Most likely over flowing float bowls. The HIF6's are tidier in the sense that they overflow internally. The HS6's just have a little vent cap at the top of the flow bowl and fuel runs down onto the generator/alternator and exhaust manifold. !!FUN!!

As for why they are overflowing - here's a couple of the usual suspects:
- fuel pump pressure - the float valves canna handle the pressure, Captain! Anything more than 5 psi or so and the fuel pump can just overpower the float valves and keep on squirting fuel into the bowls.
-sinking/bent/maladjusted float valves - the floats can crack and sink, although thi is rare. They have a little tab which pushes on the needle valve which can get bent - although usually only when someone has been monkeying with them. Worth checking to see, though.
- grit - not that any old car would have specks of rust in the fuel system - NO WAY! It probably gets in from the fuel pumps at gas stations (my Dad's diesel Land Rover had a mesh screen in the fuel filler - presumably to filter out leopard scat whil on the veldt - and it often caught surprising amounts of junk from station hoses). In any case - for whatever reason it's in the fuel system, isn't cauge by a filter, and ends up arriving at the needle valve - where it gets jammed in between the needle and seat. Holding them apart means that the valve is open and gas can keep coming in. And an overfull float bowl keeps the needle firmly pressed against the grit so nothing budges. If the needle valve would just open up farther the grit would just go through - so sometimes as a quick fix you can pinch off the fuel supply between the pump and carbs until they empty out a little (i.e. the car starts to falter at idle) then release - and the rush of fuel going past the wide open needle valves washes the offending grit past the valve...
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I'm JohnMc, and I approved this message.