Oil for SU carbs; reminder
 

Sorry to say I can't find the previous thread on this subject.
What have others found is best oil for the twin SU set-up, as used on P1800 and Amazon cars?

Good Morning!

Stuart here. You P1800 boys very kindly let me park my well worn dark blue Amazon with your lovely pristine cars at Kimbolton last year. Hopefully there ll be a few more of us this year to support you!

I use ATF fluid in mine and I understand that is usually best...why? I haven't the faintest idea but I keep mine topped up and it trundles along nicely up and down the M1 with a very decent bit of acceleration whenever I need it....which isn't often as practically all of the stretch I use has a 50 mph limit due to endless roadworks.

hope you are keeping well

Stuart

Carb oil
 

ATF is the recommended fluid i believe as stated in the Greenbooks.

Ric

ATF is what I have consistently used on Strombergs and SU`s over many years.

But as I am sure you are aware 20/50 will do, 10/40 ..even 3 in 1

The difference may be felt in the pick up on acceleration.

The green books do indeed say to use AFT however from many many years of working with SU's they do not respond well at all with ATF. They work much better with engine oil 10/40 or thicker.

Stromberg's are normally best with ATF

The thicker the oil the slower the pistons rise when you floor it. Slow rise gives more suction over the jet which sucks more petrol in giving the same sort of effect that an accelerator pump as fitted to a Weber for example. If it just gasps when you floor it, your oil is low or it's too thin.
The vertical Zenith fitted to early B18A engines has one of these and has a 2 position adjustment to give small or larger mounts of petrol when stepping on the gas. I suspect that some owners of these won't know this......

I bought some carb oil off the shelf here in sweden, was supposed to be good for zenith, stromberg and su's but not really happy with it and will be going back to engine oil...seems too thin and doesn't last long..

Does ambient temperature not play a role here? Why not try switching between engine oil in summer and ATF in winter?

I used ATF and SAE 30 in mine. I found that the ATF gets sucked out with a couple hundred miles but SAE 30 being thicker seems to resist this. Also being thicker I find that I get better performance due to the pistons rising slower.

Manuals seem to differ depending on model year and which market it's sold into.

Allegedly ATF is recommended because Engine oil changes its viscosity with heat, which of course its designed to do, and ATF isn't supposed to.

Although I cant see the temp in Carb dash-pots approaching anything like oil circulating in an internal combustion engine.