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MG MGB Technical - SU Carbs

Just restoring a road going B roadster. Does anyone have any thoughts on which SU carbs are the best to use. I have in stock HS4 and HIF4 all of which need light restoration.

Any differences in the manifolds??

Thanks

Colin
Colin Parkinson

Some like the HS 4's easy access to the float chambers and the mixture nuts. The HIF's have fuel temperature compensation, seals on the spindles and are probably slightly more efficient. Personally I rebuild the HIF's with plain butterflies rather than over-run valve efforts.............less obstruction to air flow. On the whole you won't notice a big difference in performance Manifolds are common...........I seem to remember.
Allan Reeling

Much as Allen says. Having both HS4s are easier to 'live' with, the only marginal user benefit of HIFs is that they don't tend to stall in a long idle. After that it's all downhill with float chamber access much much worse, enrichment valves to leak, and small passages to block up. Tuning to the lifting pin is also much trickier - the momentary rise and fall on mine being almost subliminal compared to my HSs. And Colortune is no help, even with two.
paulh4

Colin-
I prefer the SU HIF design. There has been a great deal of discussion of the relative merits and vices of the 1 1/2" SU HS4 Series carburetor and those of its successor, the 1 1/2" SU HIF4 Series carburetor. Advocates of the 1 1/2" SU HS4 Series carburetors point out the greater ease with which the fuel jet can be changed with the carburetor in place on the engine and the metering advantage of its concentrically mounted fuel-metering needle and fuel jet. Others feel that its remote float bowl design gives it a 'Vintage' appearance. However, the 1 1/2" SU HS4 Series carburetors are not without their vices. In the SU HS design the float is oriented so that the longitudinal hinge of the float is attached with its axis parallel to the longitudinal axis of the car. In consequence of this design flaw, when the fuel in the remote float bowl sloshes forward during deceleration, causing the float to rise and thereby close the fuel shut-off needle valve, thus preventing overflow or excess fuel when it is not needed. Upon acceleration the fuel sloshes toward the rear of the remote float bowl, which allows the float to drop and thereby cause the fuel shut-off needle valve to open, allowing more fuel in when more fuel is needed for acceleration. However, this occasionally induces a problem during extreme high-performance driving. They have a tendency to run either rich or lean (weak) under conditions of rapid acceleration and deceleration, and when the car is on a steep road. The common racer’s remedy was to fit a spacer between the float lid and the remote float bowl, and then adjust the float level upward in order to raise the float level setting and thus take advantage of the increased capacity of the float bowls. This was a satisfactory solution when running, but at idle fuel would then well up above the fuel jet, causing poor atomization that resulted in bore washing, rough idling, poor off-throttle pick-up, and horrendously rich CO2 emissions! For racers this was a tolerable, though partial solution. Adjusting the float level downward in order to lower the float level setting in order to return to the correct fuel level held within the float bowl would attend to these induced problems, but the previous problems would then return, so for a street engine it is an impractical solution.

The design of the SU HIF Series carburetors largely addressed these problems by having its float bowl integral with its body, thus allowing the laterally-hinged float to surround the fuel jet and hence more consistently meter fuel inflow under high angles of tilt, as well as during acceleration and deceleration. It also has superior performance potential due to its increased maximum flow rate, a factor that should be considered if you intend to modify the engine for increased power output. However, under conditions of very heavy cornering stresses such as are encountered while racing on a race track with sharp curves, the laterally-hinged float can also present air-fuel mixture problems, but this rarely occurs on a street engine.

In addition, the 1
1/2" SU HS4 Series carburetors require the removal of their air filter housings in order to enable the use of a pair of special short open-ended wrenches (spanners) (Burlen Fuel Systems Part # SUT 2) in order to easily adjust the air-fuel mixture, which results in a somewhat richer air-fuel mixture when the air filter housings are refitted, especially if cheap, restrictive paper air filters are fitted inside of them.

The most failure-prone feature of the SU HS4 design is that of the 'gland' seal of the fuel feed line that connects to the fuel jet where it connects to the float bowl. The problem is particularly common in cold climates. In cold temperatures the plastic of the fuel feed line becomes stiff. Of course, because it is cold, you need to use the choke. Pulling the choke lowers the fuel jet, which in turn wiggles the plastic fuel feed line in its sealing gland. After a couple of years it is not uncommon to suddenly notice an odor of fuel, and that the fuel pump chatters a bit when you switch 'On' the ignition. Investigation then reveals that a drip has begun from one of the gland seals. Tightening the gland nut is always a temperamental task. Too loose and it leaks, too tight and it leaks. The difference between too loose and too tight is approximately 1/80 of a turn!

They also have a tendency to leak fuel at the junction of the remote float bowl and the body of the carburetor, as well as from the external base around the fuel jet. The latter design feature is the result of the necessity of retracting the fuel jet downward in order to enrich the air-fuel mixture during cold starting conditions. In terms of cold starting, the 11/2" SU HS4 Series carburetors use a cable-operated lever that both lowers the fuel jet as much as 7/16" and also opens the throttle disk slightly by means of an attached fast-idle cam in order to prevent low speed stalling under the conditions of an over-rich air-fuel mixture.

On the other hand, the design of the 11/2" SU HIF4 Series carburetor uses a lower-maintenance separate fueling circuit in order to accomplish this enrichening function. Fuel for the starting air-fuel mixture is metered by means of a rotary valve spindle that has a hole drilled in it and a tapered groove machined into its shaft that gradually increases in depth, thus acting as a variable orifice when you rotate the spindle, ensuring that the degree of enrichment is progressive. That is the part that is rotated by the choke cable; it has threads on its outer end. When it is rotated, it exposes either more or less of that tapered groove to the hole that fuel enters through and into the starter valve body. The fuel then passes through a separate passageway within the body of the carburetor that terminates in the high vacuum area that is located to the right of the fuel jet aperture, immediately behind the fuel jet bridge. As an aside, the screw relief hole in the barrel must be oriented so as to give relief to the upper screw.
The SU HIF4 design also employs a thermosensitive air-fuel mixture control mechanism that compensates for changes in temperature as well as also contributing to easier cold weather starting. In order to accomplish this, a bi-metal blade in the form of an L-shaped bracket is used adjust the height of the fuel jet as needed according to the operating temperature of the engine. This L-shaped bracket is held in place with a shoulder screw and a spring so as to be an articulated assembly. If you push down on the fuel jet from above and the jet feels too springy, then the bimetallic part is probably installed incorrectly. The shouldered screw has to allow it to float against the coil spring, but still locate it securely. The air-fuel mixture adjusting screw bears against the longer leg of the L-shaped bracket, thus thrusting against it in order to change the pivoting angle of the bracket. The straight plate on the short leg of the L-bracket serves to relocate the fuel jet either upward or downward as the L-bracket pivots. Additionally, the flat plate also incorporates a bi-metallic strip that is designed to distort according to changes in temperature. This allows for compensating alteration of the Air / Fuel Ratio in response to viscosity-altering temperature changes of the fuel. This is an advantage because when the temperature within the engine compartment increases, the viscosity of the fuel passing through it becomes thinner, and thus the flow of fuel through the fuel jet increases, creating an uncompensated richer air-fuel mixture. In the case of the SU HIF design, with altering temperature, this bi-metallic strip will distort in order to either raise or lower the fuel jet slightly, consequently adjusting the fuel flow, and thus maintaining the proper air-fuel mixture over a wide range of operating temperatures. The temperature compensating mechanism rests within the fuel inside of the integral float bowl, thus the temperature of the fuel can effect the bi-metallic strip. The temperature compensating fuel jet lever closes to an angle of less than 900, causing the jet to rise approximately 0.010" to 0.015" (0.254mm to 0.381mm). This rise adjusts the air-fuel mixture in order to improve drivability when the fuel is at elevated temperatures, thus compensating for changes in the viscosity of the fuel which would otherwise result in a rich air-fuel mixture. In actual practice, the bi-metallic strip seems to have a range of movement that is approximately equivalent to a half-turn of the air-fuel mixture adjusting screw. Keep in mind that a single full rotation of the air-fuel mixture adjusting screw moves the fuel jet up or down approximately 0.030" (0.762mm). Therefore, if you are adjusting the air-fuel mixture, the 'sweet spot' could lie within one end of the bi-metallic strip’s range of motion, thus making the air-fuel mixture too rich or too lean (weak) as the fuel temperature changes if the adjustment is made when the engine is not at its normal operating temperature. This can cause starting problems when attempting to restart a hot engine as heat rising from a hot exhaust manifold can cause the bi-metallic strip to overcompensate the air-fuel mixture. In order to allow for this, drive the car until it is at full operating temperature, and only then proceed with your adjustment of the air-fuel mixture.
A positive secondary effect of this feature is that this temperature compensation also makes the HIF carburetor less prone to suffer from vapor lock, which is sometimes a problem with the predecessor H and HS Series carburetor designs during hot weather and very slow driving. This precise fuel-metering control means that once correct fueling is established by appropriate fuel-metering needle selection, the air-fuel mixture is maintained within a very wide range of normal operating temperatures. The thermally-compensated fuel jet also gives a consistent idling speed over a range of normal operating temperatures, whereas SU HS Series carburetors can tend to stall during a prolonged idle in summer when everything heats up, and can need the positions of their fuel jets to be given a tweak of enrichening in winter and leaning (weakening) in summer. Drivability with SU HIF Series carburetors is consequently enhanced, resulting in smoother running, and emissions are kept within tighter limits during the cold start and warm-up period.
Although more expensive to purchase and a bit more time-consuming to initially set up than the 11/2" SU HS4 Series carburetor, the 11/2" SU HIF4 Series carburetor is subsequently easier to adjust and has superior performance potential, providing a two to three horsepower increase at partial throttle openings. During routine adjustment, the ratio of its air-fuel mixture can be modified from above with nothing more than a simple screwdriver, hence removal of the air filter housings is not necessary as is the case in adjusting an SU HS4 carburetor.

In addition, the AUD 630 and later variants of the SU HIF carburetor design utilizes ball bearing vacuum chambers (suction chambers), a design improvement that was introduced in order to improve both fuel metering accuracy and its consistency. By incorporating a nylon cage that contains two rows of six small ball bearings between the hollow damper tube of the vacuum piston and the vacuum piston guide of the vacuum chamber (suction chamber) in order to reduce hysteresis between the vacuum piston and the vacuum chamber (suction chamber), the freedom of movement of the vacuum piston was improved, thus leading to a more rapid response to varying fuel-metering demands caused by the opening and the closing of the throttle disc, as well as improved fuel metering repeatability, especially when returning to an idling state. It should be noted that this variant makes use of a non-interchangeable complementary vacuum piston damper rod and vacuum piston which must be used with their appropriate matching vacuum chamber (suction chamber).
Stephen Strange

Stephen...........crumbs!

I'll print that off and read in bed tonight!!

Colin
Colin Parkinson

Wow - Stephen has said it all!

I do get the impression that later cars are more economical. For example, early cars on test got 23/24 mpg on average. My '72 with earlier non-swing needle HS2s and standard large inlet valve head gets 22/23 mpg in urban driving. However, I've seen anecdotal evidence on these pages of folk with rubber bumper cars, presumably HIF-carbed, getting a regular average of 30 mpg. Is that down to HIF carb efficiency or later cars being slightly de-tuned, aerodynamic(?)......?
Peter Allen

I've not had any of the problems Stephen describes, hence my preference for the HS.

The only thing I can put the low 20s reputation of the MGB down to is the 'enthusiastic' test driving of them by journalists. My HS equipped roadster has averaged low to mid 30s for the 30 years I have had the car, going up to low 40s on some longer journeys e.g touring through France. Even the V8 has averaged 30+ on long runs to Le Mans, Scotland and Lake District. What any particular car gets is predominantly down to the driving style of the person behind the wheel, as well as how carefully it has been set up.
paulh4

I prefer the simpler HS too. Not been able to match Paulh's economy figures though. I get high twenties on a run and low twenties around town. That seems to match other peoples experience. I've got a Burgess EconoTune head and have just had a re-bore to +0.040" Although the engine isn't run in yet, I've been through ignition and carb setting, so lets see how it goes. Before it was suffering from low compression but minimal oil consumption - curious ! It certainly starts better and although I haven't pushed it yet, goes better now.
It seems to me that the magic that was present in A series is missing with B series and although torquey that doesn't seam to thrive on revs. I used to get 40 mpg out of my 1275 midget.
Paul Hollingworth

I'm with Paul, my over bored 1800 never got below the high 20's and my SU equipped V8's (3.5 and 3.9) return similar mpg. I suppose part of it is down to how you drive, but a lot is down to accurate tuning. My V'8 are driven "enthusiastically" just to enjoy the sound!!
Allan Reeling

A piece in the MGOC mag this month about fitting a vacuum gauge and how it can help improve mpg by backing off slightly in a cruise without losing speed. I had one fitted for years and that's what I did, and even though it's now been in a drawer for years I still use the same technique.
paulh4

This thread was discussed between 29/03/2018 and 04/04/2018

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