This is an interesting question I wish I could answer, my interest is the fact I have a 1972 500l with a lot of engine mod work i.e Weber 40dco fast cam etc, one problem I have is every time I switch the engine off after a 15/30 minute run I get a loud backfire due to a build up of waste unburnt fuel. One suggestion from a very helpful forum member was to fit hotter plugs(you can follow my thread on classicfiat500) on the classic Fiat forum. at the moment the car is at an Abarth specialist trying to sort her out, so I will watch your thread with interest. Best of luck Barry
There's a risk here that we may now create two threads within one, but here goes anyway.
If the engine internals have been modified, it is possible that the combustion chamber may run hotter. (Or cooler) This may then need a plug that will dissipate the heat more quickly, a cooler plug, or more slowly, a hotter plug. Changing from standard would normally be triggered by evidence on the plug tip of runnign too hot or too cold. (Cold is a relative term here, petrol generally burns at around 850deg celsius. So quite warm.)
A backfire, is when fuel in the carburettor intake ignites, which is rare, and usually caused by ignition timing issues.
More likely what you are experiencing is unburnt fuel passing into the exhaust, and exploding there. For either issue, the spark plug is irrelevant.
The exhaust manifold receives the exhaust gases, which are only slightly cooler than the combustion temperature, so normally at least 800deg C. Any unburnt fuel passing through the combustion chamber is likely to burn or explode in the hot manifold. It is unlikely that it will sit as a puddle, or a cloud of vapour, waiting until the engine is turned off before it combusts. What is happening is that when the engine is turned off, the ignition stops, but as the engine turns for a moment or two, more fuel is sucked into the engine, and passes through into the hot exhaust. Pop!
This has been a potential, and occasional, problem with many engines over the last 100 years. In the 70s, and until cars all went injection, carburettors were often fitted with anti-run-on valves to alleviate this. Sometimes the fuel would burn in the cylinder, so the engine continued to run without an electric spark. Sometimes a fix (band-aid) for this was a cooler plug, not hotter. An anti-run-on valve can work in either of two ways. One type shuts off the air supply to the idle passages in the carb, whilst the other type shuts off atmnospheric pressure to the float chamber, discouraging fuel from being drawn through the jet. Fixed jet carbs tended to go with the former, whilst the latter was more for variable venturi carbs, such as SU or Stromberg.
Do you blip the throttle just before you turn off the engine? This is a poor practice, with a flawed reasoning, and serves no purpose other than to waste fuel and wash the oil from the bores. But with an engine spinning faster, the opportunity to draw fuel through is greater. With any engine, once stopped, allow the engine to idle for a few moments before turning off. This reduces the risk of drawing fuel through.