I can't get the van to run smoothly on petrol when the mixer ring is fitted.

[David James]

I purchased a van that has LPG fitted, and it runs fine on LPG. But I can't get it to run smoothly on petrol no matter what adjustments I make.
The previous owner said that when running on petrol he always had to have the choke on a bit. I have found this to be the case too. With the choke on the can accelerates smoothly and with more power, but the idle is too fast and fuel consumption is rubbish.

I have replaced the carburettor and the problem is still there. However, if I remove the LPG mixer ring from the carburettor air inlet pipe I can tune it to run perfectly on petrol.

So, I am wondering it the LPG mixer ring is reducing my air flow so much that running on petrol, is going to be impossible.

The carburettor has a 2" opening and the centre of the mixer ring is about 3/4".

I don't think this system has run correctly since it was installed, and I am wondering what I can do to fix it. Should I take it to a LPG installer, or is there anything I can do.

Any help would be much appreciated as I have run out of ideas. I am based in Sunbury, TW17.

Thanks.

[Brian_H]

More info would help, but if you suspect the mixer to be your issue (and the above suggests that you've managed to isolate it to that) then speaking to a supplier to get a more appropiate mixer might be a good idea. Tinley Tech should be able to advise you on a more suitable mixer - https://tinleytech.co.uk

Might help to know what van/engine your talking about as well?

[David James]

The van is a 1986 Fiat Ducato. 2.0 litre petrol.

I have a spare air inlet pipe, so I can swap between one with the mixer ring and one without.

I have cleaned and fitted a service kit to the carburettor, then set it up according to the manual. I can get it to run fine on petrol, until I introduce the mixer ring, then no amount of tuning can get it to idle and accelerate smoothly.

[CNG]

As Brian says, if you've got it to run well enough sans mixer, your problem is right there. There's not many Ducatos from that era about, I know this 'cos my 60s Land rover robs a Ducato disc-brake conversion from your van. I'm guessing it's a camper we're talking about.

The world's moved on a bit from single-point mixer systems, there can't be too many of us left running them. They don't get simpler, and 'faulting' one won't be too hard or pricey. If you can use a screw-driver/7mm socket and jubillee-clips, can't see you struggling.
Sadly no longer made, back in the day you'd get a BLOS mixer, some had a spring-loaded 'lift-mech'. The spring lifted the mixer, thus removed from intake-tract on petrol. That would be your ideal solution. Saving that, I suspect you need a less restrictive mixer? But... 'strangling' as it does now, your vapouriser will be getting heaps of vacuum. Something less restrictive might not provide the vacuum your vapouriser looks for.

If you can't grasp what we're on about, by the time you've read this thread you'll 'get' it.

viewtopic.php?f=33&t=15479


If we're right, and unless there's something very wrong with your carb set-up, I can't see how we could be, changing mixers is a simple enough fix.

[LPGC]

The problem will be the mixer is too restrictive or the mixer is too close to the carb, or both.

Too restrictive and the carb will usually give a rich mixture because the jets above the throttle will not only see the usual venturi effect they'll also be at partial vacuum.

Too close and the air coming through the narrower hole of the mixer can be directed at a certain area of the carb so the airflow past jets is different.

There's still a great many vehicles running mixer systems, including loads of old camper vans.

[David James]

Thanks for the advice so far.

I have made a crude adaptor to increase the distance of the mixer from the carb, and there is a definite improvement, and the van runs much better. Although with this adaption I have not got the air filter connected, so this may also be making a difference.
I have added a couple of pictures. The first showing my extension tube. Very professional I know. The second picture shows where the mixer is currently fitted and where I would like to fit it. So my question is, will the work so close to a bend in the pipe or will turbulence in the air flow be an issue. Another question, is there a set distance the mixer should be from the carb, or is it trial and error.

Thanks again, I was beginning to go insane, trying to get to the bottom of this.

https://ibb.co/BnzWP2V Click the "back" button after viewing the image.
https://ibb.co/1MrVXCH

Red line = Current mixer location
Blue line = Suggested mixer location

p.s. Struggling to add pictures.

[David James]

Update.
Extending the distance of the mixer from the carb definitely improved things.

So I have moved the mixer to the place I suggested in the air inlet pipe. The van now runs on petrol without coughing, spluttering, backfiring and kangerooing down the road. And no need to add choke.

It's amazing that just the positioning of the mixer could have been the cause of all my issues. The carb still needs a tiny bit of fine tuning, but at least now the end is in sight.

Thanks so much for the help and advice.

[Brian_H]

Photos attached, FYI they have to be under 1mb each to upload, thats probabbly why you had issues. There are better resolution ones on your links, these are just the preview images accessed from that link

2.jpg (116.63 KiB) Viewed 3951 times

3.jpg (65.69 KiB) Viewed 3951 times

It might be worth posting a couple of shots showing more of the overall setup, as if you do then someone on here might be able to advise on a better location for the mixer, does sound like your getting there with it.

Bear in mind that the lack of an air filter will have an effect, moreso on an open loop like you have got.

[David James]

I have moved the mixer to where the blue line is in the second photo, and refitted the air intake pipe correctly, so the air filter is connected.

Everything now seems to be running as it should. I just need to fine tune the carb.

The LPG system was fitted by a registered installer, but I don't think the quality of workmanship is particularly great. And I don't think the system functioned properly from day one, according to the previous owner. When I purchased the van the LPG system was no longer being used, and the petrol pipe was no longer being controlled via the switch on the dashboard. The air rubber air intake pipe had been hacked with a knife to install the brass pipe that connects to the mixer, which had caused the pipe to badly split. I had then been wrapped in cloth tape to fix the leak and hide the damage. All these things I have been discovering and fixing along the way.

I have reset the tappets, replaced the plugs, leads, distributor cap, rotor arm, coil, ignition module, and adjusted the ignition timing, and serviced and tried two carburettors, so this really has to be the last piece of the puzzle.

It's so annoying when you know there is something wrong but you can't find exactly what it is, and so you can't fix it.

p.s. Thanks for the tip about photo size.

[CNG]

As ever, LPGC has it down. I would never think pipe length would make a difference, yet clearly it does. But what do I know?
Surely Shirley, a few inches of pipe makes little difference on such a simple set-up, we're moving the same air, and adding more restriction. Thing is I like to know this stuff, and I still don't truly understand why my double-mixer abortion works as well as it does.

On SU carbs the mixer sits on the carb. Why does a short lump of pipe make the difference?

[Pinger]

As ever, LPGC has it down. I would never think pipe length would make a difference, yet clearly it does. But what do I know?
Surely Shirley, a few inches of pipe makes little difference on such a simple set-up, we're moving the same air, and adding more restriction. Thing is I like to know this stuff, and I still don't truly understand why my double-mixer abortion works as well as it does.

On SU carbs the mixer sits on the carb. Why does a short lump of pipe make the difference?

In the case of this (the OP's) set up, the difference is likely along these lines....
A gas at a given temperature has a finite energy which is further comprised of velocity and pressure and those two trade off each other. If the gas is moving fast as it is passing through the mixer venturi then its pressure is lower. When the gas is static it has the highest possible pressure. Transitions between such states are not instantaneous. What the OP has provided by spacing the mixer from the carb is distance (and hence time) for the gas emerging from the mixer at high velocity and low pressure to slow down and regain some pressure.
Thus, slowed, it is no longer travelling through the carb at the previously higher velocity accentuating the carb's venturi effect and, at higher pressure, the vacuum effect is reduced - both factors which affect fuel flow through the carb's jet(s).
There are other factors at play such as temperature changes (which may impinge on fuel vapourisation) and turbulence but the above is likely the predominant mechanism at play in the OP's set-up.

[LPGC]

Good to know someone else would have advised moving the mixer further from the carb if I hadn't Pinger

[David James]

I think it really was something as simple as the mixer being too close to the carb.
With the LPG system being "professionally" fitted, it was the last thing I wanted to tamper with in my quest to get the van running better.
I have added a picture showing how i have it set up now. It's running much better than it used to, and moving the mixer has been the single biggest improvement so far.
My next question is. Should I be worried that the mixer is now "upstream" of an engine breather pipe that feeds into the air intake pipe. The van seems to run fine on LPG, but should I be concerned that it could be drawn into the engine through the breather pipe.




1 = Engine breather pipe
2 = Petrol Pipe
3 = LPG
4 = Mixer location

[Pinger]

Good to know someone else would have advised moving the mixer further from the carb if I hadn't Pinger

Errr - not quite!

[Pinger]

My next question is. Should I be worried that the mixer is now "upstream" of an engine breather pipe that feeds into the air intake pipe. The van seems to run fine on LPG, but should I be concerned that it could be drawn into the engine through the breather pipe.

At the risk of starting a war here.... yes, you should be.
Relocate the breather pipe upstream of the mixer and you'll avoid LPG in the crankcase. Also, as it was originally configured, it is calibrated to run with the breather pipe upstream of the mixer.

[David James]

It's no big deal for me to relocate the breather pipe. So I will do that tomorrow.

If anyone has any other suggestions on what I should of shouldn't be doing, please feel free to advise me.

Thanks again everyone.
p.s. The breather comes from the rocker cover and does have a gauze oil trap filter in it. Not sure if that makes any differnce.

[Brian_H]

I think it really was something as simple as the mixer being too close to the carb.
With the LPG system being "professionally" fitted, it was the last thing I wanted to tamper with in my quest to get the van running better.
I have added a picture showing how i have it set up now. It's running much better than it used to, and moving the mixer has been the single biggest improvement so far.
My next question is. Should I be worried that the mixer is now "upstream" of an engine breather pipe that feeds into the air intake pipe. The van seems to run fine on LPG, but should I be concerned that it could be drawn into the engine through the breather pipe.




1 = Engine breather pipe
2 = Petrol Pipe
3 = LPG
4 = Mixer location

Assuming that noone has been messing with it since it was fitted of course to make it worse. Though there are (or more were) plenty of examples of good kit fitted badly and horrible results as a result. Its not unknown for badly maintained or poorly converted single point vehicles to end up blowing the air intake apart under the wrong circumstances, particually if there is lots of plastic involved there.Though if the whole system has the look of being badly fitted, then your probabbly right to blame the original installer whoever it might have been.

[David James]

My reason for thinking it wasn't a great install was mainly the hole that was cut in the air intake pipe to fit the brass pipe that fits into the mixer. I had been hacked out with a knife, so it wasn't circular. And was too small for the pipe to fit in, so when forced in, it had split the intake pipe. This had then been taped over with cloth tape.

I glued all the splits together and drilled the hole for the mixer pipe. Then sealed with rubber petrol proof tape.

I now have a new replacement air intake pipe, so when I have everything working correctly I will fit that with everything in it's correct place.

[CNG]

In the case of this (the OP's) set up, the difference is likely along these lines....
A gas at a given temperature has a finite energy which is further comprised of velocity and pressure and those two trade off each other. If the gas is moving fast as it is passing through the mixer venturi then its pressure is lower. When the gas is static it has the highest possible pressure. Transitions between such states are not instantaneous. What the OP has provided by spacing the mixer from the carb is distance (and hence time) for the gas emerging from the mixer at high velocity and low pressure to slow down and regain some pressure.
Thus, slowed, it is no longer travelling through the carb at the previously higher velocity accentuating the carb's venturi effect and, at higher pressure, the vacuum effect is reduced - both factors which affect fuel flow through the carb's jet(s).
There are other factors at play such as temperature changes (which may impinge on fuel vapourisation) and turbulence but the above is likely the predominant mechanism at play in the OP's set-up.

Thanks Pinger, the penny has dropped. Thus my advice along with Brian's to start messing with mixers, 'might' have worked, yet wasn't as wise or as simple.
Just think, our David could have found himself with something akin to this...



There's always room for another BLOS. Or there's...



I count 22 Jubillee clips - I have no shame. David. Count yourself lucky.

[Pinger]

It's no big deal for me to relocate the breather pipe. So I will do that tomorrow.

If anyone has any other suggestions on what I should of shouldn't be doing, please feel free to advise me.

Thanks again everyone.
p.s. The breather comes from the rocker cover and does have a gauze oil trap filter in it. Not sure if that makes any differnce.

What I said yesterday assumed a PCV (positive crankcase ventilation) system where (filtered) air is drawn through the pipe in question, through the crankcase, then enters the inlet manifold downstream of the carb. With that system, IMO, relocation is prudent.

Pre-dating PCV systems was a pipe fed from the crankcase to upstream of the carb to draw any gasses due to piston blow-by back into the engine. The gauze trap suggests that's what you have - but the van's age suggests PCV. With the earlier venting method the flow is away from the engine so no real need to relocate. Probably best to investigate which system you have (PCV will have an additional pipe - probably much smaller diameter connecting the crankcase, etc to downstream of the carb (and probably close to it though it could be anywhere on the inlet manifold)).