Inlet Manifold to Fit SU Carburettor to a Ruby

[Austin in the Shed]

I would advise against using a 1 1/4" Su, unless you have a reasonably well tuned engine it will be too big so metering will be even more of a compromise. You would be far better off with a 1" or 1 1/8" instrument. To clarify the reasons; for an SU to work manifold depression causes the piston to rise and fall depending upon throttle opening and engine speed. If you use an oversize Carburettor the piston will only move a small distance rather than fully open when required, therefore you are trying to meter the fuel on a very narrow band of the needle. The consequence of this is lower performance than optimum in which case you may do better with a refurbished Zenith, they work well in good condition. Although most that have been through my hands need new throttle spindles at the very least.

I have a couple of cars running on 1 1/8 Su's and both run really well from low down.I also have one fitted with the sports type log manifold and 1 1/4 SU and no matter what Alterations I've done the best I have got is power from now 1500 rpm. Not managed yet to swap carb and manifold to a smaller one to see how they compare however.