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MG Midget and Sprite Technical - Can you explain something to me?

Hi everyone!

As you should all be aware I am now the happy owner of a new sprite :)

It has a single carb conversion and the spec is as below:

1275cc engine
MG Metro manifold
single 1.5 (or 1.75, not too sure off the top of my head) carb.

Now I know the single carb means balancing is no longer an issue, but what other benefits does it bring over the standard twins?

Gratuitous picture attached :)


R Williams

It's thought to be all-round better! No tuning issues, better fuel economy but you do loose the sporty twin carb look, which I'm sure you are missing. So, tell you what, I'll swap my twins for your single! Bring a spanner on Sunday!
G Williams (Graeme)

Rylan. You've got an HIF44, aka a 1.75" equivalent single SU.

You 'might' have lost a little acceleration but you probably gained more torque, and as Graeme said, better mpg, and no longer have to balance the carbs.

Very nice. How about some pictures of the rest of the car?

Lawrence Slater


R Williams

I have always wondered about this too. Why is the HIF44 conversion for the 1275 engine such a holy grail solution?

What makes it so good? Why does it improve mpg?

Yours wonderingly...
Malcolm Le Chevalier

I hope you're not keeping it as a garage queen, or just worried about cleaning those wheels :)

are the sidelights the right way up (or down) as I sort of remember something from the photos of when you first got it

on a positive note, if you spray the tyres with 'Armor All Tyre Foam' (spray and walk away, the way all cleaning products ought to work) then they will match the hood and the clean black areas of the car will improve the appearance of the rest of the car too even if you don't touch the rest of the car -
Nigel Atkins

It might not be a HIF44/HIF6 as it looks larger than that as well as having a very short dashpot/non standard/Dolomite dashpot. I suppose it could be a HIF7 or HIF8?
Daniel Stapleton

Nigel, I already have a can of that, I just have not had a chance to apply it. The car is 100% not going to be a garage queen!

The economy makes sense as I did around 115 miles on Sunday on around 1/2 tank of petrol which seems pretty good to me!

Dan, it's a HIF for a Harley Davidson, hence the shorter dashpot, it meant that the bonnet didn't have to be altered in any way for it to fit.
R Williams

From what I understand, on a 4 cyl, 4 stroke engine each cylinder sucks in turn, over 2 revolutions of the engine. So with twin carbs with each one serving 2 cylinders each carb only delivering a fuel charge for half of the time. This causes a stop start, stop start, airflow. With a single carb it delivers to each of the 4 cylinders in turn, so is flowing the fuel charge more smoothly and it is this that gives the better economy.

And although the diameter of the single HIF44 is less than the combined size of twin carbs, since it is delivering to only one cylinder at a time,the effect is of a larger carb capacity than with the twin carbs as they never both deliver fuel at the same time.
Guy W


plus 2 down sides I see on your set up... but really not an issue to loose sleep over are

1. The intake looks a little blocky square, meaning the fuel mixture has to make.alot of hard 90 degree turns to get into the combustion chamber. where as the Duel SU carbs are more of a direct shot straight into the combustion chamber

2. is the fact that it is a BIG carb...bigger carbs do mean more air comes in but ... because the venture is much bigger in diameter it means all that air is gojng to move much slower...slower fuel charge means less your loosing some seat of your pants grunt at stop light burn offs

the BIG glaring issues...""" NO heatshield""" thats where id plan on losing sleep

id make up a sheet of aluminum ally to fit between the carb and cly head...once summer heat sets in, you will have issue with the fuel vaporising causing the engine to run lean and will give all the symptoms of a fuel pump going bad

you want sheild to lip out at the bottom so to steer the heat that comes off the exhauste manifold away from the carbs

if any thing find an old heat shield and use it as a templete or modify it to work

bottom line... you need a heatsheild before summer heat comes for a visit

Prop and the Blackhole Midget

It's a bit of a fallacy that twin carbs mean better performance. Only one cylinder is on an inlet stoke at any one time This means that on a standard twin 1.25 SU set up each cylinder is limited to drawing fuel/air through a 1.25 diameter hole even though there are two carbs. Fitting a single 1.5 or 1.75 SU means that each cylinder on its inlet stroke draws in fuel air through a 1.5/1.75 diameter hole. Thus the advantage of the single carb is the possibility of more air/fuel (which equals more power).

More power is only possible however if the engine is capable of using the potential provided by the larger carb. Thus a standard engine may not benefit very much from a HIF44 but would do so greatly if it had (a) larger valves and (b) a hotter cam. When I had a HIF44 on a fairly standard engine the power was not much different from a twin carb set up (although fuel economy was greatly improved) and a session on a rolling road suggested that a 1.5 SU might be better than the HIF44. Since rebuilding the engine with a big increase in capacity and a hot cam, the power has shot up as the potential of the big carb can now be realised.

P.S I see that I'm just repeating what Guy said!!!
Chris H (1970 Midget 1275)

NP chris...
its still more entertaining and a better read then 50 shades of gray

Prop and the Blackhole Midget

not had a chance to use the tire Foam, it only takes 6 seconds top for each tyre, you'd be the talk of the meet turning up with tyres looking like those

good news about the car not being a garage queen despite the (chrome?) wire wheels, not so good news for me that you can get 155 miles on around 3 gallons - perhaps your gauges are wrong or you've been driving slowly, possibly you have a weak right leg hence the servo, if any of these then please keep it to yourself, no gloating :)

not that you need an excuse but the more you drive it the quicker it'll get even better to drive

were your sidelight units upside down or was it on another car?

there's a heat shield, just smaller, they all seem about that size on single carb
Nigel Atkins

IIRC (book not to hand at the moment)
According to the Vizard big yellow book, the big single 1 3/4"HIF carb flowed the best of all the set-ups he tried (weber side drafts excluded IIRC). He recommended it as the easiest/cheapest way to get most power from the rest of the engine.

However he used it on a specially selected manifold, but said the MG Metro manifold was very good.

If you want max power and high revs from your modified engine, use the 1 3/4" single , if you want more low end torque and better fuel economy you can use use a smaller carb...

I had a mini1000 once, and fitted a MG Metro (1 3/4") carb and manifold and had it set up on a rolling road.
It was noticably less torquey up to around 2000 rpm, but had a noticable increase of power that you could feel from around 3000 rpm.
Andy Phillips

I think a lot of the time people fit the single carb to replace 'dead' twin carbs so comparisons are not easy. A straight shot into the ports is better than a curved shot into the ports, especially so with siamesed inlet porting. The large single does lose low rpm torque at part throttle over the standard twins. This is the effect of poorer fuel atomisation in the area under and around the piston in the large single carb. To overcome this an oval slide or oval through bore has been used for many years on motorbikes.

Anyone tried these?
oval goes siedways which is a bit odd but maybe still fuel well with the 8 million jets.
Zodiac also do special carbs.I pasted a link but it didnt work. PSI SERIES 4 GENESIS "BIG-AIR" BILLET ALUMINUM CARBURETORS

They look wicked :)

1 1/4" twins will feed a 1275 +60 race engine to 100+ at the wheels, the restrictive 1.3 max inlet valves for the regs still being the restriction I feel.

Peter Burgess Tuning


I'm with Chris and Guy. In addition, I have a similar set up to you and can vouch for the ease of balancing - zero! - the increased mpg, the easier tuning achieved and the inherent simplicity. I would also agree with Chris that unless you can release the potential of the carb - by a modified head, high lift cam, bored out block etc etc, there will not be any difference - bar mpg perhaps - over standard set up. The carb will only provide what is required and nothing more.

Nice car by the by.
Mark O

So it appears the main advantages are as follows:

- Balancing no longer an issue
- Easier to set up
- Better MPG
- May have a slight increase in torque, but may be counterbalanced by a slight drop in acceleration.

Have I got the key points?

I always wondered why a single HIF was the holy grail, and now I appear to know :)

R Williams

my understanding for the 4th point would be
"slight decrease in low end torque, but an increase in higher RPM power"

now i have the vizard book in front of me it says

" A big, , well sorted SU on an effective intake manifold, does certainly produce the goods, and as a carb for a high performance road engine , it is certainly one of the best ways to go"

adjacent to this statement is a graph comparing a single 1 3/4" SU against twin 1 1/2" SU's and 45 DCOE webber and theres next to nothing in it until 6000 rpm, (thats on a 1430cc engine so a 1275 would need less airflow so it would be even higher revs before the difference started to become significant)....

best carb for this particular engine BTW was an 48 IDE webber

so, in summary, from my sofa based engine tuning background :) , it looks like, for a 1275 and bigger engine then the single 1 3/4" is the best way to get good performance/release the max performance that your engine can acheive... especially as they were fitted to metro's and can be had cheaply...

thoroughly recommend the vizard book, very very interesting if you are interested in engines and tuning...
Andy Phillips

And remember Rylan, as Peter burgess said, the probably reason you have a single HIF, is that the twin 1"1/4 were alsmost certainly knackered, and a single HIF is(was?) a fairly cheap swap.

I assume yours is going well. Is there any reason your asking? Are you thinking of changing back to twins?
Lawrence Slater

Mine is running fine, I wouldn't change back to twins as I would have to do a fair amount of work to swap it.

I just wondered what was so good about it in comparison to the standard twins as nobody every explained it clearly!
R Williams

In swapping out from standard SUs do not forget the extra profiling work that may be required on the inlet manifold, so as to ensure the carb does not foul the bonnet. Also the cost of the manifold itself of course...As a guide, my manifold from a well known auction site was a tenner - but I think I got lucky and the refurbed good condition HIF 44 was 80 quid. The difference in mpg quickly recovers the outlay.

For me, there is a slight improvement in mid range torque over standard SUs, but simply fitting a HIF and expecting a noticeable change in torque and power will not happen unless the engine is suitably modifed.
Mark O

Rylan's carb is a Metro / Harley Davidson S.U.hybrid on a stock manifold which saves the fettling whilst still achieving underbonnet clearance.
Alan Anstead

was this the car with the upside down sidelight units then ?
Nigel Atkins

I still think you need a heat shield for this summer

on the 1500 I think the.exhaust was on the opisite side so.didnt require much of a heatbshild

if your exhauste manifold is directly under the carb / intake manifold then id think fuel evoperation will be an issue

Prop and the Blackhole Midget

Hi prop

1500 ex and in on same side and suffer from heat problems, many folk fit a heat shield to protect the float chambers and trunk cold air from beside the radiator.

We had a mini with an HS4 boil up its petrol really well on Monday, took half an hour to cool down and ru n sweet again. The customer will be fabricating a heat shield post haste.

Peter Burgess Tuning

Prop. As someone said earlier, there is a heatshield. It's small, but it works.

I've enlarged mine, but I did it just in case, rather than because I'd had a problem.

Lawrence Slater

This car has travelled across Europe, in its current spec, with reliability why alter anything?
Alan Anstead

yes but were those sidelights upside down ! or did I imagine it?

Rylan never put he wanted to change anything (not even those sidelights)
Nigel Atkins

I agree completely Alan.
Lawrence Slater

This thread was discussed between 22/04/2013 and 26/04/2013

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