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MG MGB Technical - SU carbs

Can anyone tell me what carbs "should" be on a 1974 1/2 Mgbgt. It came from the factory with twin carbs and wire wheels but through the years many things were changed. I was just about to replace the aftermarket air cleaners and go back stock but found they don't fit. They are HS carbs but appear a lot larger.

I don't mind leaving the aftermarket air filters on but will large carbs work well on a stock B engine?

Brian
Brian Smith

I would have thought that by ‘74 that you should have HIF4s fitted.

When you say that they appear larger, have you measured across the intake aperture?
Dave O'Neill 2

Surely either HS or HIF were all 1.5inch through out production. Any thing larger is a modification
Jim
jim soutar

All export models got HIFs with the then new 18V engine in 1971, 1 1/2" for both HS and HIF.

I doubt larger carbs would work 'well' on an otherwise standard engine, the V8 i.e. twice as much throughput only had HS6. With less throughput I'd expect the piston wouldn't rise so far, so choking the intake, like only having part throttle available. Even standard carbs don't rise all the way.
paulh4

My friend has fitted HS6 (1.75") in leu of his HS4's (1.5") and insists they have given him an improvement in bottom end torque. This is contrary to the advice from the likes of Roger Parker and Peter Burgess. His engine is based roughly on a 1972 18V engine so has the big inlet valves. His head has been gas flowed and he has a sports exhaust with extractor manifold. He now has K&N filters but is running with the standard 252 deg cam. The carbs and inlet manifold were from a Austin Morris 1800 S Landcrab. The factory must have thought that this set up did something for that car.
Paul Hollingworth

My friend has fitted HS6 (1.75") in leu of his HS4's (1.5") and insists they have given him an improvement in bottom end torque. This is contrary to the advice from the likes of Roger Parker and Peter Burgess. His engine is based roughly on a 1972 18V engine so has the big inlet valves. His the head has been gas flowed and he has a sports exhaust with extractor manifold. He now has K&N filters but is running with the standard 252 deg cam. The carbs and inlet manifold were from a Austin Morris 1800 S Landcrab. The factory must have thought that this set up did something for that car.
Paul Hollingworth

“The factory must have thought that this set up did something for that car.”

Maybe fitted for homologation purposes. The 1800 was used by the works rally team, I believe.

I believe the Maxi and Allegro 1750 HLs were also fitted with twin HS6 carbs.
Dave O'Neill 2

The Maxi, Allegro and some later {real) Minis had single HS6's. HS or HIF 6's allow more air flow compared too 4's, higher up the rev range at the expense of tractability low down the rev range. Good for competition cars with worked heads and lots of time on wide open throttles.
My 3.9 V8 works quite happily on Twin HIF6's. It would probably get BIT breathless if I was rev happy, but the V8's are all about torque!
Allan Reeling

My bog-standard V8 revs all too easily into the red, Unlike the 4-cylinder, although with that it's more about the sheer din than anything else. Not been into the orange with that, just sounds way too painful for it.
paulh4

I recently swapped the HIF4s out for HS6s on my '74 roadster. I don't do competition, just spirited drives through the southern Blue Ridge mountains. The car seems to perform well with the larger carbs. I probably don't tax it enough to tell a big difference but I do prefer the relative simplicity of the older HS6s.

Jud
J. K. Chapin

Thanks for the comments. The PO made a number of changes and was quick to tell me he had replaced the carbs with a brand new pair. I have the Car in my shop now getting it ready for Spring and it was when I tried to fit original air cleaners and found they wouldn't fit I began to wonder what I had. The engine is stock and while driving it last Summer it didn't run as well as my others had run, the explanation appears to be the carbs. I didn't remove them to measure the size at the choke valve side but at the intake side they are measuring out closer to an HS6. Or larger! I have now put out a request to our local club to see if anyone has an old set of HIF4 carbs that I can rebuild and install.

Generally the car is in excellent condition with only 41,000 original miles, splined hubs and overdrive. The PO installed 15 in splined hub aluminum wheels and these I like even though the spare is still a 14 in wire wheel, so some changes are pleasing!

Brian
Brian Smith

Brian, I have the HIF4s that came off the '74 roadster that I might be willing to part with. These include the intake manifold. I can't give them away but I'd entertain most any reasonable offer.

Jud


J. K. Chapin

On the carb size thing ,Personally,I love the feel of bigger butterflys, i had a pair of HD8s on my midget hillclimber for a while and I'd say that they were the best/most responsive of all the different combo's tried on it including 48'Dellorto and 45/50'Webers by far
Why you would want to take 1 3/4" SUs off a B when not worrying about originality is the question
If they still have Austin 1800 needles/springs in them, they would be running lean which might be the cause of your rough running issue--With the correct needles/springs they should run quite well
The original carbs would have been fastened to the manifold with 2 studs each ,HS6's would/should have 4studs each unless somebody has cut the carb. body away and also the air cleaner would be attached with 3 fasteners versus 2 for the original
Any pics.
willy
William Revit

“the air cleaner would be attached with 3 fasteners versus 2 for the original”

That is normally the case, but I have seen HS6s with a 2-bolt intake flange.
Dave O'Neill 2

Whatever seat of the pants may say, fitting larger carbs will never increase bottom end torque. If it seemed better maybe there was a problem with tuning or condition of the std carbs. What we find, as Paul H and Allan say, low rpm performance is very soft. The piston will lift only enough to supply air required by the engine this means the aperture presented by the 1 3/4 will be wider than the 1 1/2 as such the signal to the jet will be lower and fuelling is poorer, this is the same for wide open throttle until higher rpms are reached. It is for this reason that Japanese motorcycles were fitted with oval bore carbs and the piston featured an inverted U design to keep velocities 'just so' on the jet.
When I fitted larger carbs in the late 80s the power loss at lower rpms made the throttle response much flatter. I see the same with fitting larger single carb to Midgets over the twin 1 1/4 carbs. If the engine is capable of producing it we tend to see 8 or so bhp more at higher rpms with an MGB fitted with 1 3/4s. The sort of spec needed would be 285 cam, 10.25/10.5:1 CR good cylinder head.
Peter Burgess Tuning

Loads of holes on the V8 HIF6s but only two are needed, with the correct gasket the others are covered up as in the attached.


paulh4

"The engine is stock " ........ really ? Bigger carbs, higher lift cam ? Ported head ?
Perhaps analysis on a rolling road would save time and worry ?
Roger Walker

"oval bore carbs"

They stole the idea from Victorian sewer design ...
paulh4

Mmmmmmmmm fatbergs :)
Peter Burgess Tuning

Morning Peter, know anyone who does MGB gearboxes?
paulh4

Funny this came up-
In a mag. I was reading today there were a pair of DU6 SUs for sale
Not something you see every day
William Revit

Hi Paul sadly I don't know who to recommend
Peter Burgess Tuning

Sorry for opening Pandoras Box on the carb issue. I have had a number of MGB GTs over the years, the last was a 1974 CB and the most recent is a 1974 RB so I can still compare performance and honestly the latest GT is a lot slower getting up to speed and is not particularly smooth. When I looked at the carbs I was surprised to see how large they were. One of our local members contacted me to say he had a box of SUs and I could look through them and take what I wanted. I will hopefully find a set of 1 1/2 either HS4 or HIF4 that I can restore and use. Thanks to you all for your insight. I want the car running before the weather improves.
Brian Smith

Here we have a non-pair of HS6 carbs.

As you can see, one has the extra mounting hole at the bottom of the flange, but the other doesn't.


Dave O'Neill 2

Dave, the ones currently on the car look like the one on the right with the extra hole. Don't want to remove them until I have a set to put back on as I look forward to driving as soon as possible.

Brian


Brian Smith

Brian
IF you are going to go back to 1.5" carbs, you will have to check what inlet manifold you have
If it's the std. manifold with opened out insulators to match the larger carbs , then yes you will only need a pair of std. insulators to get back to original BUT if it's an aftermarket 1.75" manifold, fitting 1.5" carbs will create a mismatch and tuning properly will be impossible with the step causing turbulance in the manifold
Do you know what needles are in the carbs on the car--as mentioned ,if they are std Austin 1800 needles it will be lean/flat
You might only need a needle change to make it happy to drive---lot cheaper than a new pair of carbs

willy

William Revit

Hadn't considered that Willy, not sure what they currently have. Do you have any recommendation for which needles to use? Your description of performance kind of says it all.

Brian
Brian Smith

I found the following on the specification of the rally Landcrab.

The final specification of the Marathon cars was based on the newly introduced Mk2 1800S models.
The engine was not highly tuned - its capacity was increased to 1894cc, the camshaft was the standard MGB grind and the head was re-worked by Downton, the inlet was 1800s with twin 1.75" SU and the exhaust was special exiting through the rear apron. The output from this little lot went through a competition clutch to standard gearbox ratios and gave a reliable 100bhp at the lightened flywheel – about 77 at the wheels – not much for such a heavy car.

The factory must have thought that this spec had an advantge. My friends car probably has a less restictive exhuast system and air cleaners combined with long sub stacks, so he will hope to better the power figure of the Landcrab.
I cant understand quite what Peter was saying. Surely with a variable choke carb like the SU the slide will only lift as much as it needs to to satisfy the amount of air the engine is demanding thus keeping the gas velocities up.
Paul Hollingworth

Took the car for a short blast yesterday and can confirm that off the line it is not impressive. Checked on the needles and they are the standard for HS6 with the floating needles. I confirmed with the previous owner that the engine is totally stock with just over 41000 original miles. I have now obtained a couple of HIF4s to rebuild and will install once done and I have located a correct intake.
Brian Smith

Paul H
Peter is correct in his explanation-- as are you with the airflow dictating the amount of piston lift, What Peter is putting forward is that although the piston in the larger carb may lift less than a smaller carb for a given airflow the actual exposed aperture will be wider/flatter with the larger instrument, spreading the airflow out over a wider area which would have the result of less air speed at the centre needle
I've thought and thought about this and the same could be said about the smaller carb which would have a narrower hole but higher and I can't get my head around the fact that that there would be any difference in air speed at the needle/jet area
BUT
If the larger carb has the correct damper spring for the given engine, then, both sized carbs should lift at the same rate and then Peter is correct in saying airflow would be down on MrBig, however a richer needle to compensate for this lower air flow would equalize things apart from less atomisation with the lower air flow in the larger carb

I've had a lot of success with SY needles and light blue springs on MGBs with 1 3/4" carbs, they seem to be the magic combo, a tiddle rich in some areas at some times but Bs seem to like it a bit on the rich side when under load anyway, specially with our lovely fuel now

willy
William Revit

I'm surprised at the Land Crab engine outputs. 100/77 bhp doesn't seem that good, all things considered.
I had a +0.060" bored 18V in my last 1800 GT, I had worked the head myself, had three angle valve seats, bulleted bronze valve guides, done by Peter Burgess and a standard cam.
It had a decent exhaust system HIF 4's and my own polished needles.
On Peter's road it gave 131 ft/lbs and 115 bhp at flywheel, which corresponded to 119ft/lbs and 91.5bhp at the wheels.
By Downton's standards, without their flow bench, my head work was probably crude, but balanced.
"Checked on the needles and they are the standard for HS6 "??
Brian, There's no such thing as a standard needle for a carb! Needles are more specific to a vehicle and even then it's not a exactitude.
Allan Reeling

The factory must have thought that this spec had an advantge. My friends car probably has a less restictive exhuast system and air cleaners combined with long sub stacks, so he will hope to better the power figure of the Landcrab. Paul having worked on and driven these works 1800s I seem to remember that they went pretty well
Andy Tilney

Paul, There is a bloke, Ken, in the Himley area. I think something like Klassic Transmissions, who Clive Wheatley uses. (I know Klassic!!)
The chap who supplied checked out my LT77, and seems to have done a good job, was Tony Moore, he's Bromsgrove way! 07973730021.
Allan Reeling

Hi Willi,
My friends engine specification is now very close to the Landcrab rally car and his HS6 carbs and manifold came from an Austin 1800S so we would hope it had the right needles and springs in it, but I have to confess I don't know what's in his car actually.

Alan
According to the Moss catalogue KP are the standard needles and TE the rich.
Paul Hollingworth

I can't remember the wording but the SU book says putting in a bigger instrument than needed just gives under bonnet visual kudos.

Normally you use red damper spring but if the carbs are on the larger side of whats required to use the blue, weaker spring weaker mixture.

That's as much as I can remember (or know).
Nigel Atkins

Willi
I have BBW needles as they are the HS6 floating carbs and K&N air cleaners. You say you have the SY needles with blue spring, is this with stock cleaners?

I finally have them running fairly well on idle once engine is warmed up but still lacks the strong pull from a stop that my last 1974 CB MGB GT had. (It was also stock and had HIF4s)

Love the car but hoping to get better performance!

Brian
Brian Smith

Paul, Standard needles for what?
My SU "bible" gives the 1800S, the only 1800 with a twin set up, as TZ standard and CI rich. Red spring, 4.5oz.
Brian, BBW are the rich choice for the 3.5 GTV8. BBU standard. With a yellow spring. 8oz.
My 3.9 V8 runs HIF6's with BBW/ yellow.
Andy, Paul,
Horses for courses. As PB confirmed, top of the rev range is where the advantage is. That's where the rally car will have spent a large proportion of it's time, opposite lock, power sliding on a loose surface, snow etc.
Not sure Brian wants this, but Willy's needle and spring combination is worth a punt, but ultimately the ability to flow air through the carbs will be limited by head's and exhaust's ability to deal with the extra gas.
Allan Reeling

If you want to be as accurate as you can (at the time, conditions, fuel, engine/car set up, use, etc., etc.) then get it as near as possible on a car that's fully serviced and maintained and get the car on the rollers in with an experienced and knowledgeable tuner of you type of car (and use, engine, carbs). The carbs, needles and springs don't work in isolation.
Nigel Atkins

I wonder if you get better mixing at small openings with the HS4 the edges of the airflow will be further away from the jet with the larger choke size.
Stan Best

Stan, I think that was what PB was suggesting. My take is simply that the carb needs to flow sufficient air, ie., mixture, to satisfy the demands of the engine. There is always going to be a compromise with any carb, low revs v high revs, where do you want the performance/power?. I suppose that is where injection scores.
Allan Reeling

Stan, Allan, I agree with both your thoughts on this and would just like to say I respect Peter Burgess and what he says 100% and I hope he knows that by now
What I'm suggesting is a means of getting the best out of the 1 3/4" carbs that are already on Brian's car - The 1 1/2" carbs may possibly outperform them at lower speeds but the 1 3/4s can be made pretty good really all up
Brian ,
To answer your aircleaner question,I have 4 cars running around with SY needles and blue damper springs at the moment, they just seem to suit these engines and over a wide range of mods
1/ 1800MGA with very mild cam and K&N fiters
2/ 1800MGB mild cam ram tubes-no filters
3/MGB +.060"bore mild cam oiled foam cleaners
4/1950MGB,headwork,lumpy cam, K&N in cold air box
1+2 mixtures run fairly spot on
No3 -good strong mixture throughout
No4 -strong mixture, possibly a tiny little bit rich on absolute full revs with an open throttle but it's basically a competition car and economy isn't an issue at these revs

I've had a good look through the needle chart and unfortunately there doesn't appear to be a direct match for SY needles in the springy needles
There are adapters available to fit solid shank needles into springy style pistons if you want to give the SY's a try
I've never tried to convert a solid needle into being a springy one but I would imagine that could be a possability-??
The BBW needles you have are probably the closest listed, but they are quite a bit leaner in that low to mid (4th step onwards) light acceleration/cruise area than what the SY's are and that is what I suspect is causing your soft performance in that area
If you do go ahead and try the SY needles you do 'have to' match them with the light blue( 2.5oz ) damper springs otherwise you will be wasting your time

willy
William Revit

Hi Willy I can assure you the respect is mutual. You are quite right to sort out what is fitted, why waste money? I have been responding to the suggestion the 1 3/4" increased bottom end power which theoretically should not happen and empirically does not happen.
I tend to find the idle and cruise settings tend to be on the weak side and achieve good full throttle fuelling, increasing the part throttle fuelling seems to increase the full throttle too much.
Peter Burgess Tuning

Willy and Peter

Unfortunately a rolling road is not an option here and there are not any mechanics that know or understand MGBs. I have driven and worked on MGs and other British cars since the late 60s but all have been factory original daily cars. The current GT was sold to me by a club member and I have been working to get it back to factory specs as after owning a number of Bs I was not happy with performance or ride. The ride is now good (back to stock) but performance is not great. One of the changes the PO made was to go to 15" knock-off wheels and I am willing to stay with these but the carbs and intake are certainly not helping at normal speeds. I will run this setup for now but have picked up some HIF4 carbs and pieces that I can rebuild and will hopefully track down the correct intake manifold shortly.

I appreciate the comments and the combined wisdom of all respondents and will report my progress and the outcome once it is done. In the meantime I will run what is on it, knowing a solution is at hand.

Brian
Brian Smith

Brian
It may be worth going through basic checks to ensure ignition timing is ok and maybe even check cam lift in/ex at TDC
Peter Burgess Tuning

Allan - thanks for the contacts.
paulh4

Local club member had an intake manifold correct for the car that he dropped by yesterday so now just to build up a set of HIF4s and give it a go. Started snowing early this morning and everything is covered in the white stuff! Shop a little cold but it normally doesn't last long.
Brian Smith

Hi Brian
Just out of interest, do you know what brand the 1 3/4" inlet manifold is---could be interested in that (just depends)if you're going to sell it
willy
William Revit

Willy , not sure what brand/make the intake is and at this time no plans to sell. I have attached a picture of the carbs, air cleaners and intake. I am moving ahead slowly with rebuilding the HIF4 carbs, heat shield and stock intake. I probably will not change over until late Summer or Fall all going well.




Brian Smith

All good Brian, thanks anyway, just thought I'd ask
Looks a fairly neat instalation--
willy
William Revit

This thread was discussed between 25/01/2019 and 24/02/2019

MG MGB Technical index

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