1966–1973 Triumph GT6 Workshop Manual Supplement (P/N 545048)

• Covers Triumph Spitfire Mk.3 and TR4A models equipped with the US-market emission-control system, specifically the S.U. carburetor configurations developed for 1968 Federal hydrocarbon and carbon-monoxide requirements
• Factory Triumph workshop supplement, Publication Part Number 545048, 1st Edition, issued by the Service Division of Standard-Triumph Sales Limited
• Identifies S.U. emission carburetor N.A.D.A. Specification AUD 285, along with model-specific needles, jets, dampers, distributor assemblies, and emission-control settings
• Contains 20 numbered factory illustrations covering ignition timing, distributor testing, carburetor assemblies, linkage adjustment, jet setup, and crankcase emission control
• Supplements the standard Spitfire Workshop Manual P/N 511243 and TR4A Workshop Manual P/N 510322 rather than replacing the full vehicle workshop manuals

Inside the Supplement

This publication deals specifically with the changes Triumph made to the Spitfire Mk.3 and TR4A for US exhaust-emission compliance. It is not a general GT6, Vitesse, or complete-vehicle repair manual. The PDF consists of the factory supplement covering emission-related engine specifications, ignition setup, S.U. carburetor service, and crankcase ventilation, with the normal chassis and general mechanical repair material remaining in the respective base workshop manuals.

The Spitfire Mk.3 section documents the emission-version 8.5:1 compression-ratio cylinder head, Stellite-faced exhaust valves, revised 10-10-50-50 camshaft timing, special extended-range ignition distributor, crankcase emission-control valve, Champion UN12Y spark plugs, and twin S.U. emission carburetors. Triumph also explains the purpose of the throttle-disc poppet valves, mixture-adjustment restrictors, weaker metering needles, and modified piston dampers.

TR4A emission changes are narrower but clearly separated. Its twin S.U. carburetors receive revised needles and main jets, modified dampers, and throttle-disc poppet valves, while the special distributor allows a retarded low-speed setting without sacrificing the required advance characteristics at higher engine speeds.

Factory Service and Repair Coverage

The supplement goes beyond basic tune-up specifications. It identifies the factory procedures used to diagnose and repair the systems that directly affect emissions, including:

• cylinder-compression comparison and periodic compression checks
• distributor maintenance, dynamic ignition timing, and distributor performance testing
• basic and complete twin-carburetor synchronization and mixture adjustment
• S.U. carburetor dismantling, inspection, rebuilding, jet centering, and linkage setup
• float-chamber, throttle-disc, suction-chamber, piston, needle, and jet-system repair
• crankcase emission-control valve dismantling, cleaning, inspection, and reassembly
• checks using a compression tester, distributor tester, stroboscopic timing equipment, tachometer, carburetor airflow-balancing instrument, and exhaust-gas analyzer

The illustration set is particularly useful when identifying unfamiliar emission-specific hardware. Examples include Cylinder Compression Tester, Timing Marks – Spitfire Mk.3, Arrangement of TR4A Carburettors and Controls, Carburettor Jet Linkage, Checking Piston Drop, and Crankcase Emission Control Valve. Exploded component views identify individual jet, throttle, float, piston, diaphragm, spring, restrictor, and linkage parts rather than showing only complete assemblies.

Specification Highlights

The model-specific data pages give the figures needed to distinguish the two installations before tuning or distributor repair begins.

For the Spitfire Mk.3, specified idle speed is 800–850 rpm, fast idle is 1100 rpm, and ignition at idle is 2° ATDC. The emission distributor is P/N 214799, with a 0.015-inch contact gap and detailed centrifugal- and vacuum-advance test curves. Carburetor baseline data includes an 0.090-inch jet, AUC 8103 damper, DD needle, and an engine-warm idle CO range of 3.8–4.5%.

The TR4A uses an 850–900 rpm idle, 1100 rpm fast idle, TDC ignition setting, and distributor P/N 214805. Its distributor specification calls for a 0.014–0.016-inch contact gap, with separate centrifugal- and vacuum-advance limits. Carburetor data lists an 0.090-inch jet, AUC 8103 damper, QW needle, and 3.5–4.5% CO at warm idle.

Periodic service information is also specific. Distributor contact-gap and timing checks begin with the 1,000-mile service, distributor attention continues at 6,000-mile intervals, and ignition timing is checked again at 12,000 miles. The emission carburetors call for damper-oil and slow-running checks, with carburetor rebuilding or exchange recommended around the 48,000/50,000-mile point.

Several repair details matter because incorrect assembly can create faults that resemble tuning problems. The manual states that AUD 285 emission carburetors must not be substituted with non-emission-specification units. During carburetor repair, the needle must not be bent or scratched, and original orientation marks on the suction chamber and throttle components matter during reassembly. The flexible jet-tube gland is another damage point: excessive tightening can distort its sealing arrangement and produce fuel leakage.

Many of the tuning checks require the engine to operate at normal temperature while ignition and exhaust-gas test equipment is connected. Appropriate workshop precautions around hot components, rotating engine parts, fuel, and the ignition system remain necessary while carrying out these factory procedures.