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MG MGB Technical - needle puzzle

I am restoring a '72 roadster. I believe it was assembled by BL in Australia, and so there are some minor differences - e.g. paint colour.

According to the manual, the HS4 SUs should have AAU needles. It has AAB needles - so far as I am aware these are as originally fitted.

Slightly larger pistons (+20 thou), engine balanced, and head ported and polished, but no other modifications. K&N filters.

Any advice on whether it is worth changing needles (e.g. to provide slightly richer mix at 3-4000 rpm)?

In due course a rolling road session would provide useful info, but I thought someone might have some advice on a useful start point.

cheers john crighton
J H Crighton

John
Don't try and guess needles without a gas analyzer.
There are too many variables that effect mixture to get a correct needle without measuring the exhaust.
e.g.
As your engine is oversize it will run slightly richer with a given size needle compared to a std. sized engine
but the k&n filters could possibly slightly lean the mixture out especially at higher revs. Any modifications to the exhaust to free the flow will also lean it out.
Gas analyzer is the only way to go.
Spark plug reading can be another option but really I would suggest you find a rolling road and save a lot of time and money.
I can't find AAB on my chart , How do they compare to the AAU Cheers Willy
WilliamRevit

AAB and AAU have the same starting point and very similar finishing points - 448 and 440 respectively, although the AAB finishes at station 14 whereas AAU goes to 16. AAB gets slightly richer from 3 to 6, more richer from 6 to 9, then reduces in richness from there to the end.

AAB 890 850 824 785 752 715 670 610 560 537 516 493 470 448 448 448

AAU 890 850 828 806 780 745 710 675 647 620 590 560 530 500 470 440



Paul Hunt

Paul, as usual, you are a wealth of knowlege. RAY
rjm RAY

thanks everyone for the prompt response.

I noticed reference to a site on the archives www.mintylamb.co.uk - well worth a look. simply enter SU neddles codes and it gives you a graphic picture of the differences - shows, for example, at what points and by how much (say) AAB differs from AAU.

I will set the car up with the existing needles (AAB) and take it straight to someone with an gas analyser.
J H Crighton

Yes, the site is SUNEEDLES.com and, in conjunction with a companion program posted, will allow you to map current needle with an AF wideband, see where you deviate from the 12.5-14 Air/fuel mix desired, then overlay indicated needle choices-already mapped for the 12-14 stations. We've used this as a start for years. Of course some thumb polishing occassionally is required. Goodluck. Vic
vem myers

AAB's are often used as an "off the shelf" needle to complement the increased air flow given when K&N's are used. But accurately finding out whether any SU needle is right is a rolling road job. More crude, but often good enough, is driving the car at various loads and throttle openings and experimenting with various needles. You do need a few needles though!
Allan Reeling

Another excellent SU needle profile program is SUSEARCH3.xls, a zipped file one can downloaded free from the internet.

Vem
Which is the "in conjunction with a companion program posted that will allow you to map current needle with an AF wideband"?

Richard.
RH Davidson

http://www.mintylamb.co.uk/suneedle/ This allows your needle in comparison overlay with 1-2-3 others....a real eye opener when you've upgraded past the beaten path. Good Luck, Vic
vem myers

This thread was discussed between 11/11/2009 and 21/11/2009

MG MGB Technical index

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