I had this same issue a couple years back and it was the diaphrams.
I ordered a set from Joe Curto at http://www.joecurto.com
This guy knows his SU Carbs and has had a diaphram made from a better rubber
that holds up to today's gas and I have had no issues...
> Thanks to Don and all who responded,
> It looks like time to pull the carbs off for a rebuild. I suspect it is the
> diaphragm as the leak is just as Don described. Using my SU workshop manual
> I will carefully work through it a step at a time. These carbs are not very
> old; so, the throttle shafts should be (hopefully) in good shape.
> ----- Original Message -----
> From: "Don" <firstname.lastname@example.org>
> To: "'Andy Schwarz'" <email@example.com>
> Cc: "Healeys" <firstname.lastname@example.org>
> Sent: Sunday, June 19, 2005 9:38 PM
> Subject: RE: HD8 Carb Help Needed
> > Andy,
> > A torn diaphragm will leak from the bottom of the carburetor usually at
> > the junction where the diaphragm is sandwiched or a little above as I
> > recall. They are not that hard to replace even though the carbs must be
> > removed from the vehicle. I try to at least get the genuine SU branded
> > ones but some are more durable than the others. Others on the list may
> > be able to tell you of a more durable brand.
> > Don
> > HD-6's