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MG MGB Technical - AAA or AAB needles

My '76 B roadster is fitted with two HIF4 carbs and the K&N filters.

(see also the thread misfire when hot)

The carbs are fitted with the AAB needles and the advise given by several experienced MGB people is, keep the K&N filters on and replace the AAB needles for AAA needles.

My question, (I've been searchin in the archives with no result) what is the differance between these needles.

best regards, Jacob
J Halma

Here are the numbers for each needle. The numbers begin at the biggest diameter and are at 1/8" separations (15 stations) from the base to the tip of the needles. To convert the numbers to inches 890 would be .0890"

890 850 814 785 755 720 674 630 600 580 560 540 520 500 480

890 850 825 785 752 715 670 610 560 537 516 493 470 448 448

As you can see there isn't lots of difference from stations 1-10. The AAB goes richer than the AAA from station 10-15. The carburetor will only operate in the higher stations on hard acceleration or heavy loading, up a steep hill for example. I'm adding a chart from Des Hammill's (How to Build & Power Tune SU Carburetors). Print out the chart and fill in the spaces for aside to side comparison.


Clifton Gordon

Correction to station numbers, should read
"As you can see there isn't lots of difference from stations 1-8. The AAB goes richer than the AAA from station 8-15."


Clifton Gordon

If you have AAB, they are fine, the standard needle for 76-77 was ACD.

Get a copy of the SU search spreadsheet

I've done a comparison for

AAA = what we fitted with K&N
AAB = what you have (Richer than AAA, but not much)
ABD = MGB '72-'74
ACD = MGB '75 + (What I had see how lean it goes)
AAU = MGB '72

Taller is leaner, as it blocks the jet hole.

Martin Layton

I forgot to add,

Ideally, you need to have the car set up on a rolling road.
Most of us just want reasonable performance without the engine breaking, so we play around ourselves, juggling what it feels like, the plug colour, and the MPG 'till we are happy.

A skilled rolling road operator will quickly set up your car, without you having to work through a box of needles.
Martin Layton

Here is a fun site which allows you to visualize the needles.

and below is an image of such a graph.

Dave Braun

Don't you just love this image addition capability!

BJ Quartermaine

Thats the "haystack" program and it really helps. The AABs look fine to me, rich when you are nailing it but otherwise close to the AAA which is itself close to the no 5 at the first 8 stations.
Stan Best


Unless you've modified the engine further I think the AAB is way too rich.

I have K&N's (in the standard casing) on my 73 MGB and found the AAA too rich, I reverted to the standard AAU's which work fine. When researching this problem I was allerted to the nr. 6 needle which is (I think) the richer needle recommended for the older HS4 carburettors; Richer in the right places, but not as rich as the AAA.

You'll find that the nr. 6 is shorter (13 positions) and doesn't have the sleeve which is needed in the HIF4.
Both "problems" aren't. The HIF4 only uses 13 of the 15 positions and the sleeve can be fitted by the carburettor specialist C&C (?) in Gramsbergen.

They can also put the car on a rolling road and find the perfect needle for your specific engine (as rightfully suggested by Martin as the best solution); I wish I had the funds for that.....

Willem van der Veer

Sorry, not C&C (that's a brake specialist) but BCCP:
Willem van der Veer

I run the minty graph program, I stand by my comments the AAB are fine with K+Ns
Stan Best

Thanks all of you for the great amount of info I am recieving.
There is a lot of very interesting material.

After all this I started thinking that most likely I made some time back a mistake by cleaning my K&N filters (they where oily!!). Thought they had to be cleaned out, not knowing that the oil maybe is needed on the filters. Does this realy can make the differance in airflow with the K&N's. Dry and cleaned or oiled?

In case the oil is needed, does it make a great deal what kind of oil is used?

By-the-way I've got the AAA needles in the house so I can compare them with AAB needles.
I did measure the AAB needles and found no signs of wear on them.

Keep you all informed.

J Halma


I agree with Martin the only way to be sure it is set up properly is on a rolling road, a good tuner will play (emery paper) your needles untill the fueling is perfect (a good one anyway).

Graham young

They have almost no filtering effect dry, the proper K+N oil is a red colour. You are suposed to wash them from the inside to the outside then oil them to where you can just see pinpricks of light through them. Then just leave for 50K miles, as dirt builds up so the work better. I did note mine were totally dry if very dusty after 5 years so i clean and re-oil every other year now.
Stan Best

What I forgot to ask; what is the problem with the car? I couldn't fit the "misfire when hot".
Willem van der Veer

I just put K&Ns on my US-spec '73 B/GT. I got a pair of AAA needles, but first I just went back to driving the car with the stock ABD needles that were in there already. I can find no fault with the way the car runs - seems to be slightlly better as is. Pleasant surprise, but why is it not too lean?

Allen Bachelder

I tried mine with no 6 no 5 and AAA. Dave Pymme helped me on a Sunday and we set the idle mixture using his profesional gear and then scientifically drove it through a local village and then blasted up and down a hill. The no6 are rich and it went fine on them but I know from expereince it tends to soot in traffic jams. There was not much wrong with the way it went on the no 5s, even though it must be pumping about 1/2 as much again air as it did ex-works. The AAA set as nice as the 5s, ran as well at low speed and seemed to give a bit more go on the hill so they have stayted in.
Stan Best


I don't understand; with the mintylamb/haystack I notice that the 5,6 and AAA are identical until position 5, the AAA gets richer in the higher regions (which is why it is suggested with free flowing K&N's), the nr. 6 goes rather straight and the nr. 5 leans out in the higher regions. Can you explain why a nr.6 will soot in traffic when it is leaner than a AAA?

BTW, although I may sound pedantic I'm honestly not trying to be a nuisance, just enjoying a bit of needle-talk
Willem van der Veer

You will note I said "from experince" you are spot on right that all these needles look very similar at low revs. The No 6s I had were out of an MGB that was raced in the 60s so may have been worn, or possibly filed down a bit, clearly racers do not idle for long periods.
Stan Best

OK, Stan, thanks for clearing that up, I think I'll start saving for a rolling road session.....
Willem van der Veer

The way it is now;
First 30-45 min. (cold engine) is realy fun driving the car, very good response on the trottle in all rev regions.
Then it holds in when trying to accelerate from stationairy and lower ref regions.
Only by pressing the trottle very slowly it revs up.
Then at approx 3000 rpm its ok again.

My idea; It has to do with the mixture, to lean!
caused by the bi-metalic in the HIF4 carbs and the great amount of air going thru the K&N filters.
I have cleaned the K&N filters some time ago without oiling them. (did not know you have to do that)
At this moment I am waiting for the special red K&N oil (in a cleaning kit?)
BTW no differance between the AAA and AAB needles.

Jacob Halma

This thread was discussed between 17/06/2007 and 20/06/2007

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