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MG MGB Technical - SU needle query

Hi All,

My 66 MK1 with GB high compression motor, flutters (can't think of a better way to describe it) when doing 3 - 3500 RPM, 90 -100 kph and coasting or slightly backing off. Accelerates well if throttle opened.
Engine fitted with 123 distributor, there is no pinking under any load conditions (including full throttle at 35 kph in top), oil in dampers in carbys are OK. I have fitted FX needles, as per the Haynes manual. The carbys came with #5s when I got the car, but as I was having all sorts of engine shaking issues early on this change was an attempt to cure the shakes.
Carbys were set up with a colortune.
Now the ceramic of the plugs is white whilst the metal is black (sooty). This indicates running lean or excessive advance and since there is no pinking I assume that the advance is OK. The exhaust pipe is also black.

I am now thinking of putting the #5 needles back in, however, comparing the FX with #5 (see image below) indicates that at high carby piston lift they would be much leaner than the FXs, even though at low to moderate lift they are richer, if, say, they were set to be the same as the FX at the -5 station, then at the higher lift area they would be +5 leaner. (I hope this makes sense)
Would any SU guru be able to comment, please?

Thanks

Herb

Herb Adler

You are right in your assumption. The trick is to find/get a needle with the correct shape to suit your car.
I also have a 66 similar to yours and an O2 meter connected. The fluttering you describe is where/when the engine is running lean. I find #5 to rich and #21(also std. lean), although no perfect, the best I have. Keep in mind I’m in the Johannesburg area about 5000’ above sea level. I do not have a #FX to try, but see that it is richer then the #21 at the top end (which would make it to lean at the bottom and to rich at the top for my car). Just as you assumed, the #21 can be set richer using the mixture screw to compensate through the range which should give you the slightly richer lower end you require yet maintain the top end you require.
I use the comparison guide SUSURCH3.xls found on the internet. It’s a free download and very good.

Richard.
RH Davidson

Herb, try

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

It's seems a bit vague at first but just click on 090 or 100 needles at the top and select your current needle and then any other needles. It gives a clear graph of the needle stations for up to 4 comparison needles.

Tony
Tony Oliver

What condition are your throttle shafts in? Is there any play between the shafts and the carb bodies? If so, you have a vacuum leak that can be adjusted away at idle, but will become a problem at higher speeds. RAY
rjm RAY

Hi,

Thanks for your replies.
The shafts don't have any play.

However, driving the B this morning I had a Eureka experience. The figures in my first post are diameters of the needles at certain points and at higher lifts there is no difference in diameter, so there won't be extra leaning of the fuel. I was assuming that difference at one point would affect all points, sort of like an offset.

Tony
Thanks for that link, very illuminating.

I will try the #5s as I have them and they were the originals in the car.

Herb
Herb Adler

Hi Herb

What filter system are you running?

Peter
P Burgess

Hi All,

Sorry about the late reply but I've had no end of trouble with the internet, currently conecting thru my mobile phone while trying to sort out main connection problem.

Peter, using standard Coopers.

I have fitted the #5 s and the fluttering has gone. Also the exhaust, subjectively in the car, seems to drum less.

Thanks for your replies.

Herb
Herb Adler

Hi Herb
Interesting plug colouring
Is the black soot on the base of the plug dry to touch or dampish and is it black or dark grey
Also what colour is the earth tang and does it retain the same colour for it's full length or does it change as it gets further out If it changes what colour is it
When were the plugs pulled, - immediatley after the problem or when you got home. Willy
WilliamRevit

This thread was discussed between 07/06/2010 and 14/06/2010

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