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Welcome to the relaunched Carb.SU

After much delay I have relaunched the Carb.SU website. This is intended to grow over time to become a very useful resource for all models of SU Carburettors, as fitted to many British classic cars such as Jaguar, MG, Morris, Austin, and more. The main goals are to help with identification of unusual SU carbs, and to try and assist classic car enthusiasts with troubleshooting and maintenance.

OM 1” SU Carb

The first real information post is available [ here ] for the OM 1” SU carb, often fitted to early Morris and Austin vehicles.

More Information

If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: tim@carb.su

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The 1" OM SU Carb

The OM is a 1" bore SU carb fitted to prewar Morris Minors, Austin 7s and in pairs on some MGs. In rare setups triple OM Carbs are fitted. It was often fitted to the Morris Cowley as a replacement for the Smiths 5 jet. The casting number on the main body is usually 617T.

Identification: The OM has no front mounting flange, and was often run withot an air filter of any kind. The rear flange has two mounting holes, one either side of the carburet`tor bore.

It has no fast idle feature when the choke lever is pulled, and no piston damper, so only a small amount of oil is needed in the top of the bellhousing. The piston is usually of the weighted type, without a piston spring. The piston assembly from a 1" H1 carb can be fitted, which will give you a sprung piston and a damper rod for improved acceleration.

The throttle plate can be very tricky to install and remove, as it only fits into the bore in the fully open throttle position. There were not any different bore diameters made of this type, although the UB and UBA carbs are very similar to this design.

The standard piston lid was often a black bakerlite material, but many enthusiasts replace this with a brass equivalent.

Photos of OM SU Carb

More Information

If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: tim@carb.su

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The 1 1/4" HS2 SU Carb

The HS2 is one of the most commonly available SU carbs, found in single carburettor form on the Morris Minor 1000 and often the Austin A35. It is also found in twin carb configuration on the MG Midget and Austin Healey Sprite.

Identification: The HS2 has a 1 1/4" bore, front flange with two horizontal threaded holes, and manifold mounting holes are unthreaded and positioned vertically above and below the bore.

They are almost identical to the HS4 and HS6, which are larger versions of this SU Carb. The bellhousing diameter is the same diameter as some earlier smaller carburettors.

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.

The piston top is usually black plastic, but is often replaced with a brass equivalent. Many twin carb configurations had a metal heatshield to protect the carbs from exhaust heat.

The HS2 has one significant difference to the other HS style carbs, in that it has s fixed needle. HS4 and HS6 carbs often had sprung needles instead, which makes alignment of the needle in the jet a bit easier.

They are easily sourced second hand, often for £10 or less for the single carb type. Twin carbs complete with manifold cost more.

Photos of HS2 SU Carb

More Information

If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: tim@carb.su

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The 1 3/4" HS6 SU Carb

The HS6 is the same design as the HS2 and HS4 SU carbs, this one being 1 3/4" size. It was used on some bigger capacity Minis, and on the Triumph Dolomite Sprint in a twin carb layout. There are some HS6 triple carb configurations aswell.

Identification: The HS6 has a 1 3/4" bore, front flange sometimes has 2 threaded bolt holes like the HS2 and HS4, but is more often found with a distinctive inverted triangle shaped flange with three mounting holes.

The rear flange is usually of the square 4 bolt type but the HS4 style diagonal bolt holes.

The bellhousing diameter is the same diameter as the HS4 carb, although the piston is different depending on the carb bore.

For some cars such as the Triumph Dolomite Sprint, there is a rarer HS6 type with a slightly shorter bellhousing, to provide extra bonnet clearance. These are harder to find, and the bellhousing, piston, and damper are all shorter and not compatible with the normal version of the HS6.

I believe the shorter versions were only found in twin carb arrangements. They can be handy sometimes for kit cars and other modified engine situations.

Photos of HS6 SU Carb

More Information

If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: tim@carb.su

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The 1 1/2" HS4 SU Carb

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

More Information

If you have any photos or other information on SU Carburettors, I would love to hear from you. please send me a message: tim@carb.su

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