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MG MGB Technical - HIF vs HS Carbs

Thanks everyone who responded to my inquiry on exhaust manifolds. Now for a real debate, and having the choice of both HIF and HS carbs, what are the pros and cons of each and which one overall would be the best set up for converting a stock 77 B. My experience is primarily on H (MGA) HD (AH 3000) type carbs.
Scott Shirk

Well, if your experience is with the H and HD's, then you'll pretty much feel right at home with the HS4. The HIF's aren't that hard to set or adjust either so really it's just which ever you want. You'll hear arguments for or against both so use your best judgement.

Having used both, I prefer the HS4's.

Robert

Robert Rushing

I agree. I've owned and lived with cars with both HS and HIF SU carburettors over the years. I prefer the HS units, mainly because the seperate float chamber means that access to the float and needle valve is possible without removing the things. In operation, I can't say that I've noticed any real difference between the types, so it's a convenience call.
Having said that, a '77 car should have HIFs. Mine has. You may find that getting needles and jets to suit the car is easier with the HIFs owing to this.
Tim Cuthill

Scott, I have a 74B with HIF's and a 68 with HS carbs, as mentioned the HS carbs have easier float bowl access, but the choke action is very stiff with the HS carbs compared to the HIF. If I were making a decision on which to use, I would select the set that is in the best condition. Check and see which set has the least throttle free play, if in good condition you should feel little or no movement between the shaft and the carb bushings. Run a drop test for the suction chamber and piston on each carb as outlined in the Bentley book and shop manual. Going through them and selecting the best set would determine which set I would use. Good luck, Clifton
Clifton Gordon

Condition being equal, I'd take the HIF's. I, too, have taken advantage of the easy access floats on the HS on a couple occaisions and so appreciate that feature. A stuck float on an HIF does generally mean removing the carb, but that is such an easy job anyway it isn't so bad (if the carb is in good repair in the first place, it should be a rare occurance, anyway). The integral float is supposed to be superior in high g's conditions (I have not actually encountered this, though). Tim, I think Scott's car would have originally had only one carb (not an HIF?) per US spec - awful I know. Like Clifton, if I had one set in better condition, that is the set I would use. Dean
Dean Lake

I run HS4's on my '67 , Eddie runs HIF's on his POS '68 , both cars run extremely well .
I do the high g force thing all the time and never have any problems .

I prefer the HS4's my self , easier to service IMO .

-Nate
Nate

The HIF is better engineered - with shaft seals and thermostatic mixture control ( also available via waxstats on HS carbs. The choke is also a separate fuel supply rather than the crude pulling down of the jets on the HS.

IMHO the HIF carb wins hands down - but there is nothing to choose between them on performance - especially if you remove the throttle plate poppet valve on the HIF ( but late HSs had them too)
Chris Betson

Extra bits on the HIF to go wrong, additional ports and drillings to get blocked, hidden choke mechanism, poorer access to the float valve, no performance gain ...

HS, no contest.
Paul Hunt

I have a 76 that I just fitted with HS's and I havne't been happier from all angles. Working on them is easy as it can be and with the conical K&N's everything is free and clear from a setup perspective. I have posted some pictures of the before and after on this at http://www.pointerfamily.com/family/mg.htm

Josh
Josh Pointer

The one item that no one has yet mentioned concerns where you will be pulling your vacuum advance from. The HS4's allow you to use ported vacuum from the back of the rear carb, however the North American HIF4's don't have this adapter and use the vacuum from the intake manifold. If you are retaining the stock distributor from a 77 that expects manifold vacuum, then you can use either one, but you'll have to plug the adapter on the back of the HS4. If you've upgraded to a performance distributor that expects ported vacuum, then you'll want the HS4's or you'll have to modify the HIF4's to give you the ported vacuum. Optionally you can purchase new European HIF4's that have the adaptor for ported vacuum.
Personally I like the HS4's, having converted from HIF4's. Either way, the modification should put a smile on your face.
SteveO

I agree with Paul and Josh that HS's are better. My 74 came with HIF's and they never worked right as far as I'm concerned. There's no way to really test the bimettallic piece, extra circuits to richen or lean the mixture, poppet valves in the butterflies--all that means more parts to break.
My 74 had a unique problem that caused an erratic idle--the place where the needles go into the base of the piston was not cast correctly at the factory and there was 3/16 in. difference in the way the needle set into the jet at the bridge--from carb to carb. The only way to correct the problem would be to use a press to push the pipe that holds the damper through to the correct position. And if the piston breaks then I'll have to buy new carbs anyway. So I've decided to get a new pair of 70 to 71 HS's which should have the correct vacuum lines, etc., but no extra circuits or bimetallic pieces.
If you're buying new carbs, the HIF's are about $700 in Moss and the HS's are only about $400. If you're buying used, be sure to check the throttle shaft wear, etc.
My two cents.
Ken T
Ken Thompson

Scott-
Down through the years there's been a great deal of discussion on this BBS concerning the relative merits and vices of the SU HS4 carburetor and those of its successor, the SU HIF4 carburetor. Advocates of the HS4 point out the greater ease with which the jet can be changed with the carburetor in place on the engine and the metering advantage of its concentrically mounted needle and jet. Some feel that its remote float bowl design gives it a "Vintage" appearance. However, the HS4 is not without its vices. It requires the removal of its aircleaner boxes to enable the use of a special short wrench to effect mixture adjustment, which results in a richer mixture when the aircleaner boxes are refitted. It also has a tendency to leak gasoline from its floatbowl junction. In addition, it 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 HIF4 largely addressed these problems by having its float bowl integral with its body, thus allowing the float to surround the jet and hence more consistently meter fuel under high angles of tilt and under conditions of heavy cornering stresses. It also has superior performance potential due to its higher maximum flow rate, a factor that should be considered if you intend to modify the engine for increased power output. Replacing its piston and its biased needle with the earlier piston and concentric needle from the HS4 (a simple "drop-in" parts swap) improves its long-term performance further, as will removing the poppet valve from the throttle disk and brazing its mounting hole shut. During routine adjustment its mixture can be modified from above with nothing more than a simple screwdriver, hence removal of the aircleaner boxes is not necessary. Its thermosensitive mixture control makes for easier cold weather starting. Those who have converted their cars from the HS4 to the HIF4 usually report a 1-2 mpg increase in fuel economy. Unfortunately, it must be removed from the intake manifold to change the jet and its thermosensitive mixture adjustment control can cause it to run lean if underhood temperatures rise badly in heavy traffic on very hot summer days. Consequently, Jet-Hot coating of the exhaust manifold is a worthwhile investment. If you're thinking of purchasing a set of HS4s because they cost less, think again. It can be done, but it's not an easy bolt-on swap. You'll need an HS4 intake manifold, heatshield, distributor, cables, plus the linkages and a lot of other little bits and pieces that aren't commercially available anymore, so you'll spend a lot of time scrounging around trying to get them. If it's the lower price of the 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 won't be anything like what you hoped it would be. Whichever version of the SU carburetor you choose, you will find it helpful to obtain copies of the "SU Reference Catalogue" and "The SU Workshop Manual" from Burlen Fuel Systems. It's far cheaper to buy them direct from them. They have a website at http://www.burlen.co.uk/
Steve S.

Hiya

HIF4 carbs show 3-4 bhp more at the wheels on a 'Race Legal' Standard Race MGB engine.

The HIF4 carb flows more than the HS4..........both carbs flow around 10% more if the throttle spindle is 'tweaked'.........fuelling is a lot less consistent with HS carbs........engines try to stall when reversing down the ramp from my unit.......not with HIFs.

Hill Trialling with HS carbs is pretty bad compared with HIF carbs.

Peter
Peter Burgess

Clifton, re the stiff choke action, try depressing the throttle pedal before engaging the choke (HIF and HS4). By lifting the fast idle screw off the cam, much of the drag felt at the cable is eliminated. If the choke levers and jets are properly aligned, the HS4 should have a fairly smooth action, just a little stiffer than the HIF.
SteveO - Several years ago, Paul Hunt sent me the measurements necessary to drill thru the HIF vacuum port plug located on the rear carb. I anchored an intake manifold to the bed of my drill press, bored a small pilot hole then followed with a bit that matched the port hole in an HS and countersunk a hole to accomodate a press-in vacuum standpipe. I tried about a dozen dizzys from different years / makes and models, and orig from my 63 Spridget (10* advance stop) showed a marked improvement in power using the G-Tek accelerometer. Nearly 3.5 hp, as extrapolated, which is a lot of found power. When I tried a pair of fresh Apple HS4's on the same engine, I lost 2 of the previously "found" hp without messing with either dizzy or mixture as the carbs were borrowed from a freshly tuned 64; power wise, it appears to be a near-toss up, since I could have just as easily netted a loss as a gain by tweaking the carbs and timing.
mark c

Hi Mark, Thanks for the choke tip. Regards, Clifton
CLifton Gordon

Mark C., Where could I find those measurments and any other information on drilling a vacuum source on my HIF carbs? Thanks for your help.
Ernie C.

If you've got a carb port, on either HS or HIF, then use it in preference to the manifold port. Moving it was yet another USA emissions thing which IME causes the vac capsule - which is no cheapie to replace - to fail sooner.
Paul Hunt

Steve S., a couple of counterpoints. If you use K&N filters and either 'Bob" plates or the K&N-type plate with HS4s you can very easily access the jet adjusting nuts with the air cleaners in place.
You mention all the bits needed for setting up HS4s, and note that the savings won't be that great. I've found that I could buy a brand new set of HS4s ($370), a complete HS4 setup with manifold, linkages and heatshield ($100) plus a new pair of K&Ns ($70) for a total of $540. The new pair of HIFs are $700. Then, I guess you still need the used set of HSs for the pistons, plus new butterflies, plus the aggravation of tearing into brand new carbs. Then the Jet-Hot coating on the exhaust......Yep, I'll go with HS4s every time!!
Just my opinion, of course!!
Ken Lessig

Ken-
I'm amazed at your parts prices. I just went to the Advanced Performance Technologies website and checked the prices on K&N airfilters. The cheapest set for a set of SU HS4 carburetors (6" X 1.75") sell for $61.57 each. These airfilters are too shallow to avaoid sonic effects. The 6" X 3.25" are the ones to use, and they cost $63.86 each. You got pair of K&Ns for $70!?! Then you got an intake manifold, heatshield, and linkages for $100?!? Wow! A set of new SU HIF4 carburetors ordered direct from the manufacturer Burlen Fuel Systems, do not cost $700. Maybe that's what Moss Motors wants for them, but anybody who'd pay that price when he could cut out the money-grubbing middlemen deserves to get stuck for $700.

I concede that my priorities are based on a performance-oriented criteria. If somebody out there is rebuilding his engine to bone-stock specifications, has a "that's good enough" mentality, and is willing to live with all of the flaws of the antiquated design of the SU HS4, that's his choice, and he can live with it. However, if his MGB is one of the great loves of his life, putting a set of HS4s on the engine is like buying your wife a ring made of cubic zircon instead of a diamond ring. Are you a "that's good enough" guy, or are you in love? I buy my wife diamonds and I buy HIF4s for my MG because I'm in love with them both. Besides, if I did it for one and not the other, then the other would develop running problems out of jealousy. Face it, when you're in love, cost is irrelevant. It's the long-term performance that comes after the purchase that's the biggest reward.
Steve S.

Thanks everyone for their thoughts on the carb issue. Fortunatly, cost is really not much of a factor since I have complete set ups for either carb and rebuild kits are fairly close in cost. My only problem if I go with the HIF's will be to find a different exhaust manifold or make some offset washers. I made some washers in the past when I installed a header system on one of my A's a few years back.
Scott Shirk

Scott, As I understand you have intake manifolds for the HS and HIF carbs. I just looked again at the HS and HIF intake manifolds side by side, they both have holes drilled and threaded in the same location, they have plugs/vacuum takeoffs as needed for the specific application. So if you wish to use the HIF carbs and only have an HS type exhaust manifold you can use the HS intake manifold, bolt on the HIF carbs, remove the throttle/choke plate from the HIF intake and transfer it to the HS intake, remove the vacuum takeoff from the from the HIF manifold and install it in the HS manifold if needed. No shims needed and you don't have to chase more parts. You will need to install the brake servo takeoff in the manifold you use. The carbs will be 1/8" closer to the brake components with the HS manifolds. Hope this helps. Regards, Clifton
Clifton Gordon

Steve, maybe you should check somewhere other than APT for your K&N's. Vicky Brit sells the elements for $34.95 each, P/N 17-760. Use your original filter bases, spacer tubes and bolts, with your choice of top plates. You can buy HS4 setups complete with manifold, linkage and heat shield off the classified section of this site for about $100 all day long. Throw away the worn out carbs and you have all the little bits and pieces that aren't commercially available. If Burlen sells HIFs for considerably less than Moss or VB, then they probably sell HSs for considerably less as well.
Price aside, you still have to rip into your brand new HIFs to get rid of the airflow restiction of the poppet valves, you still have to deal with that silly temperature compensator, and you still have to pull the carbs off to clean out the float bowls.
Just because the HIFs are a newer design doesn't make them better. The single ZS on the all in one manifold is a newer design as well.
Ken Lessig

The throttle discs without the poppet valve are available from Joe Curto at www.joecurtoinc.com or on 718-762-7878. He is knowledgable and stocks a wide variety of genuine S.U. and Zenith spares. Just a satisfied customer, Dean.
Dean Lake

This thread was discussed between 07/11/2002 and 11/11/2002

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