SU Carbs - Quick reference needle selection
ATTENTION! This needle reference was produced quite a number of years ago now. Way before we had the new breed of rubbish fuel with ethanol in it. Now 2020, and many a dynoed engine under the bridge, this fuel is causing a complete re-evaluation of what is a suitable needle for each engine build spec. However, until a well researched new set of needle options can be offered up, the below are still reasonable starting points, but would suggest running the just possibly a little lower than usual. Not ideal as this affects idle fuelling as well, but as I said - a starting point.
Unfortunately I cannot give an all-encompassing table of what needles are needed for which engine builds. My life's collection of all the automotive-orientated work I'd ever done was stolen out of my car by some low-life scum a few years back. Not for what it was - but for what it might be. All I have is a few scribbled notes, the source of where I start now. Being an oft-voiced question on the message board, I thought I'd share it with you. Since there is already a very good source of starting points/applications for fixed needle carbs, this is for those floundering about in the wilderness that is the biased (swinging) needle example of the breed.
These are based generally on 1275cc+ motors, using a reasonably modified head, compression ratio on the sane side for unleaded road use, and uses Kent Cam profiles as the main guide so just select one that near-matches what you're using
Std filter case K&N Cone or
With K&N filter Pancake filter
MD256 AAK AAU
MD266/7 AAA AAM
MD286 AAM ABY
MD256 BDL or BAK BDK
MD266 BDK BCE
MD276/286 BBC BBC
MD296 Modified BBC……………
Twin 1.5" (assuming K&N pancakes or no filters)
MD286/296 AAM or ABY (ABY is richer towards top end).
Other references -
These are for comparison and are arranged going from leanest to richest. Makes a good visual example of what happens where on the needle profile in accordance with engine specification when the profile list of each is lined up in a row.
BEJ - standard unleaded Metro.
BER - 1.3 Pre-injected Cooper (basically MG Metro spec engine).
BFA - Alternative Metro Turbo needle, has richer cruise section, but leaner top end.
BDD - Metro Turbo standard needle.
BFY - 1.3 Pre-injected Cooper unleaded (basically MG Metro spec engine).
BDL - MG Metro standard needle.
BCE - Popular modded engine needle (see above table).
BBW - 1.3 Pre-injected Cooper with Stage One kit using K&N cone/pancake filter.
BDK - 1380 fast road spec (MD286, modded head, etc.).
When choosing a different needle, select one with the same idle figures. If this is not possible, remember -
If new needle has a leaner idle figure, the rest of the needle will give a slightly RICHER mixture than the profile suggests.
If new needle has a richer idle figure, the rest of the needle will give a slightly LEANER mixture than the profile suggests.
This is caused by re-positioning of the jet to get correct idle mixture.
Like I said - these are merely starting points - so a healthy dollop of sensibility is needed! I highly recommend buying the SU needle chart book available - part number ALT9601. Not only does it list all the needles and their profiles, nut a useful section in the front giving suggestions on how to select a different needle than the one you're using.