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MG MGA - Carburettor tuning

Took my car to the MOT last week and while I was there I checked the CO reading just out of interest - not part of the requirement for the test. The reading was about 4.5%. The tester suggested that it was a bit high. So I leaned off the carbs equally about half a turn so the reading was about 2.5%. The car ran fine on the way home which was just a few miles.

Then I started to worry that I had leaned it off too far and throught I'd set them up again as in the WM.
So went through the whole procedure with a balancer etc. on each carburettor separately and found that the mixture had to be much richer to get the idle speed to rise momentarily on each lifter. I set it all up and then with both carbs connected I now get some slight black smoke at idle and it sounds a bit lumpy.

So I am now a bit puzzled about whether I trust the WM technique or not. I did notice that the idle speed had to be around 750 to 800 rpm at most to get the speed to rise on the lifter when I set the mixture.

The car was tuned last year, with new needles, by Peter Burgess on his rolling road and the carbs were rebuilt by SU a few thousand miles ago. Any thoughts apart from why did I mess with a perfectly good set up!?
John Francis

The workshop manual technique has always worked for me (assumes that the jets are centralised, float chamber levels are OK, carb pistons are free, dashpots have oil, fuel pump delivering OK ... etc), but I also use this easy way to confirm carburettor settings ...
Go for a 30 minute drive, to get the engine thoroughly warmed;
Stop the engine with as little idling as possible;
Let the engine cool a bit;
Take out the plugs for no. 1 and no. 4 cylinders;
They should be the same colour ... grey=weak, black=rich, pale brown=just right;
Adjust the mixture by no more than 2 'flats' of the brass nut at a time (looking from above the carbs, clockwise=richer)
D Brown

Hello John, I concur with David on using the plug colour as the guide. I set my carbs using the lift pin test and they were as rich as anything with black plugs. I kept backing off 1 flat at a time and checked the plugs after 3 or 4 miles. Just kept doing it until i got the right plug colour - probably 5 or 6 flats compared to the lift pin test. Much smoother and more MPG too.
C Manley

This thread was discussed on 19/08/2013

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