Zeniths like to accumulate fuel in the recesses created by the reinforcing ribs in the tops of the throttle housings, Webers have these reinforcing webs internal to the tops which helps shed fuel since the tops are smooth. A intake backfire can ignite the fuel vapor in the air cleaner as a result.
Check:
- fuel supply pressure while engine is running, something like 3 psi is fine with plus/minus .5 psi as a tolerance
- check fuel level in each float bowl with external gauge. This is to be performed with engine running. Adjust float needle valves to achieve fuel level using shims.
- Todays's fuels boil at lower temperatures than the fuels of 50 years ago. When you shut down your engine heat can easily be conducted from the heads to the carbs and boil the fuel in the bowls. This fuel will escape the bowls via the vent pipes located between the air horns (brass tubes) and through the auxiliary venturi which spills onto the throttle valve. This conduction may be minimized by installing insulator plates between heads and manifolds. You can install more than one set but you will need to remove OEM studs & replace with longer ones.
- The boiling fuel after shutdown fills the recesses in the top cover of the carbs which makes these little fuel reservoirs that evaporate gas into the air cleaner which is then susceptible to catch on fire with an intake backfire.
- A fuel pump cut-off switch can be used to good effect to kill fuel delivery in advance of shutting down the engine. This has been used by many in the past to control boiling fuel in the float bowls. It is also a good theft deterrent device.
- Another item to consider is to drill discrete drain holes in the reservoirs in the top covers to allow fuel to drain but this would drain the fuel over the carb bodies which would stain the bodies and possibly drain onto something hot...
- As in Webers, there is the possibility to drill internal passageways to re-direct boiling fuel down the throttle bores as opposed to it going up the vent pipes. Hot re-starts are to be endured.