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Fine-Tuning MGB Carbs
Just a Simple Wrench
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Fine-Tuning the MGB Carbs
by Mike Olsen
Changing the mechanicals like aircleaners, exhaust,cam and modifying the head can increase power in the engine. To enjoy the most from these modifications a fine tuning is needed to benefit from the time and money spent.
As far as ignition timing is concerned there are racing type distributors that have a broad based application (as far as range of modification is concerned) and work quite well.
Adjusting the idle mixture is a pretty simple task but will your mixture be correct for the entire throttle range? I have worked with the spring-loaded SU needles mostly so these are the types I’ll concentrate on.
Different years of MGB’s have used different needles. Also the HIF type of carb seems to move more air and therefore uses richer needles, as compared to the HS type.
The best way to select a needle is by installing a mixture measuring gauge so you can drive the car and monitor any range you are not sure about. I didn’t want to do that so I took the long way around. I also used the HS style, which for me is easier to tune.
First I used the SU needle program to select a range of needles to work from. These appear to come in close set pairs from lean to rich AAE&AAL, AAU&ABD, AAA&AAM and a lone AAB. All these were used on stock MGB’s except the AAA, AAM and AAB.
I then entered the data into the computer so I could graph the differences. The differences are in off idle, middle or top end not a general overall lean or rich. If you were running pretty fair a small change would be to one of the associated pairs for a quick check out. This didn’t work for me but maybe you will be lucky.
My problem was a stumble off idle and at cruse a wind against the car feeling. The idle mixture was set with gunson’s color tune, which the CO meter confirmed was spot on. By riching up idle with three or four flats rich the stumble would be cleared up. This would make the idle very rich and the engine would blacken the plugs. At cruse I shifted into neutral, turned the engine off, pulled over and removed the plugs, they were white. Also at cruse I could pull the Manuel choke out very slowly and very little where the car would pick up power and run better. This is how I determined that the engine was running lean.
I changed the AAU needles to ABD which are the stock HIF needles. This gave me some improvement. Then I tried AAA, which really started to run better. Each time the mixture at idle would have to be reajusted. There was still a small problem at cruse however with that wind on the car feeling coming only after running about 20 minutes or so on the tollway . The color of the spark plugs was a gray while in town they would be a reddish brown. I then tried a set of AAM , these were way too much when idle mixture was reset to a smooth idle.
Then I decided to experiment by sanding down a set of AAA needles. This was done by spinning the needles in my fingers while holding 400 grit emery paper. Now cruse is at 1/4 to 1/2 inch down the needles. It did not take long as only about .001 inch was removed at a time. At this point the idle mixture would not be changed so it was easier to note and make changes. After .005 inch was removed the car was running great and felt like I had added a supercharger. This was also a bit of fun playing around dialing in or fine-tuning the car.
Yours may not need this much work. My compression is 190psi, mild cam, some port work, a peco exhaust, K&N filters and a small over bore. The chances are that a small needle change will do most cars but this is the fine-tuning needed to optimize the most common engine modifications.
Mike Olsen
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