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MGB SU Carb Conversions

About Us  
I am an Englishman who has lived in the US since 1984. I have been an avid British car fan longer than I care to remember. My first car was a 1947 Austin 10 which I bought for 5 British Pounds (about $10) which at that time was about two weeks wages. Every penny I earned seemed to go into cars, or parts, or both; much to the chagrin of my parents, and girlfriends.........

I rebuilt my first SU at the tender age of 16 (off my 1955 Morris Minor - very similar to the one shown here) and it's something I have enjoyed doing ever since. I was always the one that had a bench covered in other peoples carbs. When I retired from a 40+ year career in manufacturing, both here in the US, and in Europe, I turned my hobby of a lifetime into a business.  

I will make you the firm promise that you will be totally satisfied with my work, and my service. 
If you have any questions about our services, or you simply have a question about your carburetors, please don't hesitate to call, text, or e-mail me; I will help in any way I can.

e-mail me at barry@britishclassicmotors.com
Call or text me at (336) 587-9623

Frequently Asked Questions

A selection of questions from visitors to the site:

Q -
I had my MGA H4 carbs rebuilt recently (by another rebuilder I need to add) and now they are both leaking fuel from the jet. He has told me to be patient as this is common after they have been rebuilt as the cork needs to absorb fuel to swell, and seal around the jet. Have I been taken for a ride?

A -
Not necessarily, but the jets should always be assembled using cork glands that have been pre-soaked in oil to prevent this from happening. However, another common cause of this can be that the choke linkage between the two carbs is too tight, and therefore pulling the two jets towards the center which can cause them to leak. This is also an issue on the Triumph TR's. Make sure there is some "slack" in the linkage; this may well solve the problem.

Q -
My MGB has the HIF carbs, and even though I have set the mixtures exactly per your tuning guide (which I downloaded - thanks!) it still seems to be  running rich when I'm on the road. What have I missed?

A - If you are sure the mixtures are correct, then I suspect the choke O-ring. This is best checked with the carbs off the car. Undo the hexagon nut, Remove the radial spring from the choke assembly, remove the cam, cap, and seal, remove the two small screws, pull off the plate, and pull out the brass choke assembly (don't lose either the seal, of the thin paper gasket). You'll see the O-ring on the outer assembly; if it's dry, cracked, or otherwise in bad shape, replace it. You can take the brass part to the local hardware store, and find a suitable replacement. Make sure it all goes back the way it came out, with the semi-circular notch in the brass flange at the top. Always check both carbs.

Q -
I recently fitted an aftermarket fuel pump (brand name deleted - ed.) and now both carbs on my 1967 MGB are leaking fuel from the overflows. Is the pump faulty, or is it the carbs?

A - Chances are, the fuel pressure is too high. SU's should see no more than 1.5 to 2.0 psi. New SU fuel pumps are calibrated at the factory to 1.6 psi. Too high a pressure can overcome the upward thrust of the float, and cause the carbs to flood. You need to install a fuel pressure regulator, and set it at about 1.5psi, or preferably, replace it with an SU pump.

Q -
I have a 1961 MGA, and cannot prevent it from running super rich. I have tried everything I know, but no matter how much I raise the jet, it's still too rich. What am I missing?

A -
It is probably the upper cork gland in the jet assembly. In the H-Series carbs, there are two cork glands in the jet assembly; the lower one prevents fuel leakage, while the upper one prevents excessive fuel being drawn up around the outside of the jet, which creates the condition you are experiencing. Renew the cork glands, and you should be fine. Remember, to soak the cork washers in oil for 24 hours before you install them, and you will need to centre the jets again.

Q -
I have a 1960 Austin Healey 3000 with dual SU carbs.  I have installed a Pertronix ignition, adjusted the valves and rebuilt the carbs. Whenever the engine heats up it goes rich, floods out, and back fires. I had it to a licensed mechanic who has "some knowledge of SU's with an MGA"  He tried everything he knows and asked the question: have the needles been fooled with?

A - The problem you are experiencing is not a needle issue, and is due to either a sticking float (needle) valve, or incorrect float height, or a leaking float; all of which are easy to remedy. Remove the float lid by unscrewing the hexagon bolt in the center. Test each needle valve by gently blowing into the brass inlet tube, and gently lifting up the float lever; at which point the valve should close. If they don’t shut off, they, or it, needs to be replaced.

At this point, remove the float, and shake it; if you can hearing fuel sloshing around inside, replace it.

To check the float height; invert the lid, and insert a bolt (or similar round object) 7/16” in diameter under the forked lever. The lever should just touch the bolt; if it doesn’t, carefully bend the lever until it is at the correct height. Then re-assemble, and test.

Q - I have tried everything I know, but I am not able to get the idle down below 1,000rpm on my 1972 MGB. What am I doing wrong?

A - If you are sure you do not have any vacuum leaks, check to see if your carbs have a poppet valve on the butterfly. With the carbs the age they are now, it's quite common for these valves to stick open, thereby allowing more air to enter the carb. Although it is possible to solder them shut (if you are lucky) it is easier, and wiser, to replace them with the older solid butterflies.

Q - I am original owner of 73 MGB GT with a recently restored interior, and exterior. I need to replace carburetors and don't have experience with this. I assume based on various literature that I have HIF-4 carbs; but not sure how to tell for sure.  I have a mechanic who has worked on MGB's. Have heard about Weber and Stromberg products; not sure if it is wise to attempt to keep the original fuel control system (i.e try to find original carbs) or "convert" to Weber, or some other option. Do you have an opinion?

A - Your car does have the HIF4 carbs, and I always recommend staying with the original carbs; there is no better carb for our cars, for street use, than an SU. I know many people who have tried a Weber, but simply didn’t like the flat spots they experienced. I am frequently asked if I will take a downdraft Weber in part-exchange for a set of SU's. A Weber is ideal for the track, where the car is operating in a very narrow power band, but not for the street, where you need a very wide power band.

Q - I have a 1973 MG Midget, and whenever I floor the gas pedal, the car hesitates. It is becoming very annoying!

A - If you are sure you do not have a weak mixture, check that you have the correct grade of oil in the suction chamber. The ideal oil (regardless of what you may have read or been told) is SAE 20. Any thinner and the mixture will be too weak when you accelerate, and if it's too thick, it will be too rich. If you are not sure what oil you have in there now, start with SAE 20 (5w20 will do if you really can't find any) and see if that makes a difference. If it still hesitates when you accelerate, go one grade heavier until you resolve it.

Q - I have a 1967 Austin Healey 3000 BJ8 series 2.  When I turn the key on gas leaks from the back carburetor thru what appears to be a vent.  Is there an easy fix for this?  To my knowledge the carbs have never been rebuilt.  Any suggestions? 

A - Almost certainly the needle valve is leaking. When you say "vent" do you mean the overflow pipe from the float chamber? If so it is an easy fix. Simply remove the top center bolt, lift off the float chamber lid, pull out the small pin holding the float lever on, unscrew the brass needle valve, and replace it. Then re-assemble in the reverse order. I would seriously suggest that you do this for both carburetors; if one has gone the second one will not be far behind.

Q - I've just been through your DIY check list but it didn't seem to cover my problem, see what you think. HS4 Carb on a mini is recently bought. Won't start when cold, electrics ok, engine fairly new (1275cc), have tuned with a Colortune. When you rev it on the throttle mixture will enrichen but the tickover mixture is too weak and I've turned the mixture nut down 10 flats with no difference at all. So it wont start coz the mixture is too weak, runs fine when you get it going. Lift the piston and it cuts out.

I've cleaned the float chamber and flushed the jet through from the needle end and I'm confused?????? Would understand if it was worn and too rich.

A - The clue is that when you lift the piston it cuts out. It is running way too weak, and I suspect a massive vacuum leak. Such a leak will really only manifest itself at idle speeds, and startup. Double check every vacuum hose (particularly the brake booster line), and while the engine is running (as slow as possible) spray carb cleaner around the inlet manifold and carb; when the engine speeds up, you have found a leak, but don’t stop at the first one as there may be more.  Also, check that the needle is in the right position in the piston, and that the sintered collar is flush with the base of the piston.

Q - I cannot get the tick over below 1100 rpm (Sprite mk1, twin 1 1/8" SU ) and the front carb is hissing more than the back carb,I have re-bushed the throttle spindle housings and fitted new spindles, but I have not replaced the butterflies. With both butterflies closed off, if I shine a torch from the other side I can see light around the peripheral of the butterfly discs? Is It essential that the disc is a tight/close fit in the housing? If it is then I guess mine are worn and I need to order 2 new butterfly disc's?

A - Almost certainly the problem you have is that the one butterfly disc is not centralized in the bore. When you first replace the butterfly on the spindle (making sure it is the right way round - check that the angled circumference is facing the right way), it’s important to close it tightly in the bore, and check at that time to make sure it is sealing all the way round, before finally tightening the two screws. Rarely do the butterfly’s wear, and this will almost certainly fix the problem.

Q - I had the carbs on my 1960 TR3 rebuilt recently by a friend. He appears to have done a good job, but now I cannot get the car to idle; it races at about 2000 rpm, and nothing I can adjust makes any difference!

A - If you are sure that you do have a vacuum leak somewhere, you need to remove the carbs again and check the throttle disc. I suspect they have been installed the wrong way. The discs are tapered on the circumference to ensure the seal in the bore when the throttle is closed. If they are put in backwards they cannot seal and will allow large amounts of air to enter; hence the racing engine. Make sure that when you hold them up to a bright light, you can see little, if any, daylight around the outside of the disc.

(This section will be updated on a regular basis)                                                                                                                                                     

British Classic Motors - More than 40 years of SU Carburetor Restoration and Rebuilding experience..............

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