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MG Midget and Sprite Technical - MGB carbs on a Midget?

Hi

Anyone tried putting MGB HS4 SU 1.5 inch carbs on a modified 1275cc midget using suitable inlet manifold?

Cheers
Mike
M Wood

Yes Mike. I've done it. However, I never used the engine with any other carbs to have a point of comparison. What else have you done to the engine to make the 1.5 carbs a consideration?

Charley
C R Huff

Not on a Midget but had twin HIF38's on my old Metro. I can't remember what they were off but the manifold looked very similar to any other BMC twin. They were much better than the 1 1/4 HS carbs and stayed in tune ok. I think it was slightly over carbed, though it was only 1243cc so maybe better on a 1380 or similar.
john payne

I thought about it and alot of others have done it

The problem is ... bigger is not always better, its easy to over carb a smaller engine

For 1.5 inch duel carbs to work well you need alot of displacement and alot of fuel hungry modifications

The good thing about using HS4s on a fuel hungry modified engine, is apart from the needle/jet you wont need to do any modification to the carbs as it will pull more air at stock then the engine can ever use

Remember... constriction is your friend (pulling air really fast) ... you want your carbs to act like a shop vacume cleaner sucking saw dust... not a child sucking soda pop thur a 3 inch dia straw 18 inches long

Prop
Prop and the Blackhole Midget

I have a spare pair of HS6s if you're interested?
Dave O'Neill 2

(((I have a spare pair of HS6s if you're interested)))

Hahaha... do they also come with the electric turbo charger motors seen on ebay ?

If im not mistaken... I want to say the gray expermential moss supercharged midget uses 1SU HS6 carb

Prop
Prop and the Blackhole Midget

The later Cooper S inlet manifolds had a mounting flange that could be drilled & tapped for 2x HS6 carbs.

From Minispares website...


Dave O'Neill 2

Sorry dave,

I thought you were kidding....that is insane
I want to say HS6 are 1.75 inch each

Id love to see that engine it would be ultra radical

Thanks for that heads up

Prop
Prop and the Blackhole Midget

Prop
A fella I know quite well has a bugeye sprite with a 1400+cc engine with 1 3/4" su's , fairly mild cam but otherwise well worked over. It pulls hard and clean from 1500 revs right through to his self imposed 7000 limit. I've had the sniffer up the exhaust for him and on full throttle it's on between 13 to 12.5:1 right through the rev range and economical to cruise with
You would swear the engine was bigger than what it is but knowing him and what he does I imagine it is as big as they get.
I also would have thought 1 3/4 carbs would be too big for a roadie but I'm a convert since riding this thing
Actually when I come to think about it years ago a mates racer mini ran a 45mm weber with 40mm chokes so I guess it's not so surprising that the 1 3/4 SU's work
willy

I would think 1 1/2" su conversion would be fairly straight forward
William Revit

I did it with MGB HIF carbs on my 1275. Self ported cylinder head, LCB. Made 75bhp. Needles were perfect according to RR man. (from '79 MGB) And it did around 40mpg on a run. No downside at all.

The bigger SUs will only open as far as they need to so up sizing isn't a bad idea.
Rob Armstrong

<<The bigger SUs will only open as far as they need to>>

That is surely the relevant bit of information, and where an SU setup differs from a Weber. Spot on, Rob!
Guy W

Hi everyone

Thanks for the great replies.

I am thinking ahead about building a 1293cc engine with modified head (ported and larger valves), LCB exhaust manifold and RC40 exhaust plus decent air filters.

I was wondering if there is an option to the single 1.75 inch HIF44 or single twin choke Weber or Dellorto 40 carb route to upgrading from the standard twin 1.25 inch HS2 SU set up.

Looking for tractable engine for road and hillclimb use throughout the rev range (lowdown torque, not just top end) and reckoned SUs can do this, as well as the twin 1.5 HS4 setup being easily available. I guess better suited to a 1380cc offset bore engine,but the twins 1.5 SUs would not over carb due to how SUs operate?

Cheers
Mike
M Wood

I have a similar set up but still run the HS2's. When it was set up on the rolling road the tuner said it would breath better through a pair of 1.5" and develop more power. If I remember early Cooper S had them fitted as standard but I may be wrong.
Bob Beaumont

ISTR the correct inlet manifolds are either hard to find or expensive. IMO for a road modified car the big single is better.
David Smith

We set up a pair of MGB carbs on to a 1275 Midget for a customer who couldn't find an HIF44 set-up at the time. I have to say I am very impressed with the result, and while I haven't done a back-to-back if anything it feels better on the road that our own car similarly worked car but with the HIF44. As well as the good mid range and top end you'd expect, it is also very tractible.

The engine had a reasonably well worked head, tickled cam and LCB. The carbs were fitted using the standard Midget manifold and a 12mm alloy adaptor plate, all suitably opened out and blended in. Quite easy to do.
no 5 needles IIRC but I'd need to check.
Paul Walbran

Mike

I have - AFIK - a standard block, self-ported Metro head with 35mm inlets, LCB, 123 electronic ignition, decent SS exhaust with a K&N cone filter and 5 speed type 9 box. I have HIF44 fitted - with a BDK needle - standard for a Metro is BDL I believe....BDK being a little richer...

The big issue with the HIF44 is finding a suitable inlet manifold - or else machining the mating face of one so the bonnet closes. I was lucky. The one I purchased - from a well known auction web site - bolted straight on with no bonnet issues, and the carb - sourced from the same place - was in good nick.

Bit of fettling with ignition and mix yields a good low down pick up and decent top end, smooth all the way through the range, and doubtless further fiddling will probably yield a bit more. Bolt on bellmouth may assist further as perhaps will an RC40. Not monitored fuel consumption as yet, but same set up with highly ported standard-ish head c/w 33mm inlets was circa 40.


Mark O

I built an adapter plate to go from original to mgb. Made of 3/8 alloy plate. Will see if I can find a pic.
Rob Armstrong

Paul mentioned modifying the 1275 manifold to accept the 1-1/2 inch carbs. You can also go the other way and modify the MGB manifold to fit the 1275 head.

Charley
C R Huff

found it

http://robsmidget.co.uk/modifications/a-series-engine/carburettors

:)

Mine were HIFs.
Rob Armstrong

Mark O. Raised the most important issue of all in very words

Recurving the dissy to fit the carb is where the real work is going to come into play

Granted you can rev up to 4500rpm at 28 degrees, but having had several great conversations with peter burgess both public and private....this trick is just a starting point

Prop
Prop and the Blackhole Midget

I have a pair of hs 4s spare from my 1500. new gaskets, valves, throttle spindles and plates etc. about 4 years ago but then never fitted.

Malcolm
M Le Chevalier

A few years ago I acquired a pair of HS4 carbs on a altered MGB manifold that had been fitted to a A Series 1380. They went into a box as I intended to fit a Weber, but if anyone wants them, you're welcome. Collection from N. Herts. No photo as computer say, 'file too large'
Motley 5

Hi

Motley - still got those MGB carbs available please?

I know this is daft question - any thoughts on needles to get an engine going, say for a 1330 or 1293 A Series on twin HS4 SU carbs? Modded head and camshaft - spec yet to set, just gathering parts and ideas.

Cheers
Mike
M Wood

I have AAU needles on my 1293 with twin HS4's. CR 10.5:1 29mm ex 35.7 inlet.246 high lift Kent cam Maniflow inlet and exhaust manifold. Nice torquey engine will potter around 20MPH in top with a 3.7 diff but cracks on when needed.
Bob Beaumont

That is hopeful Mike! That thread is over three years old!

I have a couple of HS4s off my 1500 kicking about, not being used. I refurbished them with new gaskets/shafts etc. years ago then never used them.

Malc.
Malcolm Le Chevalier

Ha ha, just noticed I posted above Motley! Yep, I still have those! Man I need to have a clear out.
Malcolm Le Chevalier

Hi Mike,
Yes, your optimistic query was fruitful. They're still in the garage 'store area' and you're welcome to them. Let me have your email address.

Motley 5

Had to check the date as reading from the beginning I noticed a response from Prop and nearly had a fit wondering what was happening!
Jeremy MkIII

Thanks everyone.

All the advice then and now is very useful, including Bob's, Rob's and Mark O's on engine and cab spec. Prop still being helpful too!

Malc - thanks for the offer of the HS4 carbs from the 1500 - l'm tempted as a spare set under my bench unless there is a more deserving 1500 owner out there. I'll email you - the Chavalier Classics email still works?

Motley - yes please, I'd definately interested in the set of HS4 carbs on a manifold that was on a 1380 A Series Engine. My email address is Mike dot mikewood at gmail dot com

Just shows you the value of the this BBS community, including the archive.

Cheers
Mike


PS on re-reading the thread, I did not answer Charley's query of why - it is for a potential street and competition car motor build.
M Wood

This thread was discussed between 23/02/2015 and 01/07/2018

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