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MG MGB Technical - Carb problem?

Hi all. I have had some problems with my plugs blackening so much that I lose power and then wont even start. I have had to get over this problem by fitting hotter plugs obviously burning off any excess fuel. This has resulted in my 77BGT now being far more responsive than it ever has been before but I am wondering if I am hiding a problem. If things still deteriorate I have only one more step in hotter plugs I can go. Do I have to do a carb (pair) rebuild or what? I have the later type of carbs that I know are not thought well of so should I get an earlier pair and swop them or stick to the ones fitted and recondition them. Any help or advise would be much appreciated. Graham
G Palmer

If the plugs are dry and black, it sounds like an over-rich mixture, which would point to carbs. Do you have dual carbs on your 77? When you say later carbs, would that mean HIF rather than HS? Probably not a major detail for diagnosis, but would be necessary to know to help rectify the problem. Are all the plugs looking the same? It would be odd for both carbs to go out at the same time, so I would expect 1 and 2 to be affected more than 3 and 4 or vice versa. Also factor in that the middle cylinders tend to run alittle richer if I recall correctly.
If the plugs are wet with a thick black substance, I would shift suspicion to oil getting into the combustion chambers either by the rings or valve guides.

Erick Vesterback

If someone replaced the stock carb with a pair of SU's they may have the wrong jet needles for the tune of your engine. It is also possible that the carbs are just set to rich, the floats are set too high, or the fuel pump pressure is two high pushing fuel past the float needles.
John H

John. The North American Specification cars had a single Z-S. MG continued using the twin SU carbs for all other markets. Hence, twin SUs, normally HIF-4s would be factory on a car in the UK.

Graham. I am not very familiar with the HIF series carbs. I had a similar problem, with HS carbs, when the choke was used and, when released, the jets did not come fully up, leading to a drastically over rich mixture. Not a good thing. The plugs foul, as you have notice, and unburnt fuel can get into the crankcase--at least so the legend goes.

I would pull the suction chambers and pistons from the carbs and see how far below the bridge the jets are. I would also check to see if they were of equal distance below the bridges.

In addition to sticking jets, an air leak could cause the carbs to be be adjusted overly rich. As Erick notes, more information would be good. Knowing what each spark plug looks like will help to know whether there is a problem with only one carb or both.

Les Bengtson

You need to do a full set-up from scratch on the carbs, after making sure the valve clearances, plug gaps and condition, points gap/dwell and condition, timing including vacuum and centrifugal advance mechanisms, plug leads, cap and rotor condition are all good.

You shouldn't have any real problems with the later HIFs (on UK cars till the end) but whilst they are slightly kinder to the environment they can be more difficult to spot and fix problems than the earlier HSs. For example the enrichment mechanism is internal rather than external and so is more difficult to spot when it is sticking open or leaking extra fuel through permanently.

As Les indicates, during normal setting-up a vacuum leak can be 'tuned out' to give correct mixture at idle, but will then give an overly rich mixture at wider throttle openings i.e. running. The indication of this is if one or other carb has to be adjusted very far (further down) from the starting position of the jet being two turns down from being flush with the bridge, or one carb has to be moved more than the other. This will also happen if one or both enrichment valves are leaking extra fuel (jet will have to be moved up from the starting position to compensate). Also when correctly set up recheck the mixtures after opening and closing the choke to make sure the valves aren't sticking.

You definitely shouldn't need hotter plugs, and changing carbs is an expensive way of curing what may be a simple setup problem. OTOH one or both may be 'worn out' and would benefit from rebuilding.
Paul Hunt 2

Hi all. Well this really is a complicated problem for me. I am not particularly technical when it comes to this sort of thing but I will print off your comments and show them to my local garage who usually tunes the car for me. I will get to the bottom of my problem and will let you know the answer asap. Some answers as I see them at the moments. The plugs are all similar and covered with a dry black covering. I have two carbs that I believe are HS4 at least they do not have a side chamber. As to the fuel pump I fitted a brand new mechanical type about a year ago. That is about all I can add at the moment but thanks for all your input. Graham
G Palmer

What fuel are you putting in? With Optimax I find BP6s run to hot but BP7s foul in traffic. I've got a souped up engine, but I'm not sure that makes any difference.

Hi Neil. I am using super unleaded (unleaded head) and an additive as well of course. I rarely use fuel from any one manufacturer. Just thinking as I type could the additive I am using be giving me a problem. I have had to use Wynns for several months now as I can no longer find Castrol. I have been assured that there is little difference but having been around the Hi-fi world for many years I know that that is not always the case. Also what is Optimax, BP6's and 7's?
G Palmer


Sorry, Optimax was the old name for Shell V-Power.

If you don't know what BP6s and 7s are then I may have identified your problem... Champion plugs! BP6ES are the the NGK plugs for the MGB, BP7ES are colder plugs (they run colder, hence more likely to foul because they don't get hot enough to burn off deposits).

What is the additive - octane booster? If you use Shell V-Power or BP Ultimate then you don't need octane booster? Was your car tuned for super-unleaded plus additive? If not, then it is probably running too rich.


If the carbs *don't* have the float chamber on the side of the carb they are HIFs, not HSs.

I never found the octane boosted lead-replacement additive made any difference to pinking or running on, so I just use the unboosted. If you have an unleaded head you don't even need that. There is research somewhere that reckons even unmodified engines that have run several tens of thousand miles on leaded and not had new or recut valves or seats shouldn't need an additive, but it's just to be sure. But you can't mix many of the additives, not additives and LRP (if you can still get it). I also don't find Optimax, V-Power or Tesco 99 as good as the old 4-star as far as pinking and running-on goes.

With a standard engine I have never had any plug problems with any fuel. Standard plugs are Champion N9Y and their derivatives (i.e. RN9Y, RN9YC etc), Bosch W7, and NGK BP6 and their derivatives.
Paul Hunt 2

This thread was discussed between 14/02/2008 and 23/02/2008

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