Carburettors for MGB engine

Details

Written by Stephen Tickell

Choice of carburettors will depend on your relative preference for originality, practicality and performance. The main options are discussed below. If you are going for out and out performance it is best to consult a specialist MGB tuner who can advise on the more exotic possibilities for road and track.

ORIGINAL MAGNETTE EQUIPMENT TWIN SU TYPE H4

Original ZB carburettors on the original manifold will preserve engine-bay appearance used in conjunction with the later "saucepan" oil bath air cleaner on the attractive moulded alloy air duct. You will need to consult SU or a tuning specialist to establish which jet needles to use to obtain best performance across the rev range. Disadvantages generally arise from older design features. Most H4s are well worn by now unless they have been properly rebuilt to include drilling and rebushing the throttle spindles where air leaks caused by ovalling will thwart your attempts to get the mixture right.

ORIGINAL MGB EQUIPMENT TWIN SU TYPE HS4

The "son of H4" being fundamentally similar in most respects but featuring different arrangements for transfer of fuel from float chamber to main jet. The MGB manifold must be used because the mounting holes are diagonally opposed rather than horizontally. The MGB manifold is a slight improvement on the ZB having a larger diameter balance pipe. Use of MGB equipment will also bring with it the need for modified choke and throttle linkages as well as a different heat-shield. The manifold also has drillings to accommodate various needs of the MGB design such as vacuum operated valves for crankcase breather and, possibly overdrive switch. Blank off with suitable plugs or screws unless you want to use them for other purposes like a brake servo.

LATER MGB EQUIPMENT TWIN SU TYPE H1F4

These are certainly not recommended for Magnette use. By the time these were fitted to the B it was already suffering from emission controls and they were designed as part of a system that actually reduced performance.

SPECIALIST EQUIPMENT

One advantage of fitting an MGB engine is that it gives the Magnette driver access to a whole range of special tuning equipment including Weber or Dellorto carbs on manifolds designed for the purpose. However it is suggested that such exotica are only contemplated for full competition use as part of a complete job to include blueprinting of joints, airflowing of ports and raising of the compression ratio. No such work should be done without similar upgrading of the braking system and in the reassurance that the car is structurally very sound!