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MG MGB Technical - Rich front SU

A week or so ago I changed the plugs on the B. It’s a 69 B with twin SU’s. I noticed the front two plugs were quite black and the rear two were orange. I’ve been told the orange is due to lead replacement petrol. The black plugs I believe are due to too rich mixture. Is it a simple exercise to adjust the mixture of the front carburettor only without re-balancing everything? If so, how?

Gary Crutchley

Gary, this is a very easy job: I don't even need a spanner. Underneath the carburettor, assuming it is a HS4 like mine, you will notice the short fuel line from the fuel-bowl entering the (partly) plastic jet. The later has 2 nuts. One is directly at the carbs housing the other is kind of secured by a spring located aroud the jet. Both are of nearly the same diameter 18 mm or so. The lower nut is the one you should turn in clockwise direction (seen from underneath!) to achieve a leaner setting. Start with a maximum of 2/6th turns (as the nut has 6 flats for one full turn to go ) ....
Good luck.
Mark
Mark

Don't just poke and hope, you will have to do a full balance of air and mixture for best results if one carb is already out. http://www.mgb-stuff.org.uk and clicking 'Spanners', 'SU Carbs' might help.
Paul Hunt

Gary,
just a thought, is your crankcase breather attached to the front air filter. If so you could be pushing a little oil into the front two cylinders causing black plugs.

Clive
Clive Skelhorn

The best way is to take it to a rolling road, if they know what they will totalally transform your car
peter bramble

A full balance is beyond me. I’ve read everything I can in the archives and also the MG-stuff site and complexity of it puts me off – not to mention the potential for disastrous results.. I’ll check the crankcase breather. I like the idea of a dyno tune and will investigate that further. Thanks for your input.
Gary Crutchley

This thread was discussed on 25/07/2002

MG MGB Technical index

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