Overflowing Fuel

[DevonS1]

Hi, I've recently re-commissioned a 3.8 complete with new needles and seats for all float bowls. The car ran fine for a few trips and was then laid up for a couple of months.
Now, as soon as the fuel pump is initiated fuel flow straight out of the overflow on the farthest forward float bowl.
All three floats are sound, floating fine and move freely. Needles and seats are correctly adjusted and newly replaced.
Any advice for the next step of diagnostics before I torch the thing for the insurance? Thanks

[cactusman]

Either the float has sunk or the float is stuck or most likely the float valve is stuck or a bit of detritus is stuck in the valve. Take the top off the housing and investigate? Modern fuels may damage the tip of the valve as they are usually rubbed tipped...mostly Delrin tipped...although in two months I'd be very surprised...
Only other reason would be excess fuel pressure...if you have a modern pump you need a pressure of no more than around 3 psi...any more will overcome the float valve....you might need to fit a pressure regulator.
The original submerged Lucas pumps have a pressure relief at the pump...seem to recall it is set to 2.5 psi.

[DevonS1]

Thanks Julian, appreciate the advice. The fuel pump is external rather than submerged, but is still a traditional low pressure SU unit. First job tomorrow will be to remove and inspect the valve as you suggest.

[abowie]

It's probably the needle stuck in the seat.

Before you pull it apart try tapping the top of the float bowl firmly. This may dislodge the needle.

[paydase]

In addition to tapping the top of the float bowl, another tip is to turn the engine sometime to allow fresh gas flooding the bowls and wait before firing it. I have noticed that it helps.
I think that after a (hot) run and non use of the car for more than a couple of weeks (in my case), heat soaking the engine and time passing may lead to gas evaporation, gum formation and needle sticking.
Letting fresh gas come in the bowl probably helps dissolving that gum and releasing the needle.

[phoenix]

Change metal floats for new plastic ones as the brass ones tend to leak.

[bitsobrits]

Hmm....in the last 50 years or so, I've owned 60 some automobiles and motorcycles that used carburetors. I currently have 6 older cars with 16 carbs between them. I've always checked the metal floats for fuel ingress as everything I read said to do so. And yet I've never experienced a defective/leaky float.

So I think "brass ones tend to leak" might be restated as "It is possible for fabricated brass floats to leak, but likely due to poor workmanship, not age, and any leaking ones were discovered and replaced ages ago when the cars were new. So check the floats, and if good, don't worry about them."