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MG MGB Technical - SU carburetors on a '76

I have a set of SU HS-4 (AUD465) carbs. What do I need to do to retrofit these onto a 76? Is this a worth while conversion? I have a complete set of parts.
D Ferris

The SUs were always better than the Zenith, the UK always had them.
Paul Hunt

D-
The SU HS4 AUD 465 carburetor was used on the 1971 model MGBs with the 18GK engines. You'll need the appropriate intake and exhaust manifolds. The SU HS4 compatible intake manifold has no provision for distributor vacuum takeoff and a mounting flange thickness of 9/16". The HS4 compatible exhaust manifolds have a mounting flange thickness of 9/16" and can be readily identified by an external casting number of 12H 709. Be aware that the advance mechanism of the distributor used with the SU HS4 carburetor takes its vacuum from a connection on the carburetor, while the advance mechanism of the distributor used with the SU HIF4 carburetor takes its vacuum from the intake manifold. These two systems result in highly different ignition advance characteristics. The 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 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 harder 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. The appropriate distributor is the Lucas 25D4 41339 distributor. To test the centrifugal advance mechanism, disconnect the vacuum tube. You should have the following readings:

Centrifugal advance:

10 degrees @ 1,000 RPM
24 degrees @ 2,800 RPM
30 degrees @ 4,600 RPM
Steve S.

D-
Sorry, I neglected to answer your other question. Yes, it's a very worthwhile change, better in every way.
Steve S.

It's a very worthwhile conversion as Steve mentioned, he also covered what you will need to make the conversion. If you are subject to visual emiissions checks and/or actual emissions testing you may wish to check California emission requirements before doing a conversion that may not pass emission standards.
FWIW, Clifton
Clifton Gordon

If you live in an area where you have to do the biennial smog inspection (most of the state), you won't pass with the SU carbs; there are too many bits of visual inspection called out in the test which won't match up to your engine.

You would need either an old-style exhaust manifold or headers, as the stock 76 had this flow-restricting combined intake/exhaust manifold. You would also need to mount a choke cable. If you switch, label and keep all the old parts in case you have to convert it back (for sale or test).
Tom G

D, I just did this conversion myself, but it was to a 74 that already had the HIF carbs whereas yours should be the single Zenith. It's an excellent change, huge improvement over the HIF's!
As Steve says, you need the correct vacuum port. Mine has the original distributor so I'm still using the vacuum off the manifold, and it's exactly as Steve describes--hard starting. The AUD 465's are new from Moss and have the vacuum port on the rear carb. Check yours to see if there is a vacuum port sticking straight up where the carb bolts to the intake manifold.
I didn't have any problems with the thicknesses or mountings. I used the same triangle shaped bracket to hold the throttle cable and choke cable (the choke comes up from the bottom and the housing actually moves the choke!)
I only needed 2 linkage parts. The choke piece was a straight rod that could be cut from any rod of the correct diameter. You'll need the pinch clamps for each end and the center one for the choke cable. On the throttle linkage, you'll need the old style rod that has an arm that butts up against the bottom of the choke cable.
The butterflies open in opposite directions between the HIF and HS4 carbs. On the HS4, the cable pulls up from the side towards the intake. On the HIF the cable pulls up from the side away from the intake. If you don't have a throttle/choke bracket, you will need to find one from a salvaged engine or you can fabricate a copy of the HIF from 1/4 in. steel, or find a metal shop that can make you one.
I noticed a definite improvement with the change to the HS4 carbs. It was one of the best investments I've ever made on the car. And that was over the HIF carbs--imagine what it will do to improve over the Zenith!
Merry Christmas,
Ken T
Ken Thompson

Ken-
There's a simple trick to making a manifold-vacuum MGB easier to start: Simply depress the accelerator pedal to open the throttles. The vacuum advance mechanism will have no vacuum, and the ignition timing will be more appropriate. Much easier. As soon as the engine fires, close the throttles and mind the choke!
Steve S.

Thanks for all the input. I have a better picture of what I’m in for doing this conversion. I have couple of questions concerning distributors. Steve states the correct model I need to use is Lucas 25D4 41339. The PO has installed 45DE4 electronic version. Actually it’s been rather reliable for the past 4 years. Can I continue to use the 45DE4 or do I need to go to a 25D4? Depending on the answer, then I’ll have to use the correct coil. From what I’ve read, how do I wire around the resister without making a “hack-job”? the car has a new wiring harness. I’ll still need to convert back for SMOG check. Also does anyone have close up pics of the HS4s installed or a diagram? This will give me a reference to work from.

Thanks..Danny

Danny Ferris

The trick about starting manifold vacuum cars with WOT is exactly why vintage/veteran cars have hand advance and retard controls on the column - retard to start advance to run - and why distributor vacuum capsules were originally connected to the carb such that there is no vacuum and hence no advance during starting.
Paul Hunt

danny

the 25D4 is no longer available and the standard replacement for it is a 45D4 - stick with the one you've got

Chris
chris

Steve, I'm too used to EFI cars where you don't touch the gas! this morning it was about 15 degrees in the garage, but I gassed it a lot and it started after some cranking. But you're saying to leave the choke in until the engine starts. I'll try it that way on Thursday.
There is a slight hesitation when I give it gas, even when it's warmed up, that I attribute to the vacuum advance. Out of curiosity I might hook the vacuum line to that port off the rear carb to see if there is any improvement.
The 1933 Alvis setting in The Motorway (my favorite hangout!) www.motorwayltd.com has exactly the timing and advance levers on the steering wheel that Paul is describing. I was lucky enough to be there when they started it up to drive it onto a trailer to take to the Conclave in Denver. If you know how to work those levers, it will start in just a few cranks. But if you don't know how to work those levers, you'er SOL.
Merry Christmas!
Ken T
Ken Thompson

Ken-
Yep, at F15 degrees starting with SU carburetors and manifold advance sure can get to be an art form! Remember that the SU carburetor has no accelerator pump, so repeatedly pumping the accelerator pedal accomplishes nothing. No real "choke", either. Instead, pulling out what we here in the states elect to call "the choke knob" on the dashboard richens the mixture. Merely hold the throttle wide open, hit the starter, and pull out the choke knob until the engine fires, then immediately close the throttles and adjust the mixture with the choke knob. You know the eccentricities of your engine best, but I choose to push the knob in oh-so-s-l-o-w-l-y as it warms up rather than waiting until the engine starts to stumble. Familiarity with the personality of the particular engine helps. Slightly lean seems better than slightly rich, making for a little faster warm-up. In your case I would recommend folding a wool blanket and putting it on the seat so you won't have to wait until the car warms up to be able to get out of the car. If you're experiencing hesitation when opening the throttle after the engine is warmed up, check to be sure that the oil in the dashpots is at the correct level.
Steve S.

I just did this conversion on a 76 this fall and it was worth every cent. I put some pictures up the big thing is to get the conical K&N's to make sure you clear that master cylinder.

Josh
www.pointerfamily.com/family/mg.htm
Josh Pointer

Steve, that's interesting and I'll give it a try. F15 degrees here with our 20 or 30 percent humidity is easier to take than the high humidity you have in Virginia. Last sunday my son and I were out working in the garage and I wore a flannel insulated shirt-jacket. Later that evening on the weather they said the high was only 39 degrees--it felt fine to me!
The altitude changes things obviously with most US brand carbs, but doesn't have that much effect on SU's. Some people get different metering needles but for most MG's just tune it according to the lift pins and it works fine. It's amazing how something as simple as SU carbs can be so versatile. A few times I've driven over Trail Ridge Road in Rocky Mtn Nat'l Park and noticed poor running at about 11,000 ft. I'm anxious to try turning the jets back 1 flat each and see what happens.
D--good luck on that conversion. You'll like it.
Ken T
Ken Thompson

Gentlemen,
I am working on the ZS to SU conversion on my '77 roadster as we speak and this thread has been most helpful (as have all the past information) I would like to add a couple of bits for those, like me who don't normally mess with cars but love their MGs and want to do most stuff themselves. I get my parts from Matthews Foreign car parts in Birmingham AL and I was talking with John, their "B" mechanic and he steered my to an excellect couple of resources. First is: SU Carbutetters Tuning Tips & Techniques from Brooklands Books. Second and by far the most help is the video: The Overhaul, Tuning & Maintenance of SU Carburettor (H, HS, HD, HIF) Moss Number 211-035.
Even with these resources however, I still have a couple of questions. My first has been answered above referencing the distributor, thanks guys.
Next question, I will be using the K&N Cone filters but I cannot find the exact part number for the ones that fit the HIF4. K&N want me to measure the carb and give them the measurements and they will give me a part number. Fine but they are all on vacation and the factory is closed. (I'm an impatient cuss) Does anyone know the part number the the complete air filter, housing and mountinq kit?
Next on the same lines, When I put these filters on I have read much about the carb needles. The ones in my rebuild kit are marked ABD, which equates to P/N: CUD1027. I do not know the P/N of my carbs, the ID tags are missing but they are HIF4s which do not have the valve in the throttle plate.
Finally, I have read about rebuild kits that have a black jet and a white jet. I have one kit with a black jet and one with a red jet. Should I assume that red one is the same as a white one? And is that for the front or rear carb?
Sorry to be so long winded and thaks for all your help.

Happy New Year,

Bob
Bob Anderson

Bob, in the HIF's each jet is 'handed' and will only fit one way. I've never heard of having different jets for altitude, etc. Usually it's the metering needle that is different. In my new AUD465's, I'm using the standard needle at 4500 to 5200 ft above sea level and they work fine for me. I drove about 700 or 800 miles on the new carbs before my alternator quit last week (and my parts supplier is closed for the holidays too!) and the plugs were whitish tan, maybe a bit lean.
Your kits probably come with the standard metering needles. Here is a web site that will have the part numbers for the needles: www.burlen.co.uk
Some of the parts stores like NAPA, Advanced Auto Parts, Autozone or Checker (Kragen) have K&N catalogs. You might check to see if they list the stock filters and the conical kind.
Those HIF's were used mostly from 72 to 74 with a few 74 1/2 models using them. The stock air cleaners are the same as other MGB's but the carbs are threaded so the bolts screw directly into them, whereas the earlier models had longer bolts that went into a u-shaped bracket for the choke, etc.
Hope that helps!
Ken T
Ken Thompson

It's a good start. Thanks Ken.
Bob Anderson

Nobody has mentioned one of the best reasons for putting on twin SU's.
They look way cool
You can even chrome or polish them
Peter

This thread was discussed between 21/12/2002 and 31/12/2002

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