Ownership / Diagnostics
Triumph Stag troubleshooting: won't start, overheating and faults
By Jon Cosson
Answer a few questions about what the car is doing and the troubleshooter will point you at the most likely area, then read the reference below for the detail.
Interactive troubleshooter
Step 1 of 2
When you turn the key, what happens?
Most likely area
Hot fuelling or ignition, a classic Stag trait
Starting cold but refusing to restart hot is one of the best-known Stag traits. The usual causes are fuel vaporising in the carburettors over the hot engine, a maladjusted or faulty Stromberg temperature compensator, or a coil or electronic module misbehaving when hot. Try cranking with the throttle held open, then look at the temperature compensators and the coil. A coil that has become too hot to touch is worth suspecting even if it is new.
Read: Triumph Stag overheating and coolingHow does it run badly?
When does it overheat?
Most likely area
Charging system
An ignition light that stays on, or a battery that keeps going flat, points to the charging system. Unlike older cars the Stag has an alternator, so check the drive belt, the alternator output and the wiring.
Read: The Stag's electrical systemMost likely area
Battery or earth
No lights usually means no power is getting through. The Stag is 12-volt, negative earth, so check the battery is charged and that both terminals and the earth strap to the body are clean and tight. A poor earth is a common cause.
Read: The Stag's electrical systemMost likely area
Starter or battery
Lights but no crank points to the starter side: a battery too weak under load, a poor connection at the starter or solenoid, or the starter itself. Check the battery under load and the heavy cables and earth first. One symptom narrows it straight away: a rapid hammering or chattering with no rotation is the solenoid pulling in and dropping out as the voltage collapses under load, which means a supply or earth fault rather than a failed starter. Be suspicious of anything added to the earth side, such as a battery isolator or quick disconnect.
Read: The Stag's electrical systemIt cranks well. To split fuel from spark: pull a plug lead, hold it near bare metal and crank. Is there a fat blue spark?
Most likely area
Ignition timing or a heat-affected part
A misfire, especially under steady throttle, is often ignition: timing drift, a coil overheating, a tired condenser on points, or an electronic module affected by heat. Check timing and dwell first, then the coil and leads. If the misfire only appears once the car is hot there is a free test: cool the coil with chilled water when it starts, and if the car runs properly again until it heats up, the coil is the likely cause.
Read: The Stag V8 engineMost likely area
Carburettors
Poor idling comes back to the Zenith-Stromberg carburettors: mixture and idle speed, the diaphragms, the temperature compensators, or an air leak past the O-ring at the base of the carburettor. Check the carbs and for air leaks.
Read: The Stag V8 engineMost likely area
Tune, fuel or timing chains
Low power is usually tune (timing and mixture) or fuelling; if it comes with a rattle from the front of the engine, the V8's timing chains may be worn, which also retards the valve timing. Check timing and mixture, and listen for chain rattle.
Read: The Stag V8 engineMost likely area
Airflow: fan, radiator and coolant
Overheating only in traffic points to airflow and cooling capacity: a tired or wrongly-wired electric fan, a radiator that is furred or undersized, or an old coolant mix. The radiator, the fan and a correct modern coolant with the right corrosion inhibitor for the Stag's mixed alloy and iron construction all matter.
Read: Triumph Stag overheating and coolingMost likely area
The cooling system or the tune
Hot all the time points to the cooling system or the tune, and on a Stag the list is long: coolant level and the correct inhibitor, the water pump, the thermostat, a furred radiator or block, ignition timing, even leaking intake-manifold gaskets or an air leak at the carburettors. The overheating guide works through the lot.
Read: Triumph Stag overheating and coolingMost likely area
Fuel
A good spark but no start points to fuel. The Stag has a single SU electric fuel pump, in the boot area beside the tank. Switch the ignition on with the engine stopped and listen for it ticking, then check that fuel reaches the Stromberg carburettors and that the float chambers are neither dry nor flooded. A stuck float or a blocked jet is common after standing. If the pump is silent, check the inertia cut out switch on the right hand bulkhead before anything else.
Read: The Stag V8 engineMost likely area
Ignition
No spark points to the ignition. On a points system the dwell and condenser are usual suspects, with the coil, cap and rotor. Check which distributor is fitted before setting dwell. Rimmer Bros publishes these supplier figures: single points 26 to 28 degrees, twin points 29.5 to 33.5 degrees, and USA models 34 to 38 degrees. The archive has not verified them in a factory document. SC Parts lists single-point cars through commission number LD14158 and twin-point cars from LD14159, but distributors can have been changed, so inspect the fitted unit rather than relying on the commission number alone. The archive has not established a verified identification rule for the USA variant. If the car has electronic ignition, which many now do, a heat-affected module or coil is common. Start there.
Read: The Stag V8 engineMost likely area
Fuel and spark
An engine needs both. Fuel: the single SU pump ticking with the ignition on and the engine stopped, and fuel reaching the Strombergs. Spark: on points, the dwell and condenser; on electronic ignition, a heat affected module or coil. Work through both.
Read: The Stag V8 engineCommon Triumph Stag problems
The Stag's reputation was made and marred by its 3-litre V8, and most of the faults owners meet are well known and fixable. This page starts with the troubleshooter above and sets out the common problems in more detail below.
Overheating
This is the defining Stag issue. The V8's mixed aluminium and iron construction and its marginal original cooling mean overheating must be taken seriously, because a badly overheated engine can warp the alloy heads. The causes range widely: a tired or undersized radiator, a failing water pump, a stuck thermostat, an old or wrong coolant without the right corrosion inhibitor, retarded ignition timing, a weak mixture, leaking intake-manifold gaskets, or an air leak at the carburettors. The common cures are a good radiator, an uprated or correctly-wired fan, annual coolant changes with the right inhibitor, and correct timing.
Hot starting
A Stag that starts from cold but refuses to restart when hot is usually suffering fuel vaporisation in the Stromberg carburettors over the hot engine, a faulty temperature compensator, or a heat-affected ignition coil or module. Cranking with the throttle held open often gets it going while you address the cause.
Ignition
Check which distributor is fitted before using a dwell figure. Rimmer Bros publishes 26 to 28 degrees for single points, 29.5 to 33.5 degrees for twin points, and 34 to 38 degrees for USA models. These are a supplier's published figures, not figures this archive has verified in a factory document. SC Parts lists single-point cars through commission number LD14158 and twin-point cars from LD14159, but distributors can have been changed, so inspect the fitted unit rather than relying on the commission number alone. The archive has not established a verified identification rule for the USA variant. Many cars now run electronic ignition. Timing drift and a heat-affected coil or module are the usual causes of a misfire.
Carburettors
The Stag runs twin Zenith-Stromberg 175 CD carburettors, with the diaphragms, the temperature compensators and the O-rings at the carburettor base the usual points of wear and air leaks. The single SU electric fuel pump lives in the boot area beside the tank and should tick when the ignition is switched on.
Electrics and charging
The Stag is 12-volt, negative earth, with an alternator. A charging fault is usually the drive belt, the alternator or the wiring, while a completely dead car is often down to a poor earth.