Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - 1974 BGT Carbs

Hi All:

I am new to newer MGs as I have always had T-series and do not know how the carbs on my “new” 1974 MGBGT work. I have looked at the archives and can not find a problem similar to what I am experiencing.

Above 55 MPH the car surges slightly. Pulling the choke on by about 3/4 of an inch eliminates the problem. Any suggestions of how to fix this?

Yours in Safety Fast
John Crawley

Does the problem occur at a lower speed when in a higher gear? It "sounds" like fuel starvation, pulling the choke out drops the jet nearer to the fuel level and exposes a richer section of the needle.But do the basics first. Float chamber levels, needle size correst and fitted correctly. Damper spring weight correct (easily assessed even if the colour code has disappeared). Do the pistons rise and fall at the same rate or stick, when lifting with a finger and allowing to drop, damper out? Oil in the dashpots? Is the fuel pump delivering enough. Tank cap venting?
Allan Reeling

Thanks for your logical approach. I looked at the carbs did not recognize anything and went aaaaaa!!! And then forgot all I that I Knew about SUs. You are correct . . . back to basics. It has been raining here for the past few days so will try to get at it tomorrow. And no, the only time it surges is in 4th or 4th overdrive at speed.

John Crawley

I believe that a 74 MGB has HIF carbs which have a rotary choke valve feature rather than dropping the the jets. This site has some good photos whilst the vidoe from John Twist gives a good description of how to tune

Good luck!

BJ Quartermaine

'74 is on the cusp of the change, but North America could be different.
I assume from the fact that the problem doesn't show in intermediate gears that it is related to a "trailing" thottle situation. i.e., cruising, therefore high manifold vaccuum. Check for air leaks on the inlet manifold, throttle spindles (besides the obvious "do they rattle" try dribbling some thin oil or redex on the spindles at tickover, you might see it disappearing and see a bit of smoke from the exhaust)and check the distributor vacuum unit, including the vacuum pipe and rubber ends. Two effects here; 1. the mixture will be leaner than the normal trailing situation as the engine pulls in air from wherever it can get it, and 2. if there is a vucuum unit problem, the ignition won't be advancing to compensate for the weak mixture.
Allan Reeling

72-74 US/NA cars are HIF, unless somebody changed them.
The described condition is "lean surge", and Allan's recommendations are mostly good. The throttle shafts are not likely the reason here, since under these operating conditions there will be little to no air leakage there, because the throttles are open far enough to let far more air in than the leak. Gum build up on needles and jets is a possibility, especially if the car has been sitting for long periods - run some "fuel injection system cleaner" through it. It is possible that the vac chambers need to be cleaned. Or it may just have been tuned too lean. Worn needles will result in need to lean the adjustment excessively to get idle mix right, which leaves it too lean everywhere else.

FR Millmore

I am going to attempt to balance the HIF4 carbs on my 1977 MGB. After watching John Twist's video, and reading through the manual, I have one question. The manuals say to loosen the bolts on the shafts that connect the two carbs. But, Twist's video doesn't...he just starts turning adjustment screws.
Do you need to uncouple the two carbs to adjust them?
1977 MGB with twin HIF4 carbs
Randall Olson

Definitely. You may well be able to balance the airflow at idle with just the idle screws, because the butterflies should be siting on the screws and not the throttle cable. But it's far more important to have them balanced off idle as well, i.e. when they are hanging on the cable, and for that the clamps must be set so that both carbs give the same suck when they are just off idle.
Paul Hunt

This website is really useful for tuning SU HIF Carbs

Once the engine is warm, as Paul has said It's important to balance the Carbs individually and then set the clearance on the throttle interconnection rod levers to 0.012 in or 0.31 mm

Figures from MGB Workshop Manual PDF

Andy Robinson

That's a good article on the HIF balancing. I gave it a try.
Loosened the linkages so the carbs were independent.
Got the idle air synchronized with a UniSyn.
I don't think I got the mixture screws right, yet. The car was running rough, and when I accelerated the RPMs it backfired. So, I enrichened both carbs one turn, and it seemed to help.
I didn't seem to be able to change the way the car was running using the mixture screws.
This may take a while.
Thanks for the great help,
Randall Olson

Easy, but important question. Manual says "Turn the jet adjusting screw of each carb until each jet is flush with the bridge in the carb body, or as high as possible without exceeding the bridge height."
The HIF4 bridge has a cylindrical-shaped indentation where the jet protrudes. Is the bottom of this indentation where the jet is to be flush? Or, is it the top of the bridge, the flat part about 1/4" higher?
I used the bottom of the indentation yesterday, and the car started, so I'm thinking it's down in the hole where the jet needs to be flush to start.
So, what is the consensus: Bottom of cylinder-hole, or top?
Randall Olson

Did you check the air balance just off idle as well? That's more important than at idle as it affects the whole of the running range.

Book says 'flush with the bridge' and that's how I've always done it. It's the jet bearing that is lower than the bridge, and that can vary. In any case it's only to get to a position (two full turns down from flush) where the engine should start, then you set about fine tuning by rpm and lifting pin.
Paul Hunt

First I loosened the carb linkages.
Then, I balanced the air at about 1,000 RPM using a UniSyn.
Then, I turned the jet adjusting screw to weaken or enrichen the mixture without much success. I could not notice any fluctuation of the engine speed.
So, in answer to Paul's question, I didn't get to the point of raising the RPM to around 3,000, and checking the air balance.
Back to the bridge question. Do I turn the jet adjusting screw all the way? Or, do I turn it until the jet is flush with the flat part of the bridge? Or, do I turn it until the jet is flush with the lower cylindrical-shaped depression where the jet protrudes?
Thanks for the quick feedback. I think I can work on this project today while it's too rainy and cold to drive with the top down.
Randall Olson

You set the initial air flow balance at idle with the idle screws, interconnecting shaft clamps slackened, and then set the mixture also at idle.

Then after you have retightened the interconnecting spindle clamps you recheck the air flow balance just off idle. No need to go to 1000, let alone 3000, and in fact those levels are likely to make setting-up harder not easier. No need to check mixture off-idle, if it's different at other throttle openings then there is something wrong, and you won't be able to do much about it without a long time on a rolling road. If it pulls well through the range then it's probably fine.
Paul Hunt

Randy, I have copied this from the SU website on general tuning

Picture 3 on the website refers - the jet needs to be flush with the bridge of the carb or as high as it can go, but not above the level of the bridge casting


(a) Lift and support the piston clear of the bridge so that the jet is visible; if this is not possible due to the installed position of the carburetter, remove the suction chamber assembly.
(b) Turn the jet adjusting nut/screw up/anti-clockwise, until the jet is flush with the bridge or as high as possible without exceeding the bridge height (3). Ensure that the jets on multi-carburetters are in the same relative position to the bridge of their respective carburetters.
(c) Check that the sintered needle guide is flush with the underside face of the piston (4).
(d) Turn the jet adjusting nut/screw (3) two turns down/clockwise (each nut/screw on multi carburetters).
(e) Turn the fast-idle adjusting screw anti-clockwise (each screw multi-carburetters) until it is well clear of the cam (5).

Andy Robinson

I set the jets to the top of the bridge, exactly, using a straightedge to make sure.
Then, I turned the mixture screws on each carb two full turns clockwise, lowering the jets.
The engine was cold, so I started it using the choke. It ran very rough. But, it warmed up enough to restart without using the choke, I thought.
So, I loosened the connecting linkages, and disconnected the choke cable.
When I tried to start the car it would not. I checked it was getting fuel. It was.
So, I reconnected the linkages and the choke cable. It was still running rough, so I turned it off, and turned the mixture screws each another full turn. That makes three turns clockwise from being flush with the bridge.
I started it, and it seemed to run a little better.
But, the carbs are not balanced independently yet.
And, based on the UniSyn, the rear carb is drawing more air.
It seems the carbs are so out of balance right now.
Not exactly sure what to try next. But, maybe I will just get the engine going, try to balance the carbs with the linkages connected, then try to get it to run smoothly by adjusting the mixture screws.
But, I am open to any logical approach others suggest.
Randall Olson

Randy, here is another excellent site for tuning SU Carbs

The following from the site is key

1. Bring the engine up to normal operating temperature

2. Loosen both the screws on the interconnecting linkage; it's extremely important that each carburetor is able to operate independently of the other. If you don't do this, when you go to adjust the second carb, you undo the adjustments you made on the first.

I would recommend using a gunson's Colourtune to get the mixture right on both carbs.

The other obvious thing to check (that has been previously mentioned) is that you don't have a vacuum leak

Copied from the website -

One of the most common problems is rough running at idle, and before it is assumed that a rebuild is required there are a couple of easy things to check, which may solve the problem. The most likely culprit is a vacuum leak, and these account for the majority of issues reported to us by owners.

You need to check that all of the tubes and hoses connected to the carburetor(s) and the inlet manifold are connected properly, and are not damaged, or perished. And although it sounds a bit basic, check that the inlet manifold nuts are tight; you'd be surprised at just how often that is the problem. If everything looks OK, one more check is needed. With the engine running, take a can of starting fluid and carefully spray all of the hoses, their connections, and the inlet manifold gasket area. If the engine speeds up at any time, you have found a vacuum leak. Fix that leak, or leaks before you do anything else. You MUST still check the entire system after you find the first one; there almost certainly will be more than one!

If your car has a brake booster, don
Andy Robinson

"I set the jets to the top of the bridge, exactly, using a straightedge to make sure. "

It's just not that critical, it only needs to be judged by eye as it's only part of setting a starting position. You will almost certainly have to move it one way or the other from two flats down from flush to get the correct mixture. You should however aim to get them the same height relative to the bridge, as with subsequent adjustments from the '2 flats down' position, if you have to move one carb noticeably more than the other it indicates there is a problem with one or other carb. For example if the servo hose (which American cars didn't get until 1975) is leaking, it will weaken the rear carb more than the front, so that carb would need to be richened more to get an apparent balance.

Personally I couldn't get on with the Colortune with a single SU. With twin carbs unless you have two of them you are going to have to keep moving them between at least two cylinders. I found the colour variation was not as fine as the lifting pin method, which I've used ever since.
Paul Hunt


Make sure that both carbs are tightened down to the manifold properly before you continue. I've found over my many years with MG to be something easily overlooked when setting, or re-setting the carbs.


Larry C. '69 midget & '74 B/GT
Larry C '69 Midget

"try to balance the carbs with the linkages connected"

Balance them with the linkages DISconnected.
Paul Hunt

The carbs are now balanced. I think two things were causing the rough running. It was too lean, and the nuts holding the carbs to the intake manifold might have been a little loose. It's also possible a vacuum line was cracked, so it go replaced.
I drove the car about 3 miles without incident. I'll test it out more over the next few days. Then, I will pull the plugs, which were new, and see what they look like. I want to make sure it's not running too rich now.
When I raised the pistons, the RPM increased a little, and I think it flattened out again. So, I'm hoping it is about where it should be as far as the mixture goes.
All of this was done with the linkages disconnected.
Thanks for all the tips.
1977 MGB with twin HIFs
Randall Olson

"When I raised the pistons, the RPM increased a little, and I think it flattened out again"

It should, while you are still holding the pistons up that 1/32". Can be very difficult to judge with HIFs, almost subliminal. Slightly easier with HSs I have found.
Paul Hunt

The car seemed to running well, but when I turned a corner and sharply accelerated uphill it backfired, then stalled.
I restarted it, and it seemed to be OK. I drove home about 3 miles and it stalled as it was idling in the driveway.
I am thinking it is still a little to lean.
Any agreement?
1977 MGB with twin HIF4 carbs
Randall Olson

Still possible it's drawing air in from somewhere.
Servo, try clamping the vacuum pipe.
Vacuum advance unit................double whamy there!
Manifold gasket.
Allan Reeling

A backfire in the inlet can blow gaskets and also the Welch plugs in the manifold.
Paul Hunt

I started the car, and then sprayed starter fluid on every vacuum line, gaskets, manifold, servo. There was no noticeable change in the engine speed.
Next, I richened each carb a quarter turn.
I'm still thinking it might just be the mixture was too lean.
I didn't understand the procedure recommended above regarding "clamping the vacuum pipe". Can someone elaborate? Or, would the starting fluid test eliminated the need for this test?
Randall Olson

Presumably you have the dual line braking system with servo integral with the master cylinder?

If so a leak in the servo diaphragm can cause a weak mixture. Clamping or otherwise blocking the servo hose should eliminate that.

Ditto the distributor vacuum line which comes off the inlet manifold on your era. Check that in the same way, and in fact all the plumbing that is attached to the inlet manifold, like for the anti-runon valve, gulp valve and EGR valve (where fitted as you now have twin HIFs).

If you are satisfied you had the carbs correctly balanced before, check the balance again. With twin carbs vacuum leaks, depending on where they are, can affect one carb more than the other.
Paul Hunt

Yes, I have the servo integral with the master cylinder.
I disconnected the vacuum line to the servo, and plugged the hole in the intake manifold and the hose to the servo. Then, I started the car. It ran OK in the garage. I checked the air balance, and both carbs were drawing the same. Turned off the engine.
Then, I disconnected and plugged the vacuum to the advance. Again, the car started and ran OK, but it seemed a little rough. About the same as with the servo pipe plugged/capped. Air balance remained unchanged.
I tried lifting the pistons. Both carbs seemed to stall a bit. That would seem to indicate they are too lean.
What should I expect with the hoses clamped/disconnected?
Randall Olson

"What should I expect with the hoses clamped/disconnected?"

On the face of it unless whatever they are feeding is giving a vacuum leak, there should be no difference between having them clamped and having them unclamped.

However with the manifold vacuum that you should have, if you clamp the distributor hose before starting the engine, then there will be a difference in how the engine runs. This is because with manifold vacuum you go from zero vacuum advance with the engine stopped to near maximum vacuum advance with the engine idling ... unless you also have the TCSA valve which means you get no vacuum advance until 4th gear anyway! And that valve could be another source of a vacuum leak.

So the correct procedure for both vacuum lines, is to repeatedly clamp and unclamp them, first one then the other, and see if that changes the idle note.

You should only lift each piston in turn by 1/32", and it should give a momentary rise in revs then settle back down. Weak mixture, or lifted too far, will cause it to tend to stall.
Paul Hunt

Thanks Paul. That makes sense.
With clamps it didn't make much difference, although with hose to the vacuum advance clamped there was a noticeable difference...rough running.
And, I was lifting the piston more than 1/32" when it was stalling out.
I think I will put things back together and see how it drives. I'll let you know if there are any developments.
Randall Olson

The car seemed to drive OK on a short 15 mile test run.
Not sure if it's right on, though. That is, I'm not sure if it's running too lean, or too rich.
Randall Olson

This thread was discussed between 23/05/2011 and 09/05/2015

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.