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MG MGB Technical - SU needle selection -

Can anyone recommend SU carb needles for my set up:

1868 rebore / 270 Piper Cam / Burgess Stage II head / Std 1.5 Carbs.

I do intend to rolling road the car, but need some different needles to drive till then.

John
J Harle

I'd try AAA or #6 depending on the carb.
Steve Postins

Hiya John,

AAA is a good starting point but the needles need to be fine tunedin conjunction with a rolling road - take it somewhere that has experience of working with SUs and the B series - Peter Burgess would be my recommendation but there are others.

Chris at Octarine Services

Hi Chris - I thought you might pop in with a comment - thanks for the base line.

I can't get into a rolling road until late Jan 2006. I know from the plugs and exhaust gas analyizer that she is running rich and I do have a program that displays various needles from a baseline, but this has been very hit and miss with other cars I have owned. I do remember seeing members on the BBS with similar engine set ups and was hoping one of them would pitch in.

John
J Harle

John,

What do you have in the carbs at the moment?

AAA is richer than standard.

When you say it is running rich, is that measured at idle or at speed?
Chris at Octarine Services

I thought you'd only need to change the needles if you change the air filters.
John has not said if he is changing them from the standard to either K&N or Pipercross. I would advise John to change, given the mods.
Any decent rolling road company should have a stock of differnt needles so I would not purchase these beforehand John.

Cecil Kimber

Cecil,

No - you need to adjust the mixture to suit the set up of engine/carbs/timing - not only when you change to free flowing filters. Modifying the head/cam/inlet/exhaust will change the fuelling demand of the engine at different thottle openings and different speeds.

Standard needles are developed for standard engine set ups - when you modify the engine to a different spec then any standard needle is just a starting point.

A rolling road tuner will take the needles and measure whether they are lean or rich at different needle heights and then choose a leaner standard needle if the original ones are too rich (unlikely) before modifying the needles by filing flats on them to richen the mixture to the correct ratio at all stations of the needle.

You then have needles that are bespoke to the engine and it's set up.
Chris at Octarine Services

Chris thanks for your input....I did not make myself totally clear in my last posting and reading your posting I made some errors in my thinking too.
Cecil Kimber

First - thanks for all your suggestions.

At the moment I have RO needles fitted. It is defintely my intention to have the car on a rolling road. Does anyone have a recommendation for this area - Reigate, Surrey ?

John
J Harle

Do you have the Haystack program? I just ran compare with the 5 AAA and RO. I have AAA in my car and they work well, but the 6 did give better power at the top end from station 13 where they are open jet, and I see the RO is the same. The AA is richer in the mid range though.
Clearly if you were brave enough you could achieve possibly the best of both worlds with a pair of AAAs and some side cutters!
Stan Best

Stan,

No need as the AAA is available for fixed needle carbs too!

John,

The RO is richer on part throttle than the AAA and weaker on full throttle.

A pair of AAA needles may sort your rich running.
Chris at Octarine Services

Thanks guys - I will try a pair of AAA needles and might shave off material past station 13 to gain the upper accel power. I have a su excel program to compare - It is called SUSEARCH - works great !!

John
J Harle

John,

Leave the needles alone!

On a 1.5 inch SU the needles only make it to station 13 anyway!

Each station is 1/8 inch apart and thus station 13 is 1.5 inches from the shoulder.

The AAA is richer than the RO at full throttle.
Chris at Octarine Services

Chris,

Good point Chris - I was just measuring and discovered the very same as you input arrived !!

John
J Harle

Thanks to all -

I installed the AAA needles and WOW. The choke works as it should, the idle is smoother and the engine pulls much better through the entire rev range. A GREAT result. Now I have to convince my wife the GBP 200 for the rolling road is worth it....

John
J Harle

You could try www.rollingroadtune.co.uk who are in Reading, or http://www.aireytuning.com/services.asp, in Winchester both of whom have SU experience and are not too far away....

Happy Christmas
Chris Cooper

oops, you need to remove the comma at the end of the thread, http://www.aireytuning.com/services.asp
and www.rollingroadtune.co.uk

sorry...
Chris Cooper

John,

Make the excuse to drive up to Peter Burgess in Alfreton - if you mention it is one of my engines with one of his heads then he may well do the session for free - or minimal cost anyway.

The fuel cost will be lots less than £200!
Chris at Octarine Services

Hey Guys:

You all need to check this free needle charting program out...

http://www.mintylamb.co.uk/suneedle/

I have used this program and found it very helpful. I entered the RO and AAA needles and the resulting chart verifyed what Chris was saying.

It's very easy to use.

Bryan

Bryan Prindle

I just changed the CD to AAA on my 80B Le with K&n filters and it does make a diference, i wonder if iam right by doing as the book, by turning down 2 full turns of the mixture adj.screw and take it from there?
Alfredo
amf rodrigues

Alfredo,

Yes you are right - nearly all needles have the same dimension at idle.

Chris at Octarine Services

The two turns isn't critical as it is only a starting point. But if you use it and then find that one carb takes more turns than the other to get it into tune at idle it is a sign that one or other carbs has a problem.
Paul Hunt 2

This thread was discussed between 17/12/2005 and 30/12/2005

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