best place for mixer ?

[bodger]

i have a FLAPPER system as i was just wondering where is the best place to put the mixer
it front or behind the airflow meter ? , i don't have the room to mount it directly on the plenum , but it's the round type so i can put it inline in the intake pipe

i was going to go totally LPG , i did try it but it gave no more economy or performance to get rid of the airflow meter so it's back on

as it's always usful to be able to switch back to petrol incase of any problems

[camel1]

Mounting the mixer behind the airflow meter will cause idle problems on LPG mainly down to the airflow being restricted.You will have to increase the idle speed to compensate for it. Brc and other companies used to do a vacuum operated unit that opens the airflowmeter a small amount to reduce this problem when on LPG.
Other way is to use a central lockin unit to push it open a small amount 12v off the gas switched live.

This is not a problem on the later 14cux system where the mixer can mount onto the throttle itself.

[bodger]

hi , i was looking into that door loskcing thing today and most i can find only have a 1" movement , i don't think thats going to open the flap far enough

[camel1]

1" will be enough it only has to open the gate a small amount to increase the airflow through the mixer by a large amount.This will then enable you to control the fuel mixture better at idle speed.

[bodger]

that true , but i'll have to do it so it pushes it open but not attached to the flap as it will need to move freeley the rest of the sweep and when on petrol move freely too

i have a cunning plan on the go , it's in an old ambulance that has air suspension .. and i'm thinking along the lines of an air operated rod to push it open through a hole in the airflow meter

[Dangerdoc]

Hi Bodger,

Interseting this one. Reading many forums on the LPG front and also the recommendations of RPi there is a consensus that having gas mixed that far down the line gives a gas rich mixture along the entire length from the AFM to the valves.

All good and well until there is a mis-fire and you have a pretty large volume of gas to go bang.

Now having destroyed several AFM's this way I will be going down the route of a mixer very tightly fitted into the plenum inlet. This gives the least gas volume mixed to go bang.

The flap openers the ilks of those produced by Chris Perfect Components (now not doing them) and others using rods and levers to open the flap "a bit" is not really what it is about regarding idle. This helps but.... the idea of a proper flap opener is that when running on gas, a solenoid provides vacuum to a diaphragm and then that operates a crank thus pushing open the AFM flap. Should a misfire occur then the flap is not blown off it's axis and jammed in place.

Venturi gas mixers ono the air filter side of a flapper AFM reduce air flow, can have real crappy mixture effects when at idle and are not really ideal.

The 72mm jobbies are much better and can be twin fed with two vapouriser units to accomodate better running when on LPG - OK economy suffers on the LPG front a tad but you have better power.

I used a CPC AFM flap opener for ages but it started to jam open. Also there is increased risk of air leaks getting in.

The benefit of the plenum unit means that the hoses can be attached and only lightly held in place to prevent unmetered air entering the plenum but loose enough that when a backfire is experienced - and it is only a matter of time, then remember to lanyard that flexi hose from the AFM to the plenum or indeed venturi as looking for it down some wet dark road is not good.

If I remember - you've been down this route before??

Thought I'd stick my oar in on this one. Seems a common issue with flapper EFi's.

[Dangerdoc]

OK, missed some of the context - maybe rtf post properly would help - hey it's early...

You say you can't mount directly to the plenum - OK but as close as you possibly can is the real answer.

I can't see an issue with the flap position actually as the injector circuit is switched off when on GAS and so there is no "air / fuel" metering required.

The issue is backfire. RPi state that they pin the flap back and run cintinually on gas. When they occasionally need to run on petrol then the flap is unpinned and allowed to operate normally.

OK so there is a restriction in the diameter of the inlet pipe, but the VOLUME of air (remember flapper not hotwire) is still metered and on acceleration the vacuum will just pull more air through. Heck you have a K&N - plenty of volume gets through that one.

Can't see an issue with the flap, just an issue with volume of gas/air mixed that can explode hence - closest poss position to plenum.

[ChrisJC]

See last paragraph of this page for comments on mixer location, flap openers etc.

http://www.cowdery.org.uk/lpg.php

Chris.

[bodger]

thankyou once again guys ..

so if i put the mixer as close as i can to the plenum thats the best ... i know it sounds liek a sillly question ( another one ..lol)
but i have one of those back fire/flapper savers , which side of the mixer should that go ?

ps: beefy says hi Chris ..

been to the britcar thingy at wicksteed park today and he was there and MGBV8 was there too

[Dangerdoc]

The backfire valve - if I remember yours is the one with the orange band should go as near to the AFM as you can. The reason for this is to allow the air in the pipe from the backfire valve to the plenum to act as a kind of "compressive gas" and provide for a little extra to accomodate the shock of a backfire.

Make sure that the breather pipe on the same side as the linkage (goes to the metal pipe under the plenum) is not held in place with a hose clip.

Make sure that the crankcase breather hose from the flame trap to plenum is also not held onto the plenum by a hose clip but IS held onto the flame trap by one.

Of course that's only if you have EFi. The more pipes that can pop with ease without letting unmetered air into the plenum the better.

I had my plenum main inlet pipe held on by friction and tethered so it could blow off, save me loads of times on the old engine.

[ChrisJC]

You don't need the backfire thingy if you have a flap opener. I don't have one, and both times it's backfired all that's happened has been that the engine has stopped.

Chris.

[Dangerdoc]

OK I agree indeed that if you have a functioning flap opener then there is no need for a backfire valve.

However, having been the victim of a failed flap opener and that can be a vacuum roblem i.e. split pipe, electrical - duff connection or pure mechanical - dead linkage then there is a risk that the flap opener can fail, mine did just that.

Now I would have a flap opener any time AND a backfire valve. Hell when the LPG is re-setup on the new engine after it has been ran in then you can rest assured I might even tap two "popper" valves into the plenum sides themselves.

It is much cheaper to have the protection than to have a dud AFM along a dark wet road in the cold - as I say, been there - done that.

I also always carry a new / recon AFM and flexible pipe in the car just in case with the CO screw adjusted to exactly the same position as the one in the car, oh and some spare fuses. Dead LPG circuit - non-runner I found.

As always there are guys out there running all sorts of LPG configurations and getting away with it and those who just have endless trouble, just like engine setups.

Read up - review - and decide just how far you want to make your system as bullet proof as you can and also consider the more complicated, the more can bugger up.

hth

[bodger]

right i have now moved the mixer to the plenum mouth

only to find that on LPG there dosn't seem to be enough vacuum to open the airflow meter ... if i jam it open it runs fine ...
so as i have no flap opener at the moment ..it's hooked open

[Dangerdoc]

Hi bodger,

Does your nut this lot does it not

The lads at RPi do say that the flap can be held fully open if you are starting and running on LPG. The metering is not required as the injectors are switched off.

Try taking out the back fire flap if you are pinning the flap open as if it is held fully open and wedged as such there is no risk of the failures as listed above.

It seems strange that there is limited vacuum as when mine was running the vacuum was very strong indeed and that was a piddly 3.5

[bodger]

i certainly does DD , i have put i back and it runs sweet as a , i can't believe that moving it the othersids of the AFM can make that much differance
i suppose it would be ok with a HOTWIRE setup as there is no flap in the way ...

i really must stop getting bored ....lol