Any thoughts on Twin SU carbs for a 1098?

[charlie_morris_minor]

If I could just bring it back on topic, how would it be if I set twin carbs to deliver the same amount of fuel and air that one carb did previously?


The performance would be the same but surely the better flow would help economy

you would have thought so, but i recall from the vizard book him saying that some twin carb manifolds were so badly designed that they actually hampered flow. search ebay for david vizard it brings up a few of his book, it will probably answer most of your questions.

[SGTBILKO]

http://www.amazon.co.uk/Tuning-Engine-D ... 1859606202

An entertaining book even if you never intend tuning your engine.

[chrisryder]

definately a good buy.

there is a section about carbs that will tell you what needles and springs to fit in whatever carb you fit to whatever engine with whatever mods. very useful!

[Alec]

Hello all,

just to muddy the waters a bit, at one time all the top mini racers used to use two twin choke DCOE Weber carburettors with one choke machined away, essentially two single choke Webers. Prior to that they used to use, if my memory is correct, a single DCOE on a longish manifold. So there must have been a slight gain to having the straighter twin manifold, than the 'S' shaped twin for a single DCOE?

Alec

[bmcecosse]

Yes - absolutely - and twin HS6 (or HIF44) carbs are even better! But - they are NOT fitted on the ghastly cross tube manifold - they are fitted on stub manifolds with a nice straight line path into the ports. It is not possible to 'set the twin carbs to give same airflow etc' - the flow depends on the diameter of the carb throat - a big single is the way to go - HIF 38 for a 1098. HIF 44 for a 1275. The split webers etc are very much a 'full race' modification where evry last BHP is being chased regardless of cost. Of course - the Works then produced the 8 port crossflow head, running initially on twin DCOE Webers and then on Fuel Injection.
And -I don't think even Police Minors had alternators in the '60s' - - they hadn't been invented for cars then... The first ones I saw/was involved with (in the 70s??) were the Lucas 11AC units with separate 4TR and 6RA units attached. And they were available in +ve earth versions for a while.

[Alec]

Hello BMCE,

don't forget the four Amal set up, as far as air flow, size for size there is no better carburettor, especially the G.P. which has no obstruction at all on full throttle as the needle is offset out of the choke.

Alec

[bmcecosse]

ABSOLUTELY Alec - and I have often wondered if an SU could be modified to work like the Amal, by removing the throttle assembly, and regulating the flow by using the piston as the throttle - attaching a pull cable to it and blocking off the vacuum system that causes it to lift automatically in standard form. I had actually forgotten about the idea - maybe a winter project for me !

[IslipMinor]

Lucas 11AC units with separate 4TR and 6RA units

Yes, have one of them in the garage - was fitted first to my Rapier in the mid-60's and is +ve earth. Great with halogen spot light bulbs (or more correctly quartz iodine - QI). I bought a QI bulb for one of the Lucas spots that now adorn the Minor, but date back to the mid-60's, and it is still in the light and it still works fine 45 years on!!

[bmcecosse]

Oh well - they were available in the 60s then! I stand corrected - still dubious about that rocker cover.....

[chrisd87]

I have often wondered if an SU could be modified to work like the Amal, by removing the throttle assembly, and regulating the flow by using the piston as the throttle - attaching a pull cable to it and blocking off the vacuum system that causes it to lift automatically in standard form.

I did see such a carb on ebay a little while ago, from memory it might have come from an autograss Mini. Motorcycles often used to use 'slide' type carbs - they are best for top-end power but the mixture control isn't as good, particularly if the rider isn't mechanically sympathetic. They also tend to result in a rather heavy throttle action compared to constant velocity carbs.

[bmcecosse]

Interesting!

[Chipper]

When my Traveller had its original 1098cc, I tried various combinations of carbs - single 1.5" SU, twin 1.25" SUs and Weber 28/36.

I found the single 1.5" to work best, giving a useful gain over the standard single 1.25" SU. The Weber was hopeless!

After fitting the 1275cc Midget engine to it, I initially used the twin 1.25" SUs as per original MG spec, and they weren't bad but required fairly frequent fettling to keep them at their best. I then fitted a single 1.75" HIF44, and it's been running very sweetly and reliably with it for years, with far less maintenance required, for similar or better performance and economy.

Single carbs also tend to give better driveability at low speeds, due to the simpler throttle actuation - twins tended to be a little tricky to control in slow-moving traffic conditions I found - the slightest touch of throttle would have the car lurching forwards. No such problems with the HIF, as they have a very progressive cam type throttle, so you can trickle along very smoothly.

However, twin SUs do look the business under the bonnet!

[Moggie94]

I'm getting the feeling that many of you think a large single SU would be best, I will look into that firstly then.

You see I can service and tweak my car OK... but knowing what parts will work well isn't my strong suit.

[IslipMinor]

Definitely a large single SU, but of the HIF variety, not the earlier HS, HD or H series. The HIF flow much better and have good built-in connections for the crankcase evntilation, which not all the earlier ones did. For a 1098, unless very modified, an HIF4 or 38 will do well, for a 1275 then the larger HIF6 or 44 is what you will need.

[Chipper]

I'd agree with the above, but beware if buying a HIF38, as some were fitted to the economy-orientated Metro 998cc 1.0E, and will give a very weak mixture.

[bmcecosse]

But good economy - uber important these days. The 'economy' mods can be reversed if necessary.....

[Moggie94]

Are we talking a weaker mixture than the stock one?

And what was fitted as standard? I got the engine minus a carb...

[linearaudio]

Yes! The carb has a little black tube from a calibrated drilling on the engine side of the butterfly, connecting to the air space in the fuel chamber, on the opposite side to the throttle linkage, so is quite easy to spot. Idea is that when you are running a light throttle, the vacuum created lowers the fuel level slightly and so weakens the mixture- probably why my modified 1098 does 40+ mpg when cruising at 70+!
AAA seems the needle for a modded 1098, or an ADK if it bogs in the middle, as the ADK leans out a bit, and can be richened if wanted. (Very useful book by Des Hammil on modifying SU carbs shows how to calibrate a needle in a running engine!)

[Chipper]

Standard carb on a 1098cc Minor was the single 1.25" HS2 (AUD13). Also fitted to the Austin/Morris 1100, Austin A40, Austin/Morris Mini 1100 MkII.

Needles were:

Weak: EB
Standard: AN
Rich: H6

With a red spring.

These actually perform fairly well if in good condition, with excellent bottom end torque and good fuel economy, but don't tend to breathe that well if pressing on a bit.

[Moggie94]

I see, I also noticed that the MG version of the engine was rated at slightly more power, how did they achieve this on the MG models? Was it a twin carb and what other changes were made?