new install - need mixer for 2bbl Carter

[Oldskool]

Hello again, further to my intro back in February I have now acquired a s/h lpg kit.

I'm quite sure this is not the best gear available but it cost me next to nothing and I am in no hurry because the vehicle is not in regular use so my plan is to play with it until I know more about what I'm doing and then maybe change any parts I'm not happy with.

So, it's single point open loop with a BRC AT90E vapouriser. I think the main thing I am missing is a suitable mixer because the twin Rover ones that come with it are incompatible with my carburettor.

Correct me if I'm wrong but I think the AT90E picks up its demand signal directly from the mixer so I'm guessing effective operation might be quite sensitive to the mixer size/design.

The carb. is a 2 bbl Carter, circular at the top but quite a bit smaller than a 4bbl such as a Holley or anything like that.

Engine is about 5L

Any advice on choosing/sourcing/making a suitable mixer? I have an identical spare carb. so I can mock something up on the bench. There's plenty of room above the carb.

I think everything else is there: solenoids, a timed relay, changeover switch etc.

Thanks in advance.

Steve

[rossko]

There exist 'universal mixers' - a round housing with external diameter to fit your carb/throttle body, fitted with a venturi sized to suit the engine.

[Oldskool]

There exist 'universal mixers' - a round housing with external diameter to fit your carb/throttle body, fitted with a venturi sized to suit the engine.

So if the external diameter of the mixer fits the carb then that must mean the lower part of the mixer fits inside the top of the carb. But the top part of the mixer also needs to fit the air filter, and that is going to be a larger diameter by however much the thickness is at the top edge of the carb. Or does the filter still mate up with the carb and the mixer just sticks up a bit inside the filter? I can see how that might work.

I'm curious as to how the venturi is sized relative to the engine size. My experience of carburettors tells me that choosing venturi size is as much to do with service rpm as it is with engine size, and this engine is unlikely ever to need to exceed 3,500 rpm.

On that basis I'm tempted to see whether I could cut down the Rover mixers into half moon shapes and join them together to make a siamese double mixer that will fit in the top of my carb (insert light bulb smiley)

[Gilbertd]

A third of the way down this page http://www.tinleytech.co.uk/acatalog/Mi ... stems.html, Internal mixer and just select the one that will fit your carb.

[Oldskool]

[ In case classicswede is reading this - I got your PM and replied last night but it is stuck in my outbox for some reason ]

Thanks that's helpful - or rather it would be if I hadn't just realised the top of the carb. is not a complete circle; it has a flat part at one side - d'oh!

I wondered if a "male/female" one as listed below the "internal mixer" could be made to fit, but the carb. also has two posts sticking up the sides at the top which go inside the filter box to attach it, so that also seems like a no go.

I'll try and post a picture of the top of the carb.

The ID is 60mm and the OD is 67mm - not very big for a 5l motor eh.

Thanks again

Steve

[Oldskool]

The choke flap looks also to be somewhat in the way !

[rossko]

Venturi sizing is a bit of a black art, but you can find rules of thumb about it. example
http://www.chaosboyz.nl/rubriek/techniek/techlpg.htm

Cobbling it into the air path in front of the carb is often improvise as best you can. It;s better to be as close as possible, but not essential - you can have spacing collars or fit the mixer inside pancake style air filter cans etc.

[classicswede]

That is a lot smaller than a holley!

Is this just a single or are there two carbs?

I'm sure I have made a mixer for carb very much like this before. Had the make the female part much longer than a standard mixer.

[Oldskool]

Thanks chaps. It is just a single carb but it's a twin choke. As you may know, lot of American V8s were fitted as standard with very small looking 2 choke carbs. Speed freak officiandos (which includes me LOL) generally swap them for 4 barrels but since this is on a motorhome I am resisting the temptation.

I once had a 1968 455 Oldsmobile Delta (about 7.5 litre) with a tichy little carb like this on it. Plenty of torque!

It probably flows somewhere about 350-400 cfm, that's partly why I think this vapouriser will be big enough.

The Rover mixers I got look like an old fashoined design with the gas entering through a brass booster venturi in the center, very much like a carburettor. I'm wondering whether I could drill and tap the sides of the top of the carb (aka "air horn") and fit just the boosters from the Rover ones into the center of the air flow. Assuming the air flow ranges are similar it might even work!

[classicswede]

You could try those mushroom mixers in there like that if you can get them apart. They are not great but will work. A proper venturi will always work better but when you cant fit one on you have to look for other means.

[Oldskool]

Well it's going to be interesting if nothing else.

Thanks

[Oldskool]

Been doing some more figuring out on this - the mushroom mixers (as classicswede calls them) have come to pieces OK but they are just not going to fit into the top of the carb, so I would have to make up my own "top hat" to go between the carb and filter box and then fit them into that, if I am going that route.

However, I think a male/female type circular mixer might be made to fit (with a bit of modding), so I started looking at what would be a suitable mixer venturi size.

The twin venturies in my carb. are 1 1/16 inch each - which is about 27mm. So the area of each of them is about 572mm2 which makes 1,144mm2 if I add them together.

Assuming a linear relationship between flow and area (which I know doesn't actually apply) that would be the equivalent of a single carb. venturi of about 38mm diameter.

Factoring in the physical reality that two smaller ones will flow less altogether than a single one of double the area, and that optimal flow in a circular tube (with no turbulence effects) varies as the fourth power of the diameter, then it seems that the equivalent single venturi diameter of my carb. will actually be somewhere between 32 and 38mm (and it's least likely to be at the extremes of that range).

So on that basis a 32mm mixer would seem to be suitable.

Any thoughts on that ?

[classicswede]

I would say a 34 or 36mm venturi but that does depend on what vapouriser you will be using

[Oldskool]

Who thinks this will work ???

I decided to have a go at using some of the Rover parts in a home made mixer fitting to go between carb. and air filter.

Here's the carb (this one on the engine is slightly different from the spare top cover I photo'd earlier).




Notice the "O" ring I have fitted round the top - the new mixer fits snuggly onto that.

Here's the mixer I made (it ain't pretty but I can soon tidy it up if it works).




Funny how it looks like a smiley robot face - LOL

The bit that looks like the robot's mouth fits around the float bowl vent so (hopefully) it doesn't get sucked on too much by the air stream passing through the mixer.

I put a central divider between the Rover "mushrooms" so the air flow should be more centralised around each one instead of just blasting down between them. I reckon I may want to shape the passages a bit more (which will be like giving the robot some eyebrows) but I'll try it like this first.

Here's a photo from underneath - the cut out in the center divide is to clear the choke flap:




And here it is on the carb - the filter box is attached by a bolt through a stiff wire loop so I'll need to make a longer loop to fit through this:




So - who thinks it will work ??

Also, as one of my next jobs is to find a place for the vapouriser, can you remind me which way up it should go (BRC AT90E). I'm assuming I mount it so the writing on the front is the right way up.

Also, also, the vapouriser has a threaded hole in the back with nothing in it - is this just to pick up atmospheric pressure ?

Final question - I've seem some flexy high pressure LPG pipe on ebay - is it OK or do I go with copper?

Any other comments etc. are welcome of course

Cheers

Steve

[classicswede]

The mixer looks hopeful but with the BRC vapouriser you might need to put in some restriction over the top to make it suck harder.

The vaouorsier needs to be mounted North/South with the solenoid vertical.

Flexi pipe is fine but maybe put a link to what you are thinking of using to confirm

[Oldskool]

OK thanks, solenoid vertical is what I thought.

If I need more suck I was planning to start by smoothing out the corners like this (pink bit = filled)




As for the flexy pipe - this sort of stuff:

http://www.ebay.co.uk/itm/LPG-Autogas-6 ... 1082546732

[classicswede]

Faro pipe is perfectly fine and normally less hassle to fit than copper

[Oldskool]

OK cool - thanks