SU problems (I think)

[Ian Williams]

Its running too weak as revs rise (wrong needle) and nine flats down is too much making it rich at bottom end, damping is not the main issue here, you can get an SU to work well with very little damping at all, in fact that exactly what is needed in some racing situations.. I suggest reading up about how SU's work and needle selection there is a lot of good information available. I have reached a stage where I can not be bothered repeating it all as the same issue comes up again and again with lots of confusing advice for the unwary.

[Steve Clare]

Thanks for the excellent PDF John, and others for their comments.
I have ordered a GG needle, and will keep you posted!

[Hugh Barnes]

Just to add to the confusion, I have run both AN and GG needles and found little difference between the 2, eventually settling on the AN. This is with an early 1" OM series carb without all the complications of springs etc. so your experience may differ...

I think GG is just a little richer but has a very similar profile to the AN. Minty Lamb reveals all...

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

[Dennis Nicholas]

Its running too weak as revs rise (wrong needle) and nine flats down is too much making it rich at bottom end, damping is not the main issue here, you can get an SU to work well with very little damping at all, in fact that exactly what is needed in some racing situations..  I suggest reading up about how SU's work and needle selection there is a lot of good information available. I have reached a stage where I can not be bothered repeating it all as the same issue comes up again and again with lots of confusing advice for the unwary.

Well said Ian.  It is always best to understand how something works before messing with it.  A little time (sometimes a lot!) pays dividends in the long run getting things right.
A read up will show dimensions of needles and can thus even help in modifying ones needle using a file where too weak at one section.
Of course some equipment can be too technical for a lot of people, but at least a bit of effort does usually pay off.

Dennis

[Steve Clare]

Just reporting back as people were kind enough to offer support.
I sat down and read about how an SU carb. works. That was useful.
The piston is weighted, for those on the same learning curve, there's an obvious steel weighting disc on the top edge of the piston. So I removed the pale blue spring and re-introduced the damper with 3 in 1 oil.
I worked right back from the manifold- new gasket there, and all new gaskets for the carb.
I screwed the mixture screw in leaner, its at 7 flats out to run best. I then experimented with metering needles, AN, GG, and at my brother's suggestion a number 6.
The number 6 needle is hands down the best. The car didn't tick over very nicely on the GG, the AN was ok, but not as good as the 6.
I'm still fine tuning the mixture/ timing by road testing, but it's pretty good now.
It shouldn't be that every car is different with basically the same set ups, but that does perhaps seem to be the case.
Looking forward to my first speeding ticket.
Thanks again for all your helpful comments.

[Robert Leigh]

I find that the best cleaning fluid for the working parts is methylated spirits, with which I use an old toothbrush until the piston and dashpot have no signs of dark marks. Then check that the piston rises and falls with no trace of a scraping noise between the piston and dashpot. The piston should move freely by gravity and covering the top of the dashpot with your thumb should cause the movement to be considerably slower as air has to be admitted past the piston through the small circumferential clearance.. Gritty noises are unacceptable and dashpot and piston can be cleaned again if this happens using a lint free cloth with methylated spirits. Only after getting this bit right is it worth experimenting with different needles, damper or no damper, etc.