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MG MGB Technical - K & N's and needles!

I have a question, which I think I may have asked in the past but would like a definitive answer on?

We own a 1976 MGBGT and I changed from the standard "cooper cans" to K&N deep filters with MGB ally plate.

Since this change I have noticed that the performance is not 100%; sometimes sluggsh and accelation can lead to a little faltering and stuttering (particularly when cold) but not enough to make it a problem. The carbs have the original needles in them (AUD's I think?) but I have been told that with a change in filters to K&N's that a change in needle is also advisable? I use it purely on the road and the reason for changing the filters was to try to help get some additonal (if not minimal) power.

Bearing this in mind, could you tell me which would be the best needle to change to?

Many thanks!

Martin.
Martin Clegg

Its always better to set up on a dyno when mods are made, however, a good place to start with a basically standard engine, assuming the ignition system and timing have been optimised and tappets reset is with AAA needles.

Jim
jim soutar

Horses for courses. My +60 thou, stage 2 engine with K & N's and big bore has ABD's, AAA's were too rich! As Jim said, rolling road gives the definitive answer, but performance and plug colour etc., can get you very close. BUT you need needles to experiment with (expensive) or the patience and skill to modify needles.
Allan Reeling

The ABD is almost the same as the standard AAU - just slightly richer just above idle - likely to leave the engine running lean at wide open throttle.

The AAA, AAB & AAM needles are very similar up to half throttle then go increasingly rich after that.

To see the relative mixture graphs feed in the needle letters here - http://www.mintylamb.co.uk/suneedle/

90% of the fast road engines I build end up needing the AAB or AAM needle with K&Ns
Chris at Octarine Services

abds's chosen on rolling road session at Mr Burgess's. Triple A's were far too rich........as I said "horses for courses"!
Allan Reeling

Were they left as standard or did Peter do a bit of sanding on them?
Chris at Octarine Services

Martin
AAA's if it's otherwise standard
But as Allen has found , if it's oversize it will go richer

ANY given needle in a given sized engine will
A--- Run richer in a larger engine
B--- Run leaner in a smaller engine

As Allens engine is bigger than standard it will naturally run richer than if it were std. size
Also your exhaust system will effect the mixture a bit as well but not as much as engine size

There are plenty of people happily motoring with K&N's and AAA needles in otherwise std. MGB's

Cheers Willy
William Revit

Un modified ABD profile. The gas profile showed a consistent reading. Max readings, 91.5HP and 119 ft/lbs at wheels.
Allan Reeling

On my european spec 18V, with K&N's in the original housing, I had AAA's and found them to be too rich.

I returned to the standard AAU and it ran much better but ideally I should have fitted slightly richer needles, ABD's perhaps? The car is sold now, so I needn't worry.
Willem vd Veer

These are all HIF right? We got a set of rebuilt HS4 in with AAA and it kept going lean after about 1/2 up or stage #8 . Switched to Fixed needles ( Dave Anton and JOe Curto sell the fix it piece) and the #6 needles went in to provide a beautiful, flat 12-13.7 AFR throughout the range on the Innovate wideband. Just FYI. Final setting of jet height was 43 thou below the deck on both instruments. Cheers, Vic
vem myers

This thread was discussed between 01/08/2012 and 07/08/2012

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