This website uses cookies to improve navigation. No personal data is held. Do you agree to the use of these cookies?    
Classic Engines, Modern Fuel

Comments on Topic: Tests on other cars

Share this topic item by email
Submitted by The Author

I have a classic Fiat500 with hot start ( and run ) problems. The engine has been recently reconditioned ( about 50 miles) which might be part of the problem, but I have run some tests that might be of interest below.

I have fitted an Attwood in-line marine bilge fan taking air from bottom of the engine compartment onto the float chamber. The graph shows the temperature rise at idle until boiling occurs in the float chamber for three fuels, leading to miss-firing that progresses until the engine stalls. The plugs showed a white central insulator after stall, agreeing with Paul's predictions in his book. Before stalling, the plugs showed a good straw colour.

Thanks for a very useful book that has increased my knowlege of the mysteries of internal combustion engines.

Tests on Fiat500 at Idle ( May 2021 )


Tests performed at idle with electric fan thermistor ( RS 706-2743 ) on outside of float-chamber and connected to digital thermostat (W1209) set to 28 on / 26 off driving elec. fan Attwood 3000 bilge.

The thyristor and the small ( cheap ) digital thermostat printed-circuit board enables the carburettor ( or any other ) temperature to be displayed inside the car while driving which I found useful, if somewhat distracting !.

Red curve was with bonnet open, rest with bonnet closedLight blue=Anglo American ‘Storage 99 RON’ ,Dark blue=BP Super E5, Brown=BP premium 95RON E5


  1. With the bonnet open the electric fan keeps temperature within thermostat set points. With the bonnet closed the electric fan cannot cope with the trapped engine heat.
  2. With bonnet closed temperature rises until fuel boils in float-chamber and latent heat keeps temperature near to boiling point.
  3. Slight misfiring occurred at the green vertical dash line, and progressed until engine stalled at purple vertical dash line
  4. At stall the plugs showed white central insulator indicating lean mixture as predicted in Paul Ireland’s book.
  5. after stall, engine failed to restart until a delay of about 25 minutes ( ie 57min blue curve).
  6. The fixed jet Weber 28 IMB4 carburettor float chamber vents directly into the carb air intake and it looks as though this vapour is producing the restart problem ?. If inlet manifold is full of vapour presumably there is less oxygen and therefor too rich a mixture to ignite ?.

Please wait .....