SU UO Carb Needles

[daykrolik]

I recently acquired a 1967 Series 1. It seems to run very well, but on checking I found the carb needles are UO instead of UM. Should this worry me or is this a satisfactory tuning option? Could it be compensating for a problem elsewhere? It does seem to get even worse gas mileage than I expected.

[ChrisC]

Usually needles are changed to match a modification to the engine such as higher profile cams or a more free flowing air filter. If everything is standard on your car I am not sure that having richer needles would have made that much of an improvement on their own, more likely a slight detriment.

I would be tempted to build some baseline figures - such as mpg or 0-60. Change the needles to standard and re-test.

[Heuer]

Here is some reading material from our Knowledge Base:

SU Carburettor Notes: http://www.jcna.com/library/tech/tech0006.html
SU Carbs.com Tuning guide: http://www.sucarbs.com/tuning2.html
SU Search (SU needle comparison spreadsheet): http://homepage.mac.com/crscapps/CSA/Page1/Page1.html
WinSU: SU tuning software: http://www.winsu.co.uk/

I can recommend the WinSU software which is free to try but you need to register if you want it to reveal the suggested needles for your setup. Clever though as it calculates bhp and torque curves based on your inputs.

"WinSU is a computer program which contains information about SU carburettor needles. The taper of the needle inside an SU carburettor dictates how much fuel is delivered for a certain engine speed and load. It is vitally important that this taper is correct for the engine specification. WinSU will suggest the best needle and dashpot spring for your engine specification and allows different needle profiles to be graphically compared. It will also estimate power, torque, and many other performance related values. This software will be useful to owners of cars equipped with SU carburettors (Austin, Triumph, Jaguar, MG, Rover, etc).
It contains extensive databases of dimensions for over 700 different needles, for both common and rare needle codes, for HS and HIF carbs of different sizes. You can graphically overlay needle profiles to show the differences in lean/richness at various throttle openings."


This is a result screen based on my car parameters. The question marks disappear after you have bought the software - ?9.99.

More info:

[christopher storey]

It is quite common to find UO or UE needles on 4.2, particularly if the standard air filter setup has been altered. Heuer ( David Jones ) and I have discussed this on this forum many times . I am of the view that all the richer needles achieve is to consume more fuel - a lot more fuel! I have always used UM on both of mine, and get 22-23 mpg in normal use, but I am not a hard driver

A salutary lesson to me last year before the Euroclassic was to change the needles on a friend's XK150S from UE to UM, the latter being specified by SU and Jaguar. There was no effect on performance, but consumption improved from 17mpg to 22 mpg !

There are a lot of people who assume that fitting richer needles will produce more power on the basis that if you can cram more fuel in , you will get more out . By and large, it doesn't work, and in any case is theoretically unsound - it is the air consumed which governs power output

[Heuer]

In the interests of research and to be able to have a qualified dialogue with Christopher on this subject I just bought a copy of WinSU. So 'game on' - I will learn a bit more about it and start an iterative set of runs here initially based on a standard car which I will 'modify' with virtual upgrades and let's see what it produces.

And to get things started here is a comparison of three typical needles: