Come back to a Giulia Spider after a hot-day stop and it can crank without firing, or fire and stumble — it is not a fault, but the petrol boiling before it reaches the engine. It is a well-known Alfa twin-cam trait in hot weather, made worse by modern fuel, and it is very manageable once you know it.
Why it happens
Two mechanisms turn fuel to vapour in the wrong place:
- Vapour lock — fuel boils on the supply side (the pipes, the filter and the engine-driven mechanical fuel pump), so the pump moves vapour instead of liquid and the engine is starved. It cranks but will not fire.
- Percolation — heat boils the fuel in the float chambers of the Weber carburettors, flooding the engine over-rich, with a strong smell of petrol.
The Alfa twin-cam is a known sufferer: the mechanical pump sits on the hot engine, and the Webers'' float chambers are right beside it. Owners driving in genuinely hot weather — 30°C and up, especially in slow going or after a hard run — report exactly this. Modern ethanol-blended petrol, which boils far more readily than 1960s fuel, has made it more common than it ever was when the cars were new.
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
- Cool the fuel system. Park in shade and open the bonnet. If you have water, pour or wipe it over the fuel pump and the fuel line — cooling the mechanical pump with a damp cloth is the classic trick that gets a stranded twin-cam going again.
- Give it a few minutes with the bonnet up.
- 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:
- The known Alfa fix: an electric pump near the tank. Because the original pump is engine-mounted and heat-prone, the standard cure — recommended across the Alfa community — is to add an electric pump at the cool rear of the car, which pushes cool fuel forward under pressure and largely banishes vapour lock. See the fuel system and pump article.
- Insulate and re-route the fuel line away from the exhaust and manifold, use modern ethanol-rated hose, and keep any engine-bay filter out of the heat.
- Fit a heat shield or insulating spacer at the carburettors to slow heat into the float chambers.
- Keep the [cooling system](/technical-library/cooling-system-corrosion-inhibitor-and-overheating) healthy and the engine bay as cool as possible.
- Choose ethanol-free or low-ethanol super unleaded (E5) over E10.
- Make sure the ignition and carburettors are properly set up — a car already running hot or rich has less margin.
Do not misdiagnose it
It is easily blamed on the pump, the Webers or the ignition, 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. That said, on the Alfa a genuinely tired mechanical pump can be a contributing factor, so it is worth checking the pump''s condition as part of the cure.
*Written for the Giulia Spider archive. Set-ups vary by car — confirm pump type and carburettor settings against the workshop manual, and take care around hot components and petrol.*