Thoughts Appreciated: Air Flow + Mixture

[fisha]

OK, long post, here goes....

Engine is a 3.5 carbed with Stromberg carbs.

I have fitted LPG and its supplied by fairly standard mixers sitting between the small elbows of the carb and the carb body itself.

Due to the way of the coolant pipes and gas pipes, I cannot fit the original air filter box + elbows and so have instead fitted a set of cheap free-flow-style filters onto each carb.

I have removed the lift pistons + needles from the carbs so there is no restriction between the mixer and the throttle body.

The LPG setup is closed loop, so there is a stepper motor controller on the gas feed pipe to the mixers.

In reality, it looks like this...

And can be simplified to this...

Now, at idle, I can setup the LPG such that the air fuel ratio of the exhaust gas is perfect. i.e. the air/fuel flicks across the 0.45V magic value and the stepper motor sits bang on the desired default value of 90 to 100. This is good so far. B)

but

As soon as I push the throttle and the revs rise, the mixture goes lean . . . big time. Even if I allow the stepper motor to fully open in this situation, the mixture is still lean. This is also apparent during driving when pulling from low speeds in a highish gear, the car stutters slightly under accleration. It also seems to flat spot at around 4000rpm. Other than that, the whole setup goes fine, it'll trundle along smoothly, accelerate OK ( give or take a stutter ) and in general is nice to drive.

sooooooooo what can I do about this?

It would seem that the manifold vacuum is not drawing into the mixers enough and that the free-flow filters are too free flowing. I know there will be some saying " I told you so " but who cares, thats the best way I could fit the gas.

I was thinking that If i put some sort of restriction on the air flow, then the mixers will get more of the vacuum, and so draw more gas, making it richer. The original box + elbows setup must simply offer more resistance to the air, so if I can acheive the same effect, then I should be OK.

So what do people think about putting some sort of bar / restrictor plate between the filter and the small elbow of the carb?.....

In theory, this should have the effect of limiting the air flow slightly so that the mixers get a better draw. But what factors should i concern myself with regarding its design?

I could just make it a flat bar across the whole, adjusting the width to find a good balance - but would this distirb the air flow and create eddys in the flow? Does the mixer need a smooth flow across it or does it not matter?

Or should I restrict the whole cross section of the inlet by means of a mesh? That way its an even restriction and less likely to create large eddys in the flow.

What sort of mesh size do people think would be suitable??

see! lots of questions ! and your inputs appreciated.

regards, Don

[02GF74]

I have removed the lift pistons + needles from the carbs so there is no restriction between the mixer and the throttle body.

rather than mess with meshes, bars, plates etc. to restrict flow, how about replacing the piston and needle?

As you say, the piston provides a restriction but it is pulled upwards by the vacuum so the restriction reduces (or something like that - need to read the SU manual again).

Why did you remove the piston? Did the LPG installation kit say so or you thought it would work better that way?

[Niall_CSK]

I was just about to say that too, honest

This may not help but I have come across this with the wife's car.

If the air pressure going into the filter is significantly higher than atmospheric then it can fool the vapouriser, not quite sure how, into thinking there is not as much vaumn as the engine is demanding and end up throttling the gas regardless of what the stepper motor is telling it.

I proved this on wifies Astra by connecting up the cold air in scoop near the radiator and it ran like a bag of nails accelerating (difficult to tell with the Astra I know ), take the scoop off and back it came to normal rattly, put a bit of plastic infront of the intake to defelect air and it drove better than it had ever done.

I found out about this after it had gone in for a service and the nice little people in Reg Vardy took a look round the engine compartment to see what the greasy little yob doing the work had broken this time, saw it hanging off and shoved it back on (after throwing my lovely air deflector away ).

I believe this is something that has to be overcome with turbo installations by using a special type of regulator.

My suggestion, daft as it might sound, is to put a large tin can over the lovely K & N's to prevent direct air pressure.

It is worth a try, and won't cost a bean (sorry I couldn't resist that)

[fisha]

the thought had crossed my mind that the fan could be throwing air at the intake, but I had always assumed that the filter material itself would be enough to diffuse it.

I'll making a shield of some sort and see how that goes....

[Diff]

Donald,

Edited to add: free flow filters can reduce the vacuum signal from the mixers to the vapouriser and cause a weak mixture, and this is common if the mixer venturi diameter is large. The filters 'may' be affecting your signal, but if your venturi sizes aren't large, it may not be a problem, but:-

Shielding the filters from fan wash and road speed induced air flow may be important particularly at speed, but, have you set up the vapouriser?

If the diaphragm tension in the vaporiser is too high, you will not be able to get gas pulled through until higher up the rev band.

On your OMVL vapouriser, the small screw(8mm) is a bypass screw, and allows gas to flow at all speeds.

The 10mm sets the tension on the diaphragm and will only allow gas to flow from the main jet when there is sufficient vaccuum signal from the mixers to move the diapragm.

For a basic set up, you need to disconnect the stepper motor so that the valve is in the fully open position and the ECU cannot work the stepper.

Get the engine started on gas and close the 8mm screw. Keep the engine from stalling by opening the 10 mm screw if necessary. Then with the 8mm screw still closed, slowly screw the 10mm screw in until the engine just stalls.

Turn the 8mm screw out 1 turn and start the engine. The engine should idle but the speed can be adjusted by turning the screw in and out to achieve the correct idle, however you should end up with the setting between half and 1 and a half turns out.

What you are aiming for is a position where idle is being controlled only by the gas coming through the bypass (controlled by 8mm screw), BUT as soon as you push the throttle, the gas should start flowing from the main jet to ensure the mixture does not go weak.(the situation you describe).

Once this is set up, you really need to set up the max gas flow for full load. Best way is to leave the stepper and ECU disconnected as above, and connect a digital volt meter to the Lambda signal wire. Go for a drive and get the engine warm; accellerate hard up a hill and see what the volt reading is. If it goes over about .8 volts it is too rich and you need to screw in the max power valve in the gas feed to the mixers. If less than .65 volts you need to open it a bit. As a basic approximation you should be looking for around .7 to .8 volts under full load.

The mixture needs to be rich under load to ensure that there is max power, and combustion temps are not too high. When the gas ECU reads the that the throttle position is fully open, it will go into 'open loop' so that the stepper does not try to reduce the mixture to stoichometric (.45 to.5 volts) which is too lean for full load.

Setting the max flow/power valve ensures that you are not wasting gas by having an unnessessarily rich mixture at full throttle(open loop)

Once you have done this it is adviseable to recheck the first settings with the 8 and 10mm screws.

You can then reconnect the ECU/stepper motor. If you keep the voltmeter in the car as you drive you can then check that the stepper is working to keep the lambda signal to around .5 volts under part throttle conditions, and that the system is going open loop and richer when under full throttle/load.

Hope this helps,

Regards,

Diff

[FridgeFreezer]

I'd add that .5v is way too lean at idle for your engine, they like to idle quite rich. The only time the reading should be below about .65v is cruising / decelerating. Under heavy load, high rpm, and at idle, you want it richer than that.

Remember narrowband lambda sensors are there for economy and emissions.

[fisha]

ok thanks for that, i'll give it a try later on. . . .

once i've replaced the starter motor . . . . .

which has given up over the last couple of days. The final straw being lying under the car in the carpark at work in the smart work clothes, hammer in hand banging the starter!

[shiftech]

Hi Don, out of curiosity.what do you have between the mixers,A "y" or a "t" peice.Had a simular problem when using a "t" peice.Swopped for a Y and everything fell into place.My EFI will run well with just the mixer,no pipework or filter,depends on how the vapouriser is set up,the same goes for starting on gas some will and some won't.I tend to keep mine steich or close to it on full load as LPG is a bitch for washing the bores if run to rich(don't ask me how I know)

[fisha]

Its a Y-piece i have . . . shown here in the top corner.

also visible is the bracket bodge for the throttle sensor to fit on carbs.

[FridgeFreezer]

I tend to keep mine steich or close to it on full load

That's a rather risky strategy. As I said, the only times you want to run that lean are under low/medium loads and RPM.

Even the OE ECU's ignore the lambda sensors outside of these areas, and they wouldn't bother doing that unless there was good reason when they could get better economy figures by running closed loop all the time.

[hangover]

I have just fitted a similar lpg setup to my 85' carb rangie, i noticed that before i fitted the filter housing and elbows back on it wouldnt idle or run under 1000 rpm due to lack of vacumn. How thick are your mixers ? Mine are about 1/2 " and the original filter still fits. I was having probs with mine so i have just replaced the 2nd hand converter i had with a newy ands its running fine now.

Whats with the stepper motor on a carb / lpg car? you obviously dont use petrol then if you have gutted the carbs.

[fisha]

I run a closed loop LPG system on my carbs.

I plumbed in a lambda sensor into the exhaust, and a throttle sensor on the throttle levers.

I've also set the ECU so that it allows start on gas. It starts reliably every time, even in cold weather. In fact, the carbs are great in the cold since the choke mechanism still works. You can pull the choke out give the engine a supply of air to stop it from stalling when its cold. Works brilliantly.

[shiftech]

I tend to keep mine steich or close to it on full load

That's a rather risky strategy. As I said, the only times you want to run that lean are .

steich is not lean its the correct fuel /air mixture for combustion,OE don't run LPG.If you over fuel to much then you risk damage and misfires.LPG is normally set up to run rich at full throttle but kept to steich at all other times,dependent on the voltage from the tps.on some vapourisers you can set the pressure that the diaphragm reacts to(usually factory set and covered by a plastic seal

[Niall_CSK]

I've also set the ECU so that it allows start on gas. It starts reliably every time, even in cold weather. In fact, the carbs are great in the cold since the choke mechanism still works. You can pull the choke out give the engine a supply of air to stop it from stalling when its cold. Works brilliantly.

Have to have a chat about that when I get on leave Donald.

I believe MS can cope with this, what's the latest with it Fridge?

[shiftech]

Hi Don,

does the signal off the lambda cycle high/low when your idling,just seen the pics of the welding on the lambda bush. very nice.Are the valve/brass thingies on the Y piece causing a restriction in the gas line.is your filter clean.

[FridgeFreezer]

Shiftech -The very best of luck to you & your engine. Personally I'm sticking to the advice of every tuning resource I've ever seen and some much cleverer people than myself (OK, not difficult) and running richer towards the top of the map.

Niall - MS can cope with LPG at a number of levels.

It can happily be switched off or have the injectors cut by an existing LPG install

It can run switchable fuel maps if you can create a circuit to switch injectors

It can run switchable ignition maps if you want it to run your ignition too

It can run staged injectors so you can run on petrol and LPG at the same time not sure why, but you can if you want to!

Without too much twiddling it could be made to start on petrol until the engine temperature reaches a certain point and then switch to LPG (in fact I've just worked out how to do this and it's really easy)

It can add the other type of gas (NOS) 2-stage controller built in with auto fuelling & spark adjustment

I'm sure there's other stuff with the V3 boards that are out but I've not played with one yet.

[fisha]

shiftech,

yes, it does, it flicks across the 0.45V value at idle happily.

its just as soon as i come out of idle, the stepper opens right up to the max value and the O2 reading rockets to zero.

i feel its all gone out of tune a little at the moment, so i'm going to give the previous tune method a go ( nothing to lose ) and i'll see how it goes.

[fisha]

k doky

had a wee play with it this afternoon.

disconnected the stepper and adjusted it that way and it seems to idle fine.

i then took it out for a spin with the laptop reading the O2 voltage.

regardless of the stepper plugged in or not the following happens....

idle is ok, smooth ( with the stepper in its steich )

as soon as i'm out of idle, its lean

it accelerates lean ( but it accelerates OK an happily )

it cruises lean as it bimbles along.

however, if I go above 3/4 throttle to full throttle, it jumps to 0.7 to 0.8V, just where it should be.

so basically if i plant it, its spot on, other than that it runs lean.

i have tried adjusting the response screw out further, but its now hitting the back of the expansion bottle and almost feeling like its going to fall out. the response screw seems to not do all that much. i can screw it back in to get the stall as described before, but once its above stalling point, it doesn't seem to make much difference.

I wonder if i should be doing all the diaphram stuff?

[shiftech]

In your software you will have a value for the throttle pot,Its the point that the stepper opens ups for hard acceleration.There is also tick box "keep steich"

Make sure the box is not ticked,keep the voltage set for the top of the throttle pots range.You should be able to adjust the lambda by using "custom Lambda" to set the mixture up by telling the software what value to expect for steich.

[fisha]

i'm not aware of a custom lambda setting on my ECU. i can either set a 0-1V, a wideband, or nothing on my setup iirc.

regarding the throttle pot,

its all working as it should. the ECU seems to understand 4 throttle positions. idle, low, medium, full.

it automatically selects what threshold and below is the idle - you cant tell it, it works it out for itself and adjusts it as you go along and it gets it right.

then as soon as you put the pedal down a little and it detects a rev-rise, it jumps into the low tps reading. continue pressing it'll go into the medium, then at full throttle it'll show full throttle.

full throttle threshold is a level you can set and i'm happy with it.

regarding the stepper motor settings and the TPS....

i can set the band which the stepper can operate above and below its self selected default - i have this set at 255 above and 255 below, so the stepper can do whatever it wants to try and acheive stiech at idle. it seems to settle around 100ish - which is just about what you want.

i can also set the band above and below default for when its out of idle, and again, i have this set at +/- 255.

lastly, i can set what the stepper should move to at full throttle, and this is set at fully open ( 255 )

now when out on the road .....

at idle its fine, it manages steich at a stepper value of around 100.

then as soon as it hits low out of idle, the mixture goes lean, the stepper races upto 255 and stays there. this is the case as the revs build on low to medium throttle, and when your cruising along, the stepper is right open.

the only time it isn't at 255, is on a medium to high throttle opening, it suddenly jumps to a typical 0.7 to 0.8V reading, and then the stepper starts to back down from 255 to keep the steich.

At full throttle it jumps to 255 as set.

[shiftech]

I'm wondering if the open carb chambers are having a damping effect,also the shape of the venturi is designed to have the pistons in place.Have you tried it with the pistons in place but held open.with the stepper opening to 225 but running lean its doing what it should,(the stepper )but the fact that the mixture goes rich on full throttle suggest a vaccum or vapouriser problem or an inconsistencie between the vaccum/throttle position.as an after thought,if the carb chamber is larger than the area past the part opened butterfly then you will get a reduction in flow and pressure.(reduction in a negative pressure = pressure rises)Less pull on the vapouriser,

Can the carb fuel chamber vent to atmosphere,are you pulling air in through the jets,again reducing vaccum.

[Diff]

Donald,

If adjusting the diapragm tension didn't work with the the system 'open loop' as I described, then the problem is likely to be that the mixers are not producing a decent vacuum signal at low revs when you accellerate hard.

This low signal could be caused by:

1. Mixers too large

2. free flow filters on large mixers

3. Fan wash

4. Carb pistons missing 'could' cause weakening of the signal due to the large volume of air between throttle butterfly and mixer as suggested by Shiftec

5. Air leak ie through carb somewhere?

If you wanted to use the standard air cleaner, you could use the Chris perfect/Iwema mixers which fit inside the 'U' bends between the original filter housing and carbs.

Regards,

Diff

[fisha]

i think my principle problem is the free flow filters coupled with fan wash to be honest.

the pistons were a definite hinderance to low rev smooth running. Admittedly i didn't have the lift piston setup, but as i dont use petrol atall now, so i didn't see the point of even having the pistons in the first place anymore.

So to remove them, i took out the pistons, but left the rubber diaphram in place as it creates an air tight seal between the carb body and the chamber cap - so this it what i have. The mixers are shown to rough scale onthe left of the carb, they are smaller that the general diameter of the carb inlet. i can measure them a little bit more closely if required.

i can see the point about the chamber being larger than the throttle body cross section so you would get less suction within the chamber itself, but what goes out of the chamber past the throttle butterfly must surely have to get drawn in past the mixers. So surely the same suction must be passed onto the mixer cross section??? ( i.e. large chamber drawing air through a smaller inlet ( the mixer ) is surely an equal increase in suction?

i'm coming round to the idea of having maybe having to make something similar to the old air filter box. trouble is that i've used the space ( or most of it ) for the coolant pipes.

[Diff]

Donald, without the pistons in, you have a large 'chamber' which could act like a damper at low revs as suggested earlier by shiftech. If you take things to extremes and imagine a chamber the size of the albert hall, sealed except for a 50mm pipe either side of the hall. If you sucked on one pipe, you would be hard pressed to detect the same vacuum signal on the other pipe at the other end of the hall because all the air in the hall is acting like a damper. The same amount of air sucked out will be sucked in, but I don't believe it will be at the same speed.

The mixer venturi works by the speed of flow through it. If you don't have enough speed, you won't get a strong vacuum signal.

Don't forget that there is a breather in the top of the dashpot damper which could let in air+ it could be drawing in air from the float chamber via the jets and petrol pipes if these are not capped(assuming you have disconnected them.

You may have a combination of problems, and not having the pistons fitted 'MAY' be one of them.

Before redoing the air cleaner/mixers, you should eliminate the posibility of the missing pistons being the problem, otherwise even if you change the mixers and filters, you may end up with the same problem.

I would refit the pistons and either retune as I described with them working as they should on petrol, or refit them and wedge/fix them in the up position and retune as I described.

It is important that the system is initially tuned 'open loop' in order to get a set of base parameters within which the gas ECU/lambda/stepper can work once they are reconnected.

Hope this helps.

Regards, Diff

[fisha]

i see your point about the large chamber now.

i did do the tuning in the open loop mode as described. and thats what i based the previous results on lean upto mid-high throttle upon.

i'll try the pistons back in and also try wedging them up too and see how I get on.