Twin carbs...

[tedmcedd]

I was wondering...

is there something special about twin carbs, or could i make a set out of two singles?

I'd obviously get new matched needles and jets... do the dashpots have to be the same weight too??

possible, or am I mad!?

[Lord Croker]

I like this topic, many years ago I achieved making up a set of twin SU H4's using 2 individual carbs, having long tracts from the dashpot to the manifold enabled one of the float chambers to be between the two carbs. They worked well as long as you were prepared for the inevitable fuel surge under high speed cornering. I never found a way of doing this with HS types, I guess HIF carbs might be more adaptable having float chambers mounted centrally but my only experience of HIF's was fitting 4 of them to a V8 Rover engine, but the manifolding for that allowed a lot more room than on a twin set up on a Mini.
Dashpot pistons must be of a matched weight & the springs must also be of the same tension.

I don't think you're mad, it's probably something many of us have thought about, or on the other hand, people may think I'm mad for having done this! I look forward to reading a lot more on this topic.

[graham in aus]

Quite a common thing in Aus. Sold as a kit with extra carb and manifold. Also Sports 850 had two same side carbs.

http://www.miniexperience.com.au/filead ... ts_850.pdf

They use a 'w' folded steel linkage which is a bit crappy,, but could be improved now.



http://www.theminimag.itgo.com/Mar9906.html

[Lord Croker]

Excellent! I have two HS4's somewhere in my shed!

[ricardo]

The Speedwell sprint setup only contains 1 carb too, you're supposed to use your original hs2 to complete the set.

ST H4 carbs for LHD cars (AUD165 and AUD176) are also 2 similar carbs with 2 AUD4814 chambers, being one on the middle. The problem is the choke... I have a very original AUD176 set and have seen another one exactly the same - they are like the RHD setup, but have both choke levers cut. How do you make the choke work? You don't (?!). My AUD165 set has an interesting mechanism to deal with it which I believe to be original, although looking a bit home made. The jets are fixed and instead of pushing them down, you make the pistons go up with a lever.

As found (never seen other 165 set):







To Lord Croker: as for your HS4's, you may have a problem. If both carbs were originally single carbs, they have equal diagonal mounting flanges... maybe you can sort it out finding a squared flange manifold and drilling & tapping the holes to suit. They will not fit an original HS4 manifold.

[tedmcedd]

It was twin 4's i was thinking of! I think i may have to do it, as I have a few spare! ha! Ill make a manifold at work...

great stuff!!

[Lord Croker]

I couldn't agree more, it's just one of those projects that has to be done! Let us know how you get on, I would be very interested to see the manifold you make.

Ricardo, Have you actually used the AUD165 set with that choke modification? Not wishing to be critical, but my understanding of the SU carburettor suggests that lifting the piston & needle weakens the mixture. If I'm wrong I stand to be corrected, that's the good thing about this forum, we never stop learning!
You are right about the HS4's it can only be done with one of the square flange manifolds.

[ricardo]

No, I haven't used the carbs, they are stored as I got them. Don't know which car they came from. They had a set of Les Leston short ram pipes which unfortunately the previous owner sold separately.

If you push the piston up, the mixture gets lean a little bit, yes, and we need the opposite for cold start.

Putting the AUD165 number on Google, this came up:

http://www.somerfordmini.co.uk/eshop/in ... age&id=170

Note the reference for normal choke on AUD176/AUD178 (although my 176 set has cut levers (?!)) and the thermo-jet reference for the 165 set, which was only available during 1964 (Burlen site).

From Somerfords: "They are supplied with inlet manifold and linkage, or as simply a pair of carburetters (with choke mechanism) or a pair of carburetters with linkage (thermo jet)."

So, can anyone explain how these thermo jet carbs work? Am I missing some parts in my set?

[Lord Croker]

The only thermo jet SU's I know of have quite a lot of extra equipment on them, rather a lot to be missing. I once had a pair of HD8 thermo SU's from an old Jaguar engine. The workings of it are shown here: http://www.sucarb.co.uk/TechnicalDetail.aspx?id=83
The Les Leston ram pipes bring back some memories, very popular items in their day, I had one on my first Mini, a 1963 850 van. Together with a free flow exhaust, it was considered to be well tuned! Happy days!

[Toby]

ST H4 carbs for LHD cars (AUD165 and AUD176) are also 2 similar carbs with 2 AUD4814 chambers, being one on the middle. The problem is the choke... I have a very original AUD176 set and have seen another one exactly the same - they are like the RHD setup, but have both choke levers cut. How do you make the choke work? You don't (?!). My AUD165 set has an interesting mechanism to deal with it which I believe to be original, although looking a bit home made. The jets are fixed and instead of pushing them down, you make the pistons go up with a lever.

Could it be that these are a set of Fixed jet carburettors as mentioned in C-AKD-5096 the Special Tuning Manual for 1275 GT and Cooper S?

[carbon]

The later sets of twin 1.5's supplied by BL Special Tuning for the 1275GT in the early 1970's were I believe twin HS4s, rather then the earlier H4s.

I don't know when they changed over.

[ricardo]

I also think this is earlier than the GT era. The Burlen reference (1964) should be about right. A couple more pictures: