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MG MGB Technical - SU's?

Now that I've driven my ' 80 LE with the stock Zenith carb setup, I've decided that I like the '74 and earlier carbs much better. I really prefer HIF 4's. Granted, the cam and cam timing are different on earlier cars, but would it be a good upgrade to switch to HIF 4's? I have all of the stuff to do the swap. ' Just want to see what everyone else is running on their later B's.

Thanks in advance.
David Plantz

I have an '80LE. I'm running HS-4's on mine from my original '66B. Have HIF-4's on a '76B and a pair of '74BGT's. My own preference has always been the early HS-4/AUD135's attached to the appropriate intake manifold fitted with a mushroom style PCV valve. When something goes wrong with a needle valve or float, externally mounted fuel bowls are a blessing. Another plus is AUD135's aren't burdened with the hose clutter of the later model HS-4's and HIF-4's.
Steve Buchina


I have done this on a 1975 roadster that we imported from Alabama.
The result was a 82 HP engine, but you also have to change the exhaust manifold and the down tubes, the inlet manifild has to be modified that way, that the rear carb and filter can clear the breakservo (carefully grinding down the emulsion blocks and taking care to do it that way, that the heatshield can be installed still).
You will also have to change the dizzy, as the HIF 4 carbs have ported vac. and the whole setting of the ignition curve cannot be compared the that of the dizzy installed now. The ones of the erarly 70's will work well.
The cam is not that point to have a problem with it's setting. The increased low end power is fine and up to 80 MpH, there are no real differences to the earlier version concerning the car's performance.
On my car, I also took away all of this anti pollution stuff, making it more economical and giving better response too. (Just calculate how much emission can be saved by saving 30% of gas! ... you will see, that there was some unusual thinking in the eraly 70's upon emission in some states...)


Actually Dave, you can still use the manifold vacuum with the HS4s or the HIF4s to keep your stock distributor. You would probably stand to gain a few hp by going with the older type though, provided you get the right model.
Jeff Schlemmer

Hi Ralph,

Yeah, I'm looking forward to this upgrade. My car has all of the emissions stuff minus the air pump.

Hi Jeff,

Aren't you selling distributors that are curved for different applications? I'm interested to know more about them. I do have some HS4's here as well to use.

Many thanks,
David Plantz

The exhaust manifold used with the SU HS4 carburetors' intake manifold have a mounting flange thickness of 9/16" and can be readily identified by its external casting number of 12H709, while the exhaust manifold used with the SU HIF4 carburetors' intake manifold has a mounting flange thickness of 7/16" and can be readily identified by its external casting number of 12H3911. When seeking improvements in airflow capacity, things become considerably more complicated when trying to fit higher capacity aircleaners onto a Rubber Bumper car that has been modified to make use of dual carburetors. Unfortunately, the manner in which the servo-boosted master cylinder projects from the bulkhead forces most conventional owners into the use of conical airfilters when installing dual carburetors. The problem with the conical airfilters is that their shallowness creates induction pulse problems above 3,500 RPM, their small internal volume that will not allow the fitting of a set of velocity stacks, and their small surface area that offers insufficient airflow for an enhanced performance engine. The K&N airfilters all use the same filtering medium, so the smaller the surface area of the filter, the less the maximum airflow potential will be. Conversely, the bigger the surface area, the greater the maximum airflow potential. This is why the 6" X 3 1/4" deep K&N airfilters are preferred by those who go for serious power increases with a B Series engine. Induction pulse problems aside, the airflow capacity of the little conical or pancake filters is more appropriate to a mildly power-enhanced 1275cc A Series engine, such as is fitted to the MG midget or the Austin Healey Sprite. In addition to this problem, the remote floatbowls of the SU HS4 carburetor will interfere with the servo-boosted master cylinder, thus such a conversion requires the use of a set of SU HIF4 carburetors.
Retrofitting the earlier non-boosted master cylinder is the common solution, but this is not a bolt-on affair as its mounting flange is turned 90 so the mounting holes of the pedal box will not line up, and the appropriate earlier pedal box assembly is radically different, even having a different mounting hole pattern at its base that requires drilling a new pattern of holes into the body of the car.
Steve S.

"In addition to this problem, the remote floatbowls of the SU HS4 carburetor will interfere with the servo-boosted master cylinder, thus such a conversion requires the use of a set of SU HIF4 carburetors."

For the engine in my '80 LE, I fitted a tubular (steel) exhaust header, MOWOG 12H1398 intake manifold with mushroom PCV valve, AUD135 HS-4 carbs (with external fuel bowls) and a pair of K&N conical air filters. The external fuel bowls have plenty of clearance and are easily serviced. The only thing that comes close to presenting a clearance problem is the rear air cleaner. Its top cover sits about 3/16" out from the brake servo assembly. I'm also running a 25D distributor with vacuum advance off the rear carb. This engine seems to perform quite well - far better than when it was in its original RB configuration. Not as strong (on the low end) as my '66B, but far better than any other RB in our LBC club.
Steve Buchina

Are the Peco headers and exhaust systems are worthwhile upgrade?

David Plantz


if it is a LC engin, I do not think you will have any power gains, but it has a very suitible sound for a MGB.
This one box system also prevents from damaging the non existing center muffler, although it is not realy louder than the stock system.
With the manyfold, you will have to check if it is straight and does realy fit the ports. There are often restrictions, mainly at #4 cyl., due to bad weldings at the flanges or simply bended tubes. Take a gasket with you to the shop and check out what they have in stock!


best thing I ever did to my 78 NAS MGB was to ditch the weber a in favor of HS4 SUs. Even though it's a low compression engine it's MUCH peppier than before and runs and idles a whole lot better. SUs are what the car was originally designed to use...having done this conversion, now I know why. Get rid of all that emissions's a danger anyway...stupid cat converter UNDER the carb....duh!

Just saw your letter. I have HIF-4's on my car and it runs great.
You'll need the exhaust and intake manifolds, new heat shield, linkages, choke and accelerator cables, plus the carbs to make the swap. I also bought conical shaped K&N air filters and have about 1/2" clearance to the brake booster.
I ditched all the emissions stuff including the catalytic converter.
The set up was purchased new from a junk yard in England and I haven't had to do a thing to the carbs except replace one float.
No distributor change was made.
If you have any questions, or would like pictures of anything specifically, contact me off-list.
1977 MGB
randy olson

Mine is a '77, that I was just gonna fix 'er up and sell at a profit.

Well my oldest son said to me,
'Dad you've wanted another MGB for years, and this one is rust free, you'd better keep it'

So I took his advice.
And soon the HIF 1 3/4" that came from the original Marina engine, because the Z/S wouldn't run at all. Seemed to be holding back the engine from revving over 3500 rpm.
Which ended up being ignition related.

So a set of early HS4's found their way along with the exhaust manifold was installed, along with the pancake air filters , which really restrict air flow.

Huge power increase.
Well Ok, not huge, but big.

Now I have a ceramic coated header, the big rim flow valves, a mild grind cam, roller rockers, way more compression, balanced, pertronix ignitor, with the Second Strike rpm limiter/multiple sparks.

Now this car is fun!
A cam that will give power from 2500- 6500 rpm, as the current cam with 7 degrees of retard cuts torque at about 5000 rpm, more compression, and forged pistons.

Power is addictive!

DCM McCullough

Sorry I'm so late in responding. You're not the first person to report that he installed a set of SU HS4 carburetors on a Rubber Bumper model and had not experienced the fouling problem with the master cylinder. I figure that this is due to the considerable variations in the body panels that originated in the production methods of the time. Some can getaway with it, most seem to not be able to.

The gray system will fit the Original Equipment pre-1975 exhaust manifolds without modification while the red 2" Big Bore system will require the use of the Peco 2" diameter Big Bore header. Having approximately a 30% greater cross section than a 1 3/4" diameter system, the Big Bore system is actually intended for use on larger bore engines (1868cc or larger) or smaller-bore engines fitted with flowed heads and hot camshafts such as the Piper BP285. It seems to be particularly beneficial when used on engines that are tuned to produce 125 HP or more. When fitted to smaller bore engines with Original Equipment camshafts it will result in a bit more high-RPM power at the expense of some tractability at low engine speeds. This is due to the approximately a 30% greater cross section of the exhaust system reducing exhaust gas velocity, which in turn reduces scavenging effect in the combustion chambers at low engine speeds and thus increases "Pumping Losses". In addition, radiant heat from the tubular steel of the header is much greater, exposing the air in the engine compartment, the intake manifold and carburetors, and the fuel system to more heat, thus reducing fuel/air charge density and hence reducing power output. Jet-Hot coating of such headers is therefore highly recommended.
Steve S.

Hi Steve,

I guess that I'll wait on the Peco system since this is a low compression engine. I want to build it up eventually, but can't really afford to right now as I'm still in school. My ultimate goal is to install the Moss SC after adding a nice cam, exhaust, etc. Right now I have a new stock system with two mufflers. So I'll change to the earlier down pipes and leave it at that.

David Plantz

Hello to all,
I am fairly new at this great game we call MG's. How timley I got turned on to this site. I have a '79 and am considering a carb swap. I have both extra HS4 and HIF's that I could use. In terms of airflow, I have seen a set-up where a 4-inch "breather tube" is connected to an air cleaner (not sure brand) and run through a hols in front radiator shroud that I was told solves the air volume problem. My bigger question is needles. I have AUD 135 with AAL needles and AUD 405 with AAE needles. Niether of whic is stock. I do not plan on changing stock cam.
T. J. McSorley

I don't think there is an air volume problem as such, but a tube such as you describe can be used to give a cold air supply instead of the more usual hot. I've done this in the past on a single-carb car and it *seemed* to make a difference, but I may have been kidding myself. I've never got round to doing anything about it on my current cars.

As far as I'm aware neither of those carbs were used on the MGB in any market, so how they differ from the ones that were I don't know. If you can find where they *were* used that might give you a clue.

AAE and AAL needles *were* standard for the MGB, on HS carbs for the 18GH and GJ in the first case, and the 18GK in the second. The specs of the 79 engine were quite different to these.
Paul Hunt 2

Oh, I forgot to mention, I removed the '77 master cylinder, and went to the earlier type.
Ceramic coated the headers, and use the larger K&N air filters.
Along with more compression, the largest rim flow stainless steel valves,
roller rockers, port & polish, slightly bigger cam, and cam timing retarded 7 degrees.

Yes it moves!


DCM McCullough

This thread was discussed between 25/06/2006 and 06/07/2006

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