I'm now nearing the end of my Zephyr 550 rebuild. I started it up the other day and it ran well, but after a minute or so fuel started pouring out of the air box, and a few drips from an exhaust hole.

My first guess was a sticking needle valve allowing fuel into the carb body from the bowls so I stripped the carbs down, cleaned and rebuilt them, and reinstalled them in the bike, with the same thing happening again.

So I took them out, and this time tried them connected to the tank while off the bike.

What I found is that in the PRI position on the fuel tank (it's a vacuum tap), the bowls fill correctly with no leaking into the carbs, but then once the floats have shut off the feed, fuel then flows OUT of the nipple on the fuel tap which would normally be connected to the vacuum pipe to the carb body.

Now, one mistake I obviously made was having the tap permanently on PRI (on my last bike, "PRI" meant Primary, not Prime, and I'd never heard of a vacuum tap until I researched it today).

Obviously (well, it's obvious to me now I have researched it), on a vacuum tap you prime the carbs, then switch the tap to "ON" to let the vacuum do the work. Question is: is my tap working as it should, allowing fuel OUT of the vacuum nipple if I prime it too far? Or do I need to recondition the tap?

Thanks.