Starting in Extreme Heat

Why the Stag is especially prone

Two mechanisms turn fuel to vapour in the wrong place:

• Vapour lock, fuel boils on the supply side (the pipes, the engine‑bay fuel filter and the mechanical pump), so the pump pushes vapour instead of liquid and the engine is starved. The Stag’s engine‑driven mechanical pump and its engine‑bay fuel filter are classic vapour traps, sitting right in the heat.
• Percolation, heat boils the fuel in the float chambers of the twin Zenith‑Stromberg carburettors, flooding the engine over‑rich (the tell‑tale petrol reek, and sometimes a cough of flame back through the carburettors on a restart).

And here is the Stag’s particular problem: it already tends to run hot. A hotter‑than‑average engine bay means more heat soaking into the fuel, so the car that most needs its cooling in good order is also the one most likely to vaporise its fuel. Modern ethanol‑blended petrol, which boils far more readily than 1970s fuel, makes it worse again.

The heat-soak trap

The worst moment is not while driving, it is 10 to 30 minutes after you switch off. With the engine stopped there is no coolant flow and no airflow, so stored heat soaks up into the fuel system and boils the petrol sitting in it. Start straight away and it is usually fine; wait an hour and it is fine; it is that middle window, at the pub, the fuel station, the show field, that catches you out.

The symptoms

• Runs perfectly, but will not restart after a short stop on a hot day.
• Cranks but will not fire (starvation), or fires, stumbles and reeks of petrol (flooding).
• A rough, hunting idle and stalling in slow, hot traffic.
• Always comes good once cool, the sign that it is heat, not a fault.

The roadside cure

1. Cool the fuel system. Park in shade and open the bonnet. If you have water, pour or wipe it over the fuel pump, the engine‑bay filter and the fuel line to condense the vapour back to liquid.
2. Give it a few minutes with the bonnet up.
3. Clear a flooded engine by holding the throttle wide open while cranking; keep it running once it catches (a puff of smoke is normal). Do not sit endlessly cranking, which only floods it further.

Preventing it

Physical fixes work; additives rarely do:

• Cure the cooling first. On a Stag this is doubly important, a healthy cooling system, correct timing and a good fan keep the whole engine bay cooler, which is the root of the problem. This is the single most Stag‑specific step. See cooling & overheating.
• Insulate and re‑route the fuel line away from the exhausts, and move or insulate the engine‑bay fuel filter, it is a notorious vapour trap. Use modern ethanol‑rated hose throughout.
• Fit a heat shield or insulating spacer at the carburettors to slow heat into the float chambers.
• Beat vapour lock at the pump. Because the Stag’s pump is engine‑mounted, many owners fit an electric pump near the cool tank, which pushes cool fuel forward under pressure and resists vapour lock; a bleed‑back‑to‑tank line also helps by keeping fuel circulating (it makes the pump tick more, but it works).
• Choose your fuel. Ethanol‑free or low‑ethanol super unleaded (E5) vaporises less readily than E10.

Do not misdiagnose it

It is easily mistaken for a failing pump, worn carburettors or an ignition fault, and money gets spent chasing the wrong thing. The give‑away is the pattern: only when hot, after a short stop, always right once cool. On a Stag, rule the heat, and the cooling system, in first.

Common questions

Why won't my Triumph Stag restart when it is hot?

Almost always because the petrol has vaporised before it reaches the cylinders. Either the fuel boils on the supply side, in the pipes, the engine-bay filter or the mechanical pump, so the pump pushes vapour and the engine is starved, or it boils in the float chambers of the twin Zenith-Stromberg carburettors and floods the engine over-rich. The Stag's tendency to run warm and modern ethanol-blended petrol both make it more likely.

What is the difference between vapour lock and percolation?

Vapour lock is fuel boiling on the supply side, before the carburettors: the pump moves vapour instead of liquid and the engine starves. Percolation is fuel boiling inside the carburettor float chambers, which floods the engine over-rich, the tell-tale reek of petrol, and sometimes a cough of flame back through the carburettors on a restart.

Why is the worst moment shortly after switching off?

With the engine stopped there is no coolant flow and no airflow, so stored heat soaks up into the fuel system and boils the petrol sitting in it. Restart immediately and it is usually fine; wait an hour and it is fine. It is the ten-to-thirty-minute window, at the pub, the fuel station or the show field, that catches owners out.

How do I start a hot Stag at the roadside?

Park in shade, open the bonnet and let the fuel system cool for a few minutes; if you have water, pour or wipe it over the fuel pump, the engine-bay filter and the fuel line to condense the vapour back to liquid. If the engine is flooded, hold the throttle wide open while cranking and keep it running once it catches. Do not sit endlessly cranking, which only floods it further.

How can I stop it happening again?

Physical fixes work; additives rarely do. Cure the cooling system first, insulate and re-route the fuel line away from the exhausts, move or insulate the engine-bay fuel filter, fit a heat shield or insulating spacer at the carburettors, consider an electric pump near the cool tank, and use ethanol-free or low-ethanol super unleaded (E5) rather than E10.