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MG MGB Technical - 1976 engine - weaker carburettor needles

I have an MG TD into which has been fitted a 1976 MGB engine with a Ford Sierra 5 speed gearbox. The problem is that despite checking, replacing and setting most items, the engine runs very rich. The standard needles are ACD, and there are no weaker needles listed; there are other needles for other engine specifications, but I do not know if they would be weaker or richer. I just wondered if anyone has done any experimenting and determined a suitable weaker needle than ACD for the standard 1976 specification engine.
Many thanks, Roger Wilson.
Roger Wilson

How rich is it? I may be wrong, but it seems really odd for an engine to need weaker needles than standard. I'd have thought there was a pretty stable need for air/fuel against cc if everything is standard.

What have you replaced to date? Are the air filter assemblies stock and clean and getting enough air in? Leaky O-rings on the choke assembly (HIFs) can also cause rich running.

Steve Postins

I presume you mean you can get the correct mixture when idling i.e. setting-up but it becomes rich when running. An air leak on the induction side will cause this, have you checked for that? Spray carb cleaner or propane around the carb to manifold and manifold engine area and carb throttle spindles and see if the idle speeds up. AAD gets progressively weaker from station 4 upwards, AAN more all the way through, there are quite a few others. But the fact you are having problems with the stanard needles indicates a fault somewhere, the different gearbox and car shouldn't make much difference at all, assuming the carbs can breathe properly.
Paul Hunt 2

Many thanks to both of you for replying. It used to be fairly common to specify alternative rich and weak needles for SU carbs, and the early MGB 18GD and 18GG engines had both these alternatives. The installation is fairly recent, and so far I haven't been above 70mph; however, the exhaust is black and the plugs have soot on them, although the central ceramic part is a nice shade of brown. I do not think I am getting better than 15mpg; I cannot tell exactly as the odometer does not read correctly (another winter job). The engine does not have air filters as there isn't enough room to fit any. I have replaced the inner and outer rubber seals, plus the paper ring gaskets, on the choke assemblies (HIF carbs); the strange thing is that there seems to be a lot of end float here. I can push the shafts in and out by almost 0.5mm, although this is the same as before I replaced the seals. Paul, you have described the problem exactly; I can set the idle, but the running is rich. At present I am replacing the water pump plus a couple of other things (not more than an evening or two), but when they are finished I will try the carb cleaner test. I appreciate your comments about AAD and AAN needles; I think I can borrow them to experiment.
With regards, Roger Wilson.
Roger Wilson


Might also be worth checking that you have the correct size jets? Should be 0.09", could be 0.100". Could check with a 2.5mm drill - it would just about fit down a 100 thou jet, but not down a 90 thou jet.


This thread was discussed between 11/05/2006 and 16/05/2006

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