Odd fuel overflow from carbs

[paulbaker1954]

I have a very slight drip from the fuel tap when in off position and I am getting 3 carbs that will do a slight overflow overnight. (New tap on order from DSS)

What is weird though is if I leave the tap in the on position this doesn’t happen.

Am curious to figure out what is going on here. My daft theory is that if the tap is on there is enough liquid head to snap the floats closed but if there is only a slight drip going through the floats are not closing fast enough to seal the flow.

I have checked the float levels using my manometer tube and  all are fine.

Can’t get my head around this and wondered if anyone has any thoughts.

Bit fed up of going in to a fuel smelly garage each day😡😡

[McCabe-Thiele (Ted)]

I agree with your theory that when the tap is on there is sufficient fuel head/pressure for the float valves to shut off - so no leak but the pressure to shut the valve comes from the float arm not the fuel feed.

When the leaking tap is in the off position the vortice effect will mean that the resulting line pressure to the carbs will be substantially reduced rather than just the flow volume this happens when there is a substantial restriction in the valve or tap- my theory is that then as there is very little pressure on the top of the float valve it seeps as natural evaporation from the bowl occurs causing the needle valve to drop open.

As you may probably know reducing the flow in a domestic cold water tap by turning the tap head only reduces flow not pressure - this is true unless you have something that reduces the pressure. Pressure in liquids behaves in a  similar to voltage in a battery. I am guessing that the closed valve is acting as a restrictor. It's 50 years since I studied fluid technology at Uni so the detailed theory has gone! What you are left with is fluent pidgin english.