 rpm - maybe a little bit higher. Pmax on a Vortec is at 4400 rpm, running through to 5000 just exploits the lower gear ratio (I only have a 4sp gearbox).
The difference in mixer areas is + 32% (conservative calculations) which is pretty much the difference between 3500 and 4400 rpm. Providing there's no loss from using two mixers (in series) then I only need half the signal from each (and the gas from the first will generate (slight) additional signal in the second).

Very difficult to get data on two venturis in series but where a loss occurs it is due to air speed related turbulence. Larger (this is the 45mm mixer I'll be using as the upstream one) reduces velocity and I can/will tidy up its exit to eliminate turbulence as best I can. It can generate half the required pressure drop for LPG flow. (Slightly more than half as I'm working with the assumption alluded to by Gilbert,D that only the central part of the downstream mixer is contributing to LPG draw and am thus allocating flow to it in proportion to its central area alone vs that area plus any open peripheral holes (estimated to be 8 vs current 4)).

I have air speed values (from Ricardo) in valve ports as they relate to falling volumetric efficiency though I'd expect them to be higher in a larger diameter inlet duct and am still a long way shy of actual 'choked flow' velocity so it's only (turbulence generated) energy loss in the venturi(s) manifesting as a pumping loss by the engine that has significance here - as far as I can ascertain.

[Gilbertd]

I'm trying to get my head around why you think two mixers will work better than one. My thoughts on this are your original mixer wasn't really a mixer at all, all it had was stub of pipe in the airflow, no real venturi to accelerate the airflow and give a proper signal. The replacement has a big lump of metal in the middle so the actual cross sectional area that the air can flow though is only small. If you where to plug all the holes around the outside you should get plenty of signal but the engine would be strangled while opening them up improves the airflow but reduces the signal. One thought though, you don't have a free flow, K&N type air filter fitted do you?

[Pinger]

Because if one mixer generates a signal from the air passing through it then the same air can then get a another dose of LPG when it passes through the next mixer (in series not parallel - in case there's any doubt there).
Each mixer (being larger) has a low(ish) signal but the LPG flow they draw will be doubled up.
The original mixer has a curved entry - I intend improving the exit to make a passible venturi out of it. Work on that has begun.
The new mixer works with 4 of the peripheral holes open (the equivalent area of an ID of 35.6mm) and while it runs cleanly to 5000 rpm it isn't making the power it should. With 5 of the peripheral holes open (an area increase of a mere 8% ) it is hopelessly lean at all rpms above idle.
No K&N - stock GM filter.

I've calculated the pressure drop from the new mixer that works with 4 of the peripheral holes open (as per above). From that I've subtracted the pressure drop available from the old (45mm) mixer (when properly configured) and the balance has to come from the new mixer. That should be with 8 of its peripheral holes open. That should have an equivalent ID of 40.8mm - a useful increase from equiv 35.6mm in relation to airflow.
I'll hopefully get a chance this week to try the two in series (on the driveway) this week. If it looks like working I'll recheck the calcs across all rpms (specifically that there is sufficient combined signal just out of idle) and if that checks out make a proper job of installing the old (modified) mixer and properly roadtest that.
It's got be worth a shot and I wont know that it will or wont work without trying.

[LPGC]

It may not quite be so simple with the multiple inline mixers fed by a single reducer.

If they both provide the same signal the signals may be cumulative and with 2 points of gas entry the engine may get twice as much gas. We know this works because it's common to fix 2 reducers to a V engine such as on an older Rover V8 with twin SU type carbs (one per cylinder bank). With this kind of setup, if the reducer was totally linear to signal we'd expect to be able to remove one of the 2 mixers from the engine to run one bank of cylinders on petrol while the other bank ran on LPG. But the results (in terms of correct mixture) wouldn't be the same if we didn't block/plug the gas feed from the reducer to the mixer we had removed from the engine because some signal from the fitted and operational mixer would see air entering through the mixer we had removed from the engine. We could look at it the other way and think we could fit 2 low power reducers to feed a high power engine through a single mixer, all we have done here is swap the additional reducer for a mixer that has been removed from the engine but is still connected to the system and is open to atmospheric pressure with no air flowing through it. Back to the proposed situation with 2 mixers in series, if one provides hardly any signal while the other provides a lot of signal, should we expect a cumulative signal, should we expect the average, or should we expect something unintuitive?

Going back to basics, signal is the pressure that the reducer sees on it's outlet pipe connection that's very slightly lower than atmospheric pressure. We can generate this signal by partial exposure to manifold vacuum and/or by causing airflow passing a shielded (from incoming air) gas outlet point to be at high speed, bit like an aircraft wing generating lift by having airspeed above the wing lower than airspeed below it.

Some averaging has to be expected because if we have 2 different signals and join them together, some air will pass from the lowest signal (highest pressure lowest vacuum) point to the highest signal (lowest pressure highest vacuum) point - We have effectively created another very minor way for intake air to reach the throttle body through a pipe loop between the front and rear mixer, just that we have attached a connection to the reducer using a T joiner in the middle of that pipe. You wouldn't get the same with a conventional petrol carb that has more than one jet because each jet is fed from the float bowl which isn't itself effected by signal, so to go with this analogy each jet in a carb effectively has it's own reducer. But signal to a mixer system reducer flexes the diaphragm to the point that pressure at the outlet of the reducer equals atmospheric pressure. To look at it in another way , in relation to the carb analogy this is like the signal kind of effecting the level of fuel in the float bowl, the 2 jets perhaps both having their own independent effect on level of fuel in the float bowl, while level of fuel in the float bowl will affect how much both jets flow.

[Pinger]

It may not quite be so simple with the multiple inline mixers fed by a single reducer.

If they both provide the same signal the signals may be cumulative and with 2 points of gas entry the engine may get twice as much gas. We know this works because it's common to fix 2 reducers to a V engine such as on an older Rover V8 with twin SU type carbs (one per cylinder bank). With this kind of setup, if the reducer was totally linear to signal we'd expect to be able to remove one of the 2 mixers from the engine to run one bank of cylinders on petrol while the other bank ran on LPG. But the results (in terms of correct mixture) wouldn't be the same if we didn't block/plug the gas feed from the reducer to the mixer we had removed from the engine because some signal from the fitted and operational mixer would see air entering through the mixer we had removed from the engine. We could look at it the other way and think we could fit 2 low power reducers to feed a high power engine through a single mixer, all we have done here is swap the additional reducer for a mixer that has been removed from the engine but is still connected to the system and is open to atmospheric pressure with no air flowing through it. Back to the proposed situation with 2 mixers in series, if one provides hardly any signal while the other provides a lot of signal, should we expect a cumulative signal, should we expect the average, or should we expect something unintuitive?

Going back to basics, signal is the pressure that the reducer sees on it's outlet pipe connection that's very slightly lower than atmospheric pressure. We can generate this signal by partial exposure to manifold vacuum and/or by causing airflow passing a shielded (from incoming air) gas outlet point to be at high speed, bit like an aircraft wing generating lift by having airspeed above the wing lower than airspeed below it.

Some averaging has to be expected because if we have 2 different signals and join them together, some air will pass from the lowest signal (highest pressure lowest vacuum) point to the highest signal (lowest pressure highest vacuum) point - We have effectively created another very minor way for intake air to reach the throttle body through a pipe loop between the front and rear mixer, just that we have attached a connection to the reducer using a T joiner in the middle of that pipe. You wouldn't get the same with a conventional petrol carb that has more than one jet because each jet is fed from the float bowl which isn't itself effected by signal, so to go with this analogy each jet in a carb effectively has it's own reducer. But signal to a mixer system reducer flexes the diaphragm to the point that pressure at the outlet of the reducer equals atmospheric pressure. To look at it in another way , in relation to the carb analogy this is like the signal kind of effecting the level of fuel in the float bowl, the 2 jets perhaps both having their own independent effect on level of fuel in the float bowl, while level of fuel in the float bowl will affect how much both jets flow.

I'll try and explain it as I see it.
All flow is dependent on there being a pressure differential. Without that, no flow occurs. The greater the pressure differential, the greater the potential for flow.
For the sake of this argument lets assume the pressure at the reducer's outlets is as atmospheric and that the case for which I present figures is WOT at 5000 rpm. My engine gets sufficient LPG flow when (in the aforementioned case, the calculated pressure drop at the (single) mixer is 0.25 bar. This should be the pressure differential causing flow of LPG from the reducer. But, as Gilbert.D made obvious, this pressure differential is only communicated to the reducer in the mixer's central core - not from the (4 open) peripheral holes. Thus only a proportion of the pressure drop can influence LPG flow and that proportion is the core area divided by the sum of the core and (4 open) peripheral holes. Therefore I assume that the LPG drawn sees an equivalent pressure differential of 0.17 bar. Therefore a pressure differential of 0.17 bar is sufficient to flow sufficient LPG for the engine's requirements at WOT and 5000 rpm (ie, if all the air flowing through the mixer contributed to creating a pressure differential, that pressure differential would be 0.17 bar).

A mixer with a 45mm ID at the same WOT and 5000 rpm conditions can create a pressure differential of 0.1 bar and that should draw LPG from the reducer in the ratio of 0.1/0.17 = 0.59 ie, a pressure differential of 0.1 bar should deliver 59% of the engine's requirements. The balance of 41% required has to come from the second mixer. The pressure differential required though is not 0.7 bar as it can only act on LPG at that mixer's core (which will be 52% of total area available for airflow). The upshot of that is that a pressure differential of 0.145 bar is required to act over 52% of that mixer's flow area which will cause LPG to flow at a rate that it would were only a 0.7 bar pressure differential applied where all the airflow had the opportunity to influence the LPG's flow.

(A simplified version of the above where fact that the second mixer's peripheral holes do not communicate with the reducer is ignored, ie, it is assumed that they do gives figures of 0.25 bar pressure differential required from a single mixer or, (as in previous case) a 0.1 bar pressure differential from one mixer and a pressure differential of 0.15 bar from the other. Remarkably similar.
What hasn't been accounted for is that the total gas flow (air and vaporised LPG) entering the downstream mixer would be higher by circa 4%.Trivial).

So, the upshot of all this would be that each mixer would draw slightly different amounts (in the ratio of 40:60) from different pressure differentials (again, in the same 40:60 ratio) and my expectation would be that the reducer can balance this slightly disparate delivery requirements. But it shouldn't have to as the two outlets will merge downstream from the actuator, upstream of the mixers and thus be balanced the other side of the actuator - which is what the reducer will sense.

[LPGC]

I understand the points you're making, I see the gist behind all of them and agree with some of them. I don't pretend to have all the answers! But I think there's a lot more going on besides.

E.g. The size of the holes for gas to enter the air must play a part, all other things being the same a single pin prick size hole would still generate the same vacuum signal as a larger hole if we were to connect a pressure sensor. But if we have a reducer connected to a pin prick sized hole and all gas flow entering the engine has to pass through the pin prick the pin prick will limit restrict gas flow from the reducer, there will be a pressure drop between the reducer and the outlet side of the pin prick which sees a drop in signal at the reducer. With a pin prick sized hole it probably wouldn't flow enough gas or petrol even if we fed the pin prick with liquid at 5 bar differential pressure, never mind the small fraction of 1 bar with vapour that mixer systems operate with.

We can see manifold pressure on a lot of engines because they have manifold pressure sensors fitted which we can read via OBD. At full throttle with no restriction (such as a mixer) fitted the manifold should reach 1 bar at WOT even at bhp max. With a well matched mixer fitted the map might still reach (say) 0.95 bar at WOT at the rpm where the standard engine makes bhp max, from this we can take it that a well matched mixer setup can deliver enough gas for correct (suitably rich) mixture at WOT across the rpm range for a manifold pressure drop of only 0.05bar. Not sure how much exposure to manifold vacuum plays it's part in signal but it seems likely that speed density also plays a part with a well designed mixer.

It would be interesting to connect sensitive pressure sensors to the manifold and to the signal port on various mixers on various installs to see how the readings compare. Could run tests with the engine running on petrol (to check vacuum signal when the reducer isn't supplying gas) and again when the engine is running on gas (check vacuum signal when the reducer is supplying gas).

One of my points in my previous post concerns this effect https://en.wikipedia.org/wiki/File:VenturiFlow.png Imagine there were no water in the side pipe of this venturi tube, there would be a flow of air through the side tube, how that would effect your 60/40 calculation using a single reducer I'm not sure, with 2 reducers independently feeding your 2 series mixers I expect it would effect your 60/40 calculation a lot less.

There are 'shield type' mixers which are effectively just a post/restriction over only a small area of an intake pipe with the gas outlet on the exit side behind the shield.

I've driven thousands of miles in a Jeep using a mixer I made from a baked beans can! Removed the lid and made a hole in the bottom, increased the size of the hole to be as big as I could get it while still getting enough signal. Gas entry point just an open ended brass fitting screwed through the side, it ran great lol.

[Pinger]

I understand the points you're making, I see the gist behind all of them and agree with some of them. I don't pretend to have all the answers! But I think there's a lot more going on besides.

E.g. The size of the holes for gas to enter the air must play a part, all other things being the same a single pin prick size hole would still generate the same vacuum signal as a larger hole if we were to connect a pressure sensor. But if we have a reducer connected to a pin prick sized hole and all gas flow entering the engine has to pass through the pin prick the pin prick will limit restrict gas flow from the reducer, there will be a pressure drop between the reducer and the outlet side of the pin prick which sees a drop in signal at the reducer. With a pin prick sized hole it probably wouldn't flow enough gas or petrol even if we fed the pin prick with liquid at 5 bar differential pressure, never mind the small fraction of 1 bar with vapour that mixer systems operate with.

Think of the pressure differential acting on the area of the hole and that will determine flow. Up to the point where the orifice hasn't sufficient area for the fluid velocity and becomes 'choked'. Choked flow has strict definitions re 'critical pressure' and occurs when said fluid velocity is at the local speed of sound. Up to that point it is turbulence that creates the pressure loss and is defined by Reynold's Number. (Fluid moving in a duct does so in the manner of a telescopic ariel being extended. Fastest at the centre, stationary at the duct wall (boundary layer) and differing between those extremes. That shearing enabling the speed differential is the source of the friction).
I will at least be doubling up the mixer entry area - which always look a bit restrictive relative to the 19mm ID pipe that feeds them.

We can see manifold pressure on a lot of engines because they have manifold pressure sensors fitted which we can read via OBD. At full throttle with no restriction (such as a mixer) fitted the manifold should reach 1 bar at WOT even at bhp max. With a well matched mixer fitted the map might still reach (say) 0.95 bar at WOT at the rpm where the standard engine makes bhp max, from this we can take it that a well matched mixer setup can deliver enough gas for correct (suitably rich) mixture at WOT across the rpm range for a manifold pressure drop of only 0.05bar. Not sure how much exposure to manifold vacuum plays it's part in signal but it seems likely that speed density also plays a part with a well designed mixer.

I see MAP data on the truck forum for my engine as a few there play around with aftermarket ECUs with extensive data logging and tuning possibilities. 0.85-0.9 bar is attainable. I based my calculations on a volumetric efficiency of 0.85. I view the pressure differential across the mixer as due to venturi action though with a restrictive mixer, engine generated vacuum is a possibility. The further I go in the direction of increased airflow, the more relevant does the venturi principle become.

It would be interesting to connect sensitive pressure sensors to the manifold and to the signal port on various mixers on various installs to see how the readings compare. Could run tests with the engine running on petrol (to check vacuum signal when the reducer isn't supplying gas) and again when the engine is running on gas (check vacuum signal when the reducer is supplying gas).

MAP data would be interesting to observe - especially given how easy a baseline voltage from a MAP sensor can be interpreted. On the to-do list.

One of my points in my previous post concerns this effect https://en.wikipedia.org/wiki/File:VenturiFlow.png Imagine there were no water in the side pipe of this venturi tube, there would be a flow of air through the side tube, how that would effect your 60/40 calculation using a single reducer I'm not sure, with 2 reducers independently feeding your 2 series mixers I expect it would effect your 60/40 calculation a lot less.

By introducing a third element ie, flow from the reducer via the actuator I expect the flows will be proportionate to the pressure differentials at each mixer and the reducer will see the sum of the flow demands. Also, what you linked is a single venturi with only one pressure drop. Were the widest diameter there not the widest but narrower with an even wider section up stream there would be vacuum also at the point where you see pressure. I get the point you are making though and it would be a bigger concern if my mixer feeds didn't (or don't) converge before communicating with the reducer.

There are 'shield type' mixers which are effectively just a post/restriction over only a small area of an intake pipe with the gas outlet on the exit side behind the shield.

I've driven thousands of miles in a Jeep using a mixer I made from a baked beans can! Removed the lid and made a hole in the bottom, increased the size of the hole to be as big as I could get it while still getting enough signal. Gas entry point just an open ended brass fitting screwed through the side, it ran great lol.

That's kind of what my original mixer is/was. Intending to add a divergent duct to its downstream side to create a proper venturi. If it generates a lot of turbulence I will have gained nothing in air flow.

[Pinger]

Well theory is a fine thing!
Tried a lash up set-up on the drive way with an upstream (my old one unmodified) fed via a power valve (not through actuator).
In any test I could get the actuator down from its usual in idle position of 100 to 20. But that's just the gas coming in under pressure from the reducer.

Out of idle the upstream mixer will not deliver LPG but permits unblocking of the perimeter holes in the downstream mixer.
With only the one mixer with 4 holes open, at 3000 rpm the actuator sits at 141. With the upstream mixer in place it is the same no matter that I open its PV.
With only the one mixer with 7 holes open, at 2500 rpm the actuator is already fully open. When I mount the upstream mixer without any LPG supply to it the actuator goes to 141 at 3000 rpm. That would imply a drivable set-up - where with only 5 holes open it is chronically lean when driven. Throughout, the default never shifted from its earlier 171. (All of this was done without an air filter in place but surely that can't influence things to this degree. It was mounted directly upstream from the (hot wire) MAF sensor).

I had expected to see some LPG flow through the upstream mixer which I could then improve upon by modifying it to behave more like a venturi. Even as it is, this is the mixer that that engine has (AFAIK) ran with since 2002 but resolutely refused to draw LPG even when the downstream mixer is configured such that it otherwise makes the engine undriveable.
It seems I can de-restrict the downstream mixer by creating a restriction up-stream. TT suggested this as a possibility (and the K&N 'issue' validates it) but just how restrictive is this approach? Will I be able to completely de-restrict the downstream mixer (which I'd like to do but expected to settle for 7 or 8 holes open which looks possible already with zero fueling contribution from the upstream mixer) merely by having a 45 mm orifice upstream from it (45mm ID being as big as I expected but with a better flow coefficient than with what is little more than a plate).
So, my options appear to be to attempt to induce LPG flow through the upstream mixer by making a venturi out of it, Using it as it is (without LPG supply) to enable de-restriction of the downstream mixer, a combination of those where I get a reasonable airflow rate from both restrictions while feeding only the downstream one with LPG.

Guys, treat all the above questions as rhetorical and just have a laugh - let it cheer you up - before you see another 20,000 new Covid cases on the news tonight....

[Gilbertd]

Rather than cheer me up it's all gone completely over my head. RF and electronics is the field I worked in until very recently. I'm just going on experience. I've sorted out quite a few singlepoint systems that weren't working correctly. A couple of those used a mixer similar you what you had originally and didn't work. In order to get sufficient gas at idle, the vaporiser had to have the idle bleed set to give a constant flow of gas, but then it would still go lean out of idle, suggesting insufficient signal (or 'suck' as I prefer to refer to it). Looking at the one you have at the moment, my point from earlier stands. The holes around the outside allow airflow to bypass the venturi and, although I don't know the exact size of the venturi, half of it is plugged up with the bit in the centre so, to me, it looks like it reduces the cross sectional area too much. I'd try a conventional venturi mixer with a larger orifice but all of the airflow passing through it. Works fine for me.

[Pinger]

Rather than cheer me up it's all gone completely over my head. RF and electronics is the field I worked in until very recently. I'm just going on experience. I've sorted out quite a few singlepoint systems that weren't working correctly. A couple of those used a mixer similar you what you had originally and didn't work. In order to get sufficient gas at idle, the vaporiser had to have the idle bleed set to give a constant flow of gas, but then it would still go lean out of idle, suggesting insufficient signal (or 'suck' as I prefer to refer to it). Looking at the one you have at the moment, my point from earlier stands. The holes around the outside allow airflow to bypass the venturi and, although I don't know the exact size of the venturi, half of it is plugged up with the bit in the centre so, to me, it looks like it reduces the cross sectional area too much. I'd try a conventional venturi mixer with a larger orifice but all of the airflow passing through it. Works fine for me.

I get what you are saying but I'm already at an equivalent diameter of 35.6mm (and that is driveable - but there should be more top end power) and the largest conventional mixers available are 36mm. There's headroom for more from my mixer - it's just a case of exploiting it.

[Gilbertd]

So are you saying that even if you got the 80mm OD one like this https://tinleytech.co.uk/shop/lpg-parts ... nal-mixer/, it still only has a 36mm venturi? The spare one I have here is 36mm venturi but 78mm OD but is intended as a male/female fitting directly onto a throttle body. When I tried to use that I found 36mm too large for a 195 bhp, 4.6 litre V8 (same problem you are having, fine at idle but going lean out of idle) and had to go down to a 34mm venturi.