Should I rebuild my SU carburettors or just tune them?

Tune them if nothing is worn. Rebuild, or send them to a specialist, when the throttle spindles or jet bearings are worn, the jet glands weep fuel, or the mixture will not hold. A worn throttle spindle is a permanent air leak and no tune will hold.

Which needle does an MG T-Series SU use?

The standard needle for normal running is commonly marked ES, with a 0.090 in jet whose head is marked 9. Verify both against the workshop manual for your car. A weaker needle suits sustained running above roughly 6,000 ft, and engine modifications may need a different profile.

Are HS or HIF carburettors correct on an MG T-Type?

No. The T-Series used the earlier H-type SU. The HS and HIF are later designs; HS carburettors are sometimes found on T-Types as replacements, and their adjustment details differ.

SU Carburettor Rebuild: Parts, Needles & Overhaul Guide

Every MG T-Type runs on SU H-type carburettors. The HS and HIF are later designs. This guide covers what a rebuild involves on the H-type and when it is needed at all.

For how the carburettor works, read how carburettors work; for the component guide and servicing routine, the SU Type H carburettor guide; and for setting the pair up afterwards, setting up twin SU carburettors.

Rebuild or tune?

The H-type is one of the easiest carburettors on any classic engine to live with. A light service, topping up the damper oil and cleaning the pistons and suction chambers once a year, keeps most in order, and a tune is enough when nothing is worn.

A rebuild is due when the wear is mechanical: throttle spindles or their bearings worn, a jet gland weeping fuel, a float chamber that floods, or a car that will not idle slowly or lean out once air leaks have been ruled out. A worn throttle spindle is a permanent air leak and no tune will hold. Badly worn spindles, worn jet bearings or a full rebuild are best left to a carburettor specialist with the correct reamers and gauges.

The parts a rebuild renews

SU carburettors and parts are still made, so needles, jets and rebuild kits are readily available. Kit contents vary by supplier; check the list against the parts the archive renews whenever the carburettors come apart:

jet glands and gland washers;
gaskets and seals disturbed in the strip;
fibre washers and split pins, always new;
two new screws for the throttle disc;
the float needle valve, if dirty or worn, and the float if it is punctured;
any perished fuel hose, replaced with ethanol-resistant hose.

The parts trade is listed in Parts and Specialists.

Jets and needles

The needle profile is chosen during engine development and is correct for a standard engine in normal conditions. Two situations call for a change: sustained running at high altitude, above roughly 6,000 ft (1,800 m), where a weaker needle is fitted; and engine modifications such as a camshaft, compression, air cleaner or exhaust change, which may need a different needle for the new airflow.

The standard needle is commonly marked ES, and the standard jet is 0.090 in, its head marked 9. A 0.100 in jet marked 1 is usually wrong for a standard engine. Verify these against the workshop manual for your car. Refit the needle with the shoulder on its shank exactly flush with the face of the piston. On a TD Mk II the piston return spring is red, and on the TF and TF 1500 it is blue.

Pistons and suction chambers are matched pairs: never swap a piston into another chamber.

Float and float chamber

Flooding at the float chamber or jet usually means an incorrect fuel level, a dirty or worn needle valve, or a punctured float. Clean or renew the needle valve and renew any float that rattles or sloshes when shaken. Check the float level and jet centring when tuning, and do not assume they are still right after work elsewhere in the fuel system.

Reassembly cautions

Mark the suction chamber to the body before separating them.
Lift the chamber straight off without tilting, or the needle bends.
Keep track of the piston spring and any thrust washer under it.
Leave the jet locking nut slack until the jet has been re-centred.
Do not disturb the cork washer under the jet locking nut.
Mark the oval throttle disc before removal and spread the split ends of its new screws.
Never use abrasives on the piston or chamber; they are a precision fit.

Common questions

Should I rebuild my SU carburettors or just tune them?: Tune them if nothing is worn. Rebuild, or send them to a specialist, when the throttle spindles or jet bearings are worn, the jet glands weep fuel, or the mixture will not hold. A worn throttle spindle is a permanent air leak and no tune will hold.
Which needle does an MG T-Series SU use?: The standard needle for normal running is commonly marked ES, with a 0.090 in jet whose head is marked 9. Verify both against the workshop manual for your car. A weaker needle suits sustained running above roughly 6,000 ft, and engine modifications may need a different profile.
Are HS or HIF carburettors correct on an MG T-Type?: No. The T-Series used the earlier H-type SU. The HS and HIF are later designs; HS carburettors are sometimes found on T-Types as replacements, and their adjustment details differ.