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MG MGB Technical - HS4 vs HIF

I need some opinions.

I'm running a GK engine in a 68 GT with HS4s that have seriously worn throttle shafts and bushings. This makes any tuning a challenge.

I've got a set of HIFs that have low miles and tight throttle shafts, but they have been sitting around since about 1985.

It seems that the parts (I do my own labor) would be a lot cheaper to rebuild the HIFs since they have good shafts/bushings.

I would lean toward the HIFs for the dollar savings, but I've never had a set of those apart. I might also lean toward the HIFs since David Vizard speaks well of them in his A-series book in terms of a slight advantage in fuel economy.

I would lean towards the HS4s because I have rebuilt and tuned HSs several times, and so am more comfortable with them. I have never tried to tune a set of HIFs, and have never driven a car equipped with them.

Another possible consideration is the dash pots and pistons. Are they interchangeable between the two? At this point I have no idea of the condition of them on the HIFs, but my HS4s have the sweetest set I have ever owned. Their drop time, with the air plugged, approaches infinity on both, and it is not because they are jammed.

Your comments will be appreciated.

If you want to suggest a switch to Weber, bite your tongue. I will try one of those when I can pick one up at a swap meet for about $20, but I won’t spend big bucks to leave the elegant simplicity of the SU.

Thanks,
Cheap Charley
C R Huff

Charley,
Another benefit of the HIF is that it is less prone to leaning out the mixture in those tight left-hand u-turns while autocrossing ;-) The biggest problem with the HIF in street applications is that it is hard to detect an overflowing fuel bowl as it simply runs down the engine's throat like a post-nasal drip. Other than that, it is a fine carb.
David "HIF6 in my drawer waiting for my midget" Lieb
David Lieb

Well, David, you might be smiling, but in fact that might be a consideration. I've got a square body Sprite for autocrossing, but dealing with the trailer is a bit of a pain. I may return to autocrossing with the BGT. My last foray there with the BGT was when I lost the oil pressure and had to rebuild the engine.

If the HIF does run the gas down the throat, does it run so much that you can tell that it is happening by how it runs, or is it more subtle than that?

Wahoo, HIF6s on a 1275? And I thought I was cool with HS4s on my 1275!

Charley
C R Huff

Charley
The HIFs are as easy [or difficult] to rebuild and tune as HS4, the only drawback I've found with HIFs is if there is a problem with the float or needle you need to remove the carb to repair it. If an HIF overflows the engine misfires due to flooding and the petrol should also run out the overflow pipes down the side of the block.
Ron
R. Algie

As Ron says, the HS is much easier to get at the float valve and main jet with the carbs in situ compared with the HIF. The HIF has a bottom cover which is submerged so you almost always have to replace the large O-ring when getting at the aforementioned float valve or jet.

In my experience HSs are slightly easier to tune. Judging whether the revs have risen, fallen or stayed the same when using the lifting pin requires enough practice on an HS as it is, on the HIF is is pretty-well subliminal.

The HIF has an internal enrichment valve so you cannot see what it is doing, it can block, and it has O-rings which can leak. The HS system is far simpler in that it just moves the main jet and is all external so you can see if it is working or not.

As far as the float valve leaking goes agin in my experience they are exactly the same - they overflow, and the overflows must be piped down past the exhaust for safety.

The only benefits of the HIF for all year round road use are the temperature compensated jet means the mixture doesn't have to be set a smidgen richer for winter and back again for summer like my HSs seem to need. And because of the internal bypass port the HIF doesn't suffer from the ghradual reduction in idle revs to stalling point if stuck in traffic a long time. Also because the jet is situated in the middle of the float chamber it is less likely to suffer from fuel starvation in high G situations, but I've never experienced that in road use anyway.

I'd go with HSs every time.
Paul Hunt 2

I'm interested in a swap from HIFs to HS4s. I've been plagued by problems with flooding in my 1980 roadster rear HIF. I've fitted a grose needle, new float and pivot adjusted float to spec and it's still flooding! Fuel pressure is good and now it seems to be leaking at the O rings as well.
Can anyone confirm that to change to HS a matching manifold is required? Are there any issues with the flange thickness on the exhaust/inlet manifold?
Phil

I don't know if the manifold would need to be changed for an HIF-to-HS swap, but I do know a bit about the potential of flooding through one carb. Depending on the year of car, and whether the carbon canister is installed and properly routed, you can easily wind up filling the canister with gasoline before the extra fuel ever chokes out the engine. Alternatively, as was noted, the overflow could end up running down near (or over!) the exhaust manifold.

More often than not, any flooding that may happen - and it is NOT any kind of epidemic, by the way - is caused by a stuck float. Inside the float chamber is a little ledge, and occasionally the float body gets snagged under this little shelf. All that's needed for permanent correction is a tiny brass washer placed along the float's swivel shaft. This will block the float from traveling in that direction.

Flooding has happened to me a grand total of two times in the 2 years I've owned this car. The first time was when the Pot-Hole from Hell jumped out and grabbed the car. The second time was when - with the car tuned far too rich - I wailed on the accelerator to see how hard I could push the engine. The engine did really well, right up to the point where it flooded and died. The first incident was a fluke, and the second was 100% self-imposed. Not a bad record.

The HIF's and HS's are both decent carbs. Each has advantages over the other, but the only reason I'd bother exchanging one for the other is if my current set was shot and I could install a good working pair (of whatever kind) for a decent price.

Rick Stevens

Thanks for the advice, guys. I guess I might be better off rebushing my HS4s. They would probably be a little easier to deal with out on the open road. Now if I could figure out what I did with my ream.

Charley
C R Huff

If the vent/overflow gets blocked on any SU, charcoal canister or not, the float can't rise to shut off the valve and that *is* when fuel runs down the inlet. This is more likely to happen on canister cars.

I was plagued with occasional flooding on one HIF, until I discovered the float had some fuel in it (not easy to see). Never did discover where the leak was despite heating it under water to look for bubbles or squeezing it. With a new float it has been fine since.
Paul Hunt 2

I was also plagued with one overflowing HIF, I tried new float needles, grose jets and even a pressure regulator all to no avail, the problem always came back, I eventually replaced a perfectly good float mainly because I couldn't think of anything else and I've had no problems since! I must hold the record for the fastest remover/replacer of HIF SUs in the west!!
Ron
R. Algie

Yep Ron I sympathise! Have been talking to MGOC technical line today. They were suggesting I go back to an SU fuel pump or try a regulator. Paul Hunt's preference for HS's seems to make economic sense too.
A new pair of HS4s are 279 pounds against a recon set of HIFs at 259 pounds.
The o rings on one of my recon HIFs have only lasted a short time, so I am looking seriously at a new set of HSs.
I want to be able to drive my MGB and not be worried about getting home!

































Phil Barton

Phil, Bracknell, UK-
No, you don't need to change intake manifolds to mount the SU HS4s in place of a pair of SU HIF4s. You will, however need the HS4-appropriate heat shield as the mount for the throttle cable is in a different lateral position.

Harley-
There has been a great deal of discussion of the relative merits and vices of the 1 SU HS4 carburettor and those of its successor, the 1 SU HIF4 carburettor. Advocates of the 1 SU HS4 point out the greater ease with which the fuel jet can be changed with the carburettor in place on the engine and the metering advantage of its concentrically mounted fuel-metering needle and fuel jet. Some feel that its remote float bowl design gives it a Vintage appearance. However, the 1 SU HS4 is not without its vices. It requires the removal of its air filter boxes to enable the use of a pair of special short wrenches (Burlen Fuel Systems Part # SUT 2) in order to effect mixture adjustment, which results in a richer mixture when the air filter boxes are refitted. It also has a tendency to leak fuel from its floatbowl junction and from the base of its fuel jet. The latter is the result of the necessity of retracting the fuel jet downward in order to enrich the fuel-air mixture during cold starting conditions, causing wear of the sealing glands. In terms of cold starting, the 1 SU HS4 uses a cable-operated lever that both lowers the fuel jet and also opens the throttle disk slightly in order to prevent low speed stalling under the conditions of an over-rich mixture. The 1 SU HIF4 design uses a lower-maintenance separate fueling circuit in order to accomplish this function. Mixture enrichment is accomplished by means of a separate fuel path within the body of the carburettor between the float chamber and the constant depression area close to the fuel jet aperture. A rotary valve, effectively a long, plain shank screw with a slot in it, controls metering. In addition, the 1 SU HS4 has a tendency to run rich or lean under conditions of rapid acceleration and deceleration, during hard cornering, and when on a steep road. The 1 SU HIF4 largely addressed these problems by having its float bowl integral with its body, thus allowing the float to surround the fuel jet and hence more consistently meter fuel under high angles of tilt and under conditions of heavy cornering stresses.

Although more expensive to purchase and more time consuming to set up than the 1 SU HS4, the 1 SU HIF4 is easier to adjust and has superior performance potential due to its higher maximum air flow rate which endows it with somewhat better performance at high engine speeds. During routine adjustment its mixture can be modified from above with nothing more than a simple screwdriver, hence removal of the air filter boxes is not necessary. Its thermosensitive mixture control makes for easier cold weather starting. A bi-metal blade is used adjust the height of the fuel jet as needed according to the operating temperature of the engine. This precise fuel-metering control means that once correct fueling is established by appropriate fuel-metering needle selection, the mixture is maintained over a very wide operating temperature range. The thermally compensated jet gives a consistent idle speed over a range of temperatures, whereas SU HS series carburetors can tend to stall in a long idle in summer where everything heats up, and can need a tweak of richening in winter and weakening in summer. Drivability with SU HIF series carburetors is consequently enhanced and emissions are kept within tighter limits during the cold start and warm-up period.

Those who have converted their cars from the 1 SU HS4 to the 1 SU HIF4 usually report a 1 to 2 mpg increase in fuel economy. Unfortunately, rejetting requires that it be removed from the intake manifold and its thermosensitive mixture adjustment control can cause it to run lean if underhood temperatures rise badly in heavy traffic on hot summer days. Consequently, Jet-Hot coating of the exhaust manifold is a worthwhile investment, as is the fabrication of U-shaped heatshields to insulate the runners of the intake manifold.

It can be done, but it is not the easy bolt-on swap that some presume that it might be. You will need an HS4 heatshield, distributor, cables, plus the linkages and a lot of other little bits and pieces that are not commercially available anymore, so you will spend a lot of time scrounging around trying to get them. If it is the lower price of the 1 HS4 that seems attractive, be aware that when you get through buying all of the hardware necessary to do the installation correctly, the difference in cost will not be anything like what you hoped it would be. Whichever version of the SU carburettor you choose, you will find it helpful to obtain copies of the SU Reference Catalogue and The SU Workshop Manual from Burlen Fuel Systems.
Steve S.

Steve,

Thanks for the good write up. Well, now more food for thought.

The HIFs that I have are complete, and still bolted to the manifold with the heat shield. So, I wouldn't need to scrounge a bunch more pieces. In fact, I have the whole engine that they came from, and for that matter, the whole car. However, I believe it had an engine swap because I don't think the GK engine came with HIFs.

In your last paragraph you mention that the distributor is different. Do you mean the ignition distributor?

Charley
C R Huff

"the 1 SU HS4 uses a cable-operated lever that ... opens the throttle disk slightly in order to prevent low speed stalling under the conditions of an over-rich mixture. "

This is the fast-idle for use when the choke is in use and applies to both HS and HIF. It prevents stalling when the engine is not yet up to temperature, not when it is 'over-rich'.

"The 1 SU HIF4 design uses a lower-maintenance separate fueling circuit in order to accomplish this function."

Lower maintenance? I don't see how, you can't see the HIF enrichment working and it can leak. It's additional components and fuel path are just more to go wrong and get blocked for no useful gain.

The HIF *is* easier to adjust as you say, although I can tweak my roadster HSs just by reaching under the air cleaners without using any tools. I really doubt the mixture variation between air cleaners off and on, at idle when the carbs are being adjusted the air flow is minimal and hence any restriction will have no effect. If it *did* have an effect at idle, the effect at wide throttle openings would be huge. I know K&Ns have less restriction (and require the use of a slightly 'richer' needle as a result) but the gain is at wide throttle openings, not idle.

You certainly don't *need* to change the distributor when changing carbs. You may need to move the vacuum take-off point between rear carb and inlet manifold if the existing carb has the port but the replacement cabs don't, or vice versa, but that is all. Only if significantly changing the characteristics of the engine would you consider recurving the distributor.
Paul Hunt 2

Charley-
The vacuum advance control capsule of the distributor used with the pre-1971 North American Market 1˝” SU HS4 carburettor takes its vacuum from a connection on the carburettor, while the vacuum advance control capsule of the distributor used with the North American Market 1˝” SU HIF4 carburettor takes its vacuum from the intake manifold. These two systems result in highly different initial ignition advance characteristics. Manifold vacuum continuously varies as the throttle is being opened. Only when the throttle is wide open is the vacuum at a minimum, but even then there is still some present because of restrictions in the throat of carburettor and air cleaner housing. It depends on the specification of the vacuum advance control capsule as to when vacuum advance ceases to be applied and can be as low as 3” Hg or as high as 10” Hg, depending on which vacuum advance mechanism it uses. Manifold vacuum distributors have maximum vacuum at idle, and hence have maximum advance at idle because this allows a smaller throttle opening and hence lower emissions for the same idle speed, at the expense of ease of starting and initial throttle response. Carburettor or ported vacuum distributors have no vacuum at idle and hence no advance at idle. However, as the throttle opens the vacuum rapidly increases to become the same as that produced by the gradual fall in vacuum in manifold vacuum distributors. Thereafter they are the same. The pre-1971 North American Market 1˝” SU HS4 system uses vacuum produced when the throttle opens to advance the ignition timing, resulting in easier starting and quicker off-throttle response. The North American Market 1˝” SU HIF4 system uses manifold vacuum to advance the ignition timing while the throttle is closed, resulting in harder starting and slower off-throttle response, but lower exhaust emissions and better fuel economy while idling. The hard starting problem of this system can be easily overcome by simply opening the throttle all the way while cranking the engine. Once the throttle opens, the vacuum is the same on both types. If you prefer to use a set of 1˝” SU HIF4 carburettors while retaining the advantage of the superior off-throttle response of the ported 1˝” SU HS4 ignition advance system, the UK/European market versions used the ported vacuum of the 1˝” SU HS4 and can be ordered from Burlen Fuel Systems. Of course, your distributor’s vacuum advance capsule will have to be compatible with whichever version of the vacuum system you choose to employ.
Steve S.

Currently with the HS4s, I do have the vacuum take off from the rear carb. However, since this car has been put together from parts over the years, I need to check my distributor and vacuum canister to see if they are correct for the engine and vacuum port location. I've found lots of mistakes already, so there is no reason I should expect that to be right. The engine is a GK, but I don't know the year. I think they were only used for a couple years.

When I get the time to deal with it I will probably look around for a ream to do the throttle shafts on the HS4s. They are a little less mysterious to me, the cost to repair the HSs will probably be about the same as rebuilding the HIFs, and it sounds like the pros and cons are about a push.

Thanks, Charley
C R Huff

The distributors and engines changed many times over the years for North America to cope with the ever-tightening emissions requirements. The UK kept the same engine and distributor for the whole of rubber bumper production. The position of the vacuum source i.e. carb or manifold is irrelevant as far as choice as distributor goes, and you certainly don't need to consider changing the distributor or vacuum capsule if changing the vacuum source. The only difference between carb and manifold is at idle, where manifold gives near maximum vacuum and carb zero. This advance at idle makes for more efficient combustion so the idle screw can be turned out a bit so using less fuel and meking less emissions for the same idle speed. As mentioned elsewhere the irony is that during 1977 the vacuum was controlled electrically so that was only applied to the distributor in 4th gear, effectively putting it back to being carb vacuum again at idle in neutral.

The 18GK was used in 1971 only, with a 25D4 41339 distributor. This had a curve giving 10 degrees *additional* (to static) advance at 1000 rpm, 24 at 2800 and 30 at 4600. The vacuum vacuum capsule started addinng advance at 7 in.Hg, and gave a maximum of 10 degrees at 13 in.Hg. But that is all pretty irrelevant anyway as today's fuels are very different from the original and hence the requiremen ts are different. Not forgetting that from 1967 North America increasingly had curves that were more about emissions than performance. If you don't have to meet the emissions targets anymore you can almost certainly do better with different curves, but you will only be able to determine what they are on a rolling road.
Paul Hunt 2

Paul,

Hmm, kind of surprising that the GK was only built for one year, and I ended up with one in my 68GT and another in my early 74GT. Maybe 71 was a popular year in the states, thus making them plentiful.

I just went to look at the dist and vac in the 68 (the 74 has been dead since many years before I even owned it) and it does have 25D 41339 distributor. It doesn't have the 4 after the D, but I guess that just means 4-cylinder?

However, the vacuum can is a 7 - 13 - 5. So, I guess I'm being cheated out of 5 degrees of vacuum advance. I'll have to rummage through some of my boxes and see if I can come up with anything else. Am I correct that the loss of that 5 degrees would result in worse fuel mileage?

I did read about your check using an in cockpit switch over for the vacuum sources, so I'm not too concerned with that. I have many more serious deficiencies to worry about.

Charley
C R Huff

I should have said that what is stamped into the vacuum can is distributor degrees, which must be doubled to get crankshaft degrees. I quoted crankshaft degrees because that is what you see with a timing light.

According to various manuals the USA had 18GF, GH or GJ from 67 to 70, 18GK for the 71 model year, then the 18V was used from the start of the 72 model year. According to Clausager 71 was only 7th in numbers of chrome bumper roadsters produced, 3rd in GTs but much smaller numbers.
Paul Hunt 2

Thanks,

Since the vac can looks like it was replaced recently, I'm glad to hear that it was at least correct for the engine in stock trim at the time of produciton.

Charley
C R Huff

This thread was discussed between 24/03/2008 and 01/04/2008

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