Inlet Manifold to Fit SU Carburettor to a Ruby

[Ian Williams]

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.