Just had the same issue with HIF SUs on my +8 The float neele valve has a filter above it and this was clogging with bits of rubber/plastic debris. Cleaned the filter and problem sorted. Just need to track down the source of the debris, I assume it due to corrosion because of the E10 fuel.
Have an in line filter but it's near the fuel tank
Luckily my carbs have the overflow tubes that vent below the sump
