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MG MGB Technical - Fuel air mix

My 1977 UK spec BGT is running far too rich. I have looked at my Haynes manual, checked Paul's website and gone through the archive but none of the instructions are simple enough for me.

Can anyone tell me, in idiot proof language, how to weaken the mixture (other than paying someone to do it for me).

Please bear in mind that my mechanical aptitude is such that I have the potential to shear manifold studs whilst changing windscreen wipers.

Chris
Chris Thompson

Hi Chris.

Hope this isn't too obvious...

Are the chokes releasing properly ?.
Clogged air filters will cause rich running.. have yours been changed recently ?.
How do you know it is running rich ?... black plugs can be caused by other things.

HTH.. Don
Don

Hi Chris,

About an inch up from the bottom of the carbs, facing the wing, are two cast "tubes" - they stick out on the corner of the carb and are angled slightly upwards.

Peer into the one on the front carb and you will see that a screw lives in the tube - it has two slots in it at right angles - it is not a Phillips or Posidrive screw - just an ordinary flat bladed screwdriver is required.

Insert the screwdriver into either slot and turn it ANTICLOCKWISE - ie out - to weaken the mixture.

Turn BOTH carbs screws exactly the same amount - I suggest trying HALF a turn on each one to start with.

Just above these screws, on the underside of the casting where the top bolts on is a small pin. If you press it upwards 1/16 inch it will lift the piston of the carb.

If your mixture is rich, with the engine idling, when you lift the piston the engine will speed up. If it is weak it will slow down. If it is just right it will speed up a little then slow back to what it was.

Be warned - it is as easy to get this wrong as it is to get it right!

As a good known starting point it is a good idea to undo the three screws that hold the top on the carb (the suction chamber) and lift off the top with the piston inside - look into the body and you will see the main jet - wind the mixture screw ANTICLOCKWISE till the jet is level with the bridge it sits in - you can feel this best with a finger. Then wind the jet down (CLOCKWISE on the screw) for TWO whole turns - do both carbs.

This will give you a reasonable mixture which can be fine tuned as above - but you should not need to go far from that setting - if you have to make a large adjustment there is probably something else wrong.
Chris at Octarine Services

This thread was discussed between 25/04/2004 and 26/04/2004

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