Triumph · 1970–1977The Stag Archive

How-To & Repairs / Fuel/Carburettors

Hot-starting and vapour lock

How to fix the classic Stag hot-start problem: the car starts perfectly cold or warm, refuses to restart after a short stop in hot weather, then frees up after 15–30 minutes' cooling. The cause is fuel vaporising in heat-soaked lines and carburettors, and the fixes are all about heat management — not more fuel pressure.

Tools needed

  • Basic spanner and screwdriver set
  • Pipe cutter and flaring tool (if reworking metal fuel line)
  • Thermal / heat-reflective sleeving
  • Multimeter (to rule out a hot coil)
  • Clean rag and small container to catch any fuel spillage

Parts needed

  • Non-return (anti-drainback) valve for the fuel line
  • Phenolic insulating spacers to suit the Zenith-Stromberg carbs (optional)
  • New fuel filter, mounted away from the engine bay
  • Fresh fuel hose and clips

Warnings

  • Work on the fuel system with the engine cold and no ignition sources nearby.
  • The Stag requires low fuel pressure only — never fit a higher-pressure pump in an attempt to "cure" hot-start.

Steps

  1. 01. Confirm it really is vapour lock

    The signature is a car that starts flawlessly cold or warm, but refuses to restart after a short stop in hot weather — the classic petrol-station no-start — and frees up after 15–30 minutes' cooling. The electric fuel pump often ticks continuously as it tries and fails to prime.

  2. 02. Rule out a hot ignition coil

    A heat-soaked coil can mimic vapour lock exactly. Before touching the fuel system, confirm there is a healthy spark when the fault is present. If spark disappears when hot and returns as the coil cools, treat that separately — it is not a fuel problem.

  3. 03. Address any underlying overheating

    Vapour lock is much worse on a car that is running hotter than it should. If the cooling system needs attention, deal with that first — otherwise you are treating a symptom, not the cause.

  4. 04. Relocate the fuel filter out of the engine bay

    A fuel filter mounted in the engine bay is a heat trap. Move it to a cooler location on the chassis, upstream of the engine, so the fuel entering the carbs has not been sitting next to hot metal.

  5. Owner-uploaded photo slot
    Fuel filter in its new location. The fuel filter relocated out of the engine bay to a cooler spot on the chassis.

  6. 05. Reroute and insulate the fuel lines

    Route the fuel lines away from the exhaust manifolds, heater pipes and any other hot components, and sleeve the sections that must pass close by. The aim is to keep the fuel below its vapour temperature all the way to the float chambers.

  7. Owner-uploaded photo slot
    Fuel line routing and heat sleeving. The fuel line rerouted away from heat sources, with any necessary heat sleeving fitted.

  8. 06. Fit a non-return valve

    Fit a non-return valve near the pump so fuel cannot drain back down the line when the engine is shut off. This keeps the carbs full and shortens hot cranking time considerably.

  9. 07. Consider phenolic spacers under the carbs

    Phenolic insulating spacers between the carburettors and the inlet manifold reduce the heat soaked into the float chambers after shutdown. They are a proven, low-cost improvement on the Stag.

  10. Owner-uploaded photo slot
    Phenolic spacers under the carbs. Phenolic insulating spacers fitted between the Zenith-Stromberg carbs and the inlet manifold.

  11. 08. Do not fit a higher-pressure pump

    Owners sometimes swap in a modern high-pressure fuel pump and make the problem worse. The Stag needs only low pressure — around 2.7 psi. Excess pressure floods the carbs when hot and gives you a fresh no-start problem to chase.

    Warning:Do NOT fit a higher-pressure replacement fuel pump. Confirm the correct pressure specification from the workshop manual.

Related faults

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Further reading

Figures and procedures should be confirmed against the official Triumph workshop manual; for safety-critical or specialist work, consult a specialist.

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