The 1 1/2" HS4 SU Carb7th of November, 2019
The HS4 is the same design as the HS2 and HS6 SU carbs, this one being 1 1/2" size. It was commonly used on later Minis in single carb setup, and on MGB B series engines in twin carb layout. I am not aware of any triple HS4 setups, other than perhaps aftermarket modifications.
Identification: The HS4 has a 1 1/2" bore, front flange with two horizontal threaded holes, and either 2 or 4 manifold mounting holes unthreaded. On the two bolt version, the bolt holes are placed diagonally opposite, and on twin carb setups this prevents installation of the two carbs the wrong way round.
The bellhousing diameter is the same diameter as the larger HS6 carb, although the piston is a different size to match the bore of the carb.
As with the HS2, when found in twin carb configuration, one carb is "left handed" having the float bowl on the other side, usually configured with both float bowls on the outside. Some more compact setups have both float bowls on the inside, between the two carburettors.
Some HS4 and HS6 carbs had a waxstat style jet holder, which is much larger than the previous one designed to weaken the mixture once the engine was warm to improve economy. They are often replace with the earlier fixed type jet, as they are harder to tune and in some hotter countries people have complained about the mixture being too weak, or idling problems.
Unfortunately the control mechanism for raising and lowering the jet is different for this type of carb, so swapping the jet style involves either an adapter piece attached to the older style jet, or taking several parts from an older style carb.
Photos of HS4 SU Carb
If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: firstname.lastname@example.org