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MG MGB Technical - Setting Fuel Mixture SU HS4 Carbs

I have a 1968 MGB with standard SU HS4 carbs. When fitting a pair of K&N deep filters I changed the carb needles to No. 6 as advised.
To retune them I followed the book procedure. I started by turning each jet downwards 2 full turns and then raised each one equally until I was convinced they were at the correct level. Ie. when lifting the piston 1/32 inch or 30 thou. the revs rose slightly then resumed the same speed.

I was surprised how much I had to raise the jets. Two full turns lowered the jets by 60 thou. and my final setting had them finishing up at 40 thou below the jet housing, so I thought the mixture might even be running a little weak.

At the MOT today the examiner pointed out my CO2 reading was 5.2% and suggested it was too rich.

So if anyone has their carb dashpots off for any reason can you tell me if carb jets sitting about 35 to 40 thou. down is about the right setting please.

Thanks everyone.


a b

You seem to have the tuning method correct. If the car is running well and the plugs are a pale brown/white colour, why worry? The tester can't fail it for having a highish CO level, in fact he shouldn't have tested the exhaust at all. The only thing a 1968 car has to comply with is that it mustn't make "excessive smoke". I reckon these ancient engines run better a little bit rich.

Things you can check - take off the air cleaners and with your finger lift each piston up a little. Do they fall down smartly with a definite click when you let them go? If not, clean the pistons and chambers with petrol (no abrasives) and re-centre the jets.

Make sure you have the correct springs in the dashpot chamber and make sure you have a small amount of oil in the dashpot damper.
Mike Howlett

I generally start at 9 flats down, having found that the generally recommended 12 is too rich. I usually don't have to go more than 1 flat either way from the bench setting, normally 1 down if it's winter, 1 up in the summer. AKA right where you have it! They will usually run fine a flat lean, giving best economy but needing more choke on start, but the "snap" on accel is off then, and I wouldn't want to run sustained high speed like this.

FRM
FR Millmore

5% is spot on for these engines!
Chris at Octarine Services

Many thanks to everyone above. Seems like I have nothing to worry about. The MOT examiner´s comment was probably well meant advice as he passed the car.
a b

I can't say I've noticed having to wind the jets up much from 2 flats down, but if it is maybe it's *deliberately* making it rich so as to start without choke.
P Hunt

Paul -
I find that most people who set the carbs up reasonably within range have them too rich. I think it's a matter of lifting the piston too far when checking and/or misinterpreting "a slight increase in engine speed". Any MG that will start readily below 50F w/o choke is too rich, and it costs fuel and bore wear and oil contamination.

FRM
FR Millmore

I thought on a 68 the MOT emissions test was visible smoke only.
c cummins

It is, hence Mike's comment earlier. Measured limits were only applied to cars *first registered* (note not the build date as applied to eligibility for 'Historic' i.e. free road tax) on or after 1st August 1975, and is 4.5% CO max and 1200ppm HC max.
P Hunt

This thread was discussed between 01/12/2009 and 04/12/2009

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