SU's

[DavidThompson]

The twin SU's adorning the 4.5 engine of my car are probably 90 years old, and they look as if they are in need of some TLC.
The pistons have a good deal of play in them, making it impossible to centre the needle. This, presumably, can't be right, or is it what I should expect?
The dash pots are empty of oil (or at least I can't see any) and the pistons do not seem to be damped. The legend on the top vent cover reads "Use Wakefield Oilit" . Any idea what viscosity oil I should use, and how far up the dash pot should it come?
The leading carburettor has some richness / weakness adjustment left, but the rear one has not, as its spring is completely tight. I guess I can reduce the size of the spring?
or do I need a new needle? The engine starts promptly and pulls OK but is running richer than I would like.
All responses greatly appreciated.

[Nicholas Simons]

It sounds as if your carbs need a well earned overhaul. Everything you mention is unacceptable.

[John Murch]

I do agree with Nicholas that a rebuild of the carbs would be a good idea.
With the SU carbs fitted to our cars, I always understood that one does not fill the dash pots with oil, but just the occasional few drips of 3 in 1 to ensure that the action of the ‘piston’ is smooth.
It is amazing how much wear there can be everywhere and still they function surprisingly well. At the very least replacement of the needles and seats would be wise. There is bound to be wear on the throttle spindles which can allow air in as well, which makes accurate tuning difficult.
The float chambers also suffer wear in all areas, so a full rebuild is not cheap.

[Philip Edginton]

May I suggest you try these people. let us know how you get on please.

http://sucarb.co.uk/contacts/

[DavidThompson]

I suspect you are correct and the way forward is a rebuild. Not quite what I anticipated when thinking I would just weaken the running mixture and balance the carbs.
I have had a number of cars using SU's, but mainly in period and not very old, so never really needing attention. However, I had always thought the dash pot contained a reservoir of oil to stop the piston flying up when suddenly opening the throttle.
An exception was a more recently owned AH 3000 Mk3 which had the carbs dangerously at the same side as the exhaust manifold. I remember a leaky float causing fuel to be pumped out of the overflow, straight onto the hot manifold, fortunately without any dramatic consequence.
I've ordered an SU manual which hopefully will guide me through the basics, but suspect will have little reference to anything as old as mine.

[DavidThompson]

Thanks Philip, I’ve used Burlen (same people) in the past and they have been helpful.

[Nicholas Simons]

Some early SU's ( maybe all ) don't have dashpots to give damping. Unscrew the knob at the top and see if there is a thin rod with a brass piston on the end. If not, you have no dashpot. My 1934 SU's don't have a dashpot. Lubricate with a drop of light machine oil.

[Stephen Blakey]

The dashpot on the SU is the transient enrichment device and has the same role as the accelerator pump on a fixed-choke carburetor.

When you put your foot down, the sudden increase in manifold pressure causes the vapour to condense leading to a temporary weakening of the mixture, hence the need for a system to temporarily add more fuel.

The SU system is cheap and efficient. The dashpot holds the piston down momentarily, which causes an increase in velocity over the jet increasing the pressure differential across the jet, thanks to Msr Bernoulli's equation, and hence more fuel flow. Clever.

Engines without a transient enrichment function have to be set to run rich all the time, or suffer from a momentary hesitation on tip-in (the industry term for a step change in throttle. A constant manifold-pressure increase being a "crowd")

[Nicholas Simons]

Do we know when the dashpot was introduced?

[John Murch]

It was 1936 when the H type SU was introduced which was modified in 1938 to include the enrichment device.