MG M Type SC 6436

[Ian Grace]

Even more interesting - thanks Simon. So does this mean SU defined their carbs by the diameter at the butterfly end? And I wonder why the butterfly end would be designed to be larger than the air inlet end. Is this the general case for all carbs? If we're not careful, we could learn something here!

[DF9053]

Yes I agree, DF is 7/8 on the intake side, 1 inch on the manifold side. I would have thought that widening the size of the choke would have slowed the air speed down as it moves through the carburettor, on modern cars this reduces torque at slow engine speeds hence the VVTi idea of variable throttle appetures, but I think this is all too technical on a Minor as the manifolds are hardly matched up to the carb and they further strangle the engine. Makes you wonder what a well designed manifold would achieve with a standard carb. Kimber clearly gave up and redesigned the head for the J type, would have been good to have had his perspective as I am sure this is old ground!

Cheers
Jeremy

[Toby]

I've got one of those mesh things in my manifold which I believe is meant to improve fuel consumption, although I've been assured that it doesn't make any difference I don't think this helps performance? Anyone got any experience of this? Is this what is meant by variable intake, if you remove it it varies the size!

[David Whittle]

I believe the size of an SU carb is measured at the intake diameter. The increases in diameter at the butterfly end is to take into account the recuction in volume caused by the spindle and the butterly and the two screw heads. In theory the maximum possible speed and volume of gas on full throttle should be similiar both sides of the jet. In practice I doubt that it is, which is why people shave the spindle and file down the screw heads. If you look at the standard obstructions in a 7/8" SU I would guess you only get 3/4" at full throttle ! Read the Book on Power Tuning an SU and you will see what I mean!

Edited to add - they are wonderful carbs, SUs - I would not want any other make!

[Ken Martin]

Funny how this one has developed into a debate on carbs! I am entering the proceedings late with an observation. Having agreed with the findings that the Minor side float chamber carb is 7/8 inches at the outer end and 1 inch at the manifold, I have just checked my Type MM integral annular float chamber carb fitted to early Minors and found that both ends are 1 inch diam. The attached picture proves it! Ian has I believe the ex-Roger Payne Type MM carb, which I let Roger have many years ago for his Minor project (!), and Tony Gamble has a Type MM carb fitted to his 1928 saloon and so my findings can be easily confirmed. I also have what I believe to be an MG M Type carb and it has the same diameter at each end - 1 and 1/8 inches.

[David Whittle]

Ken

So early Minors had the edge on performance then?

I have seen this early type Carb on one of Brian Maers Minors, I wonder if it was fitted to other cars in the 1928 era?

SU dropped it pretty quickly for the later type side float 7/8" so I presume the float chamber was a weak point.

All these carbs do seem to be troubled with too much heat from the exhaust manifold at times. As Jeremy mentions, the J2 style crossflow head is much neater for many reasons.

Has any one tried twin carbs on a Minor or an M type? I have seen a home made inlet manifold (just two short tubes clamped in place where the old inlet part of the manifold was) which allow two side draft 1" carbs to be squeezed in to the space. I think it was a period tweek for M type and D type MGs but someone must have tried it on a Minor!

[Ian Grace]

McEvoy offered a twin carb. conversion for OHC Minors, I think with two RAG carbs.

[Ken Martin]

Ian
Yes, it does seen odd that Morris effectively detuned the Minor when it changed carbs as I am fairly sure the inlet and outlet ends of SU carbs are normally the same diameter. Until this debate I had't realised there was a difference of 1/8 inch on the Minor one end to end. Even so it wouldn't have made any difference if the smaller diameter had continued right through to the unchanged manifold would it? And why downsize the diameter anyway? I cannot find any info at all on this issue.
The early carb was evidently a special one for the Minor as suggested by the Type designation 'MM'. I don't know of any other cars that used it and it was soon taken out of production - probably in early 1929? As I mentined above, Tony Gamble still has one on his 1928 car.
Martin Redmond has got a twin SU set up on a Minor head in his highly modified trials special Minor.

[Ian Grace]

The improved carb. was fitted from chassis MM1101, so late in 1928. It didn't last long!

[Ken Martin]

Interesting: I wonder whether Type MM had a fundamental flaw - ie it leaked - and if Morris replaced old with new types under warranty? [Was there a warranty?] I guess that the cars still so equipped have had a very gentle life!

[David Whittle]

It has often been said that a smaller diameter choke imroves low down torque and 'lugging' ability on vintage engines. As Minors were more often than not 'lugging' along in hilly country possibly carrying 4 adults and some luggage, perhaps the increase in torque was felt more valuable than the odd second in the 0-35mph dash. The MG M type relies on revs and light weight, so it does not need the lugging ability of a 4 seater!

I believe the reason it works is related to the velocity of the charge when drawn through a smaller diameter hole, being superior to that when drawn through a larger hole, and hence mixes and swirls better to fill the combustion chambers. Bigger is not always better!

[Ken Martin]

David
I think you are right. Few Minors went as fast when new than they frequently do now! I must say that I often find myself travelling along faster than I would by choice to avoid holding up traffic more than necessary. When I turn off a main road onto a counry lane and relax and slow down, I feel it must be more like the 1930s!
Drivers of non-sports cars in 1930 didn't like changing gear and laboured along in top - this was a consideration when a car was tested in those days. Big carbs and inlet valves are only worth having if the engine is to be pushed to high revs