Where is it leaking from ? If the seat O ring is bad - it will rise to the air holes in the air box side and leak from there.
If the bowl gasket is bad - it will just leak from there.
Cool.
Buddha.

".Needle O rings"  Didn't need them? That is one of THE most consistent leakage spots on Suzi CV carbs. You can't see anything wrong with them , they just leak.

Sorry for the delay. When at their more 'natural' angle as they would be on the bike, the fuel seemed to be leaking from the middle of the carbs where the slide and needle jet are.

Then when directly upright, it also seemed to be leaking from the air holes on the lower airbox side (not sure which, or if both).

The float bowl gasket appeared to be fine.

Strangely I also noticed some fuel leaking from the hose circled in the image attached. I thought this was the air vent hose so I was very surprised to see this happen?

Quote from: gregjet on January 22, 2018, 01:21:55 PM
" Sometimes they may have a thumb print on em too."
Possibly taking creative pride a touch far?

Not as bad as this... https://www.theguardian.com/uk-news/2017/dec/13/surgeon-admits-marking-his-initials-on-the-livers-of-two-patients

Quote from: max on February 21, 2018, 11:21:28 AM
...but the conditions in my garage were unworkable...

Isn't this why we have kitchens?

Quote from: user11235813 on February 25, 2018, 05:39:03 PM

Quote from: gregjet on January 22, 2018, 01:21:55 PM
" Sometimes they may have a thumb print on em too."
Possibly taking creative pride a touch far?

Not as bad as this... https://www.theguardian.com/uk-news/2017/dec/13/surgeon-admits-marking-his-initials-on-the-livers-of-two-patients

Quote from: max on February 21, 2018, 11:21:28 AM
...but the conditions in my garage were unworkable...

Isn't this why we have kitchens?

It's a shared student house - my housemates don't like the smell of petrol as much as us   

Quote from: max on February 25, 2018, 05:18:34 PM
Sorry for the delay. When at their more 'natural' angle as they would be on the bike, the fuel seemed to be leaking from the middle of the carbs where the slide and needle jet are.

Then when directly upright, it also seemed to be leaking from the air holes on the lower airbox side (not sure which, or if both).

The float bowl gasket appeared to be fine.

Strangely I also noticed some fuel leaking from the hose circled in the image attached. I thought this was the air vent hose so I was very surprised to see this happen?

Looks like float seat O ring is bad.
That can caise the fuel to rise into that top vent hose, as well as leak from the slide and needle area.
Cool.
Buddha.

Quote from: The Buddha on February 27, 2018, 12:09:42 AM

Quote from: max on February 25, 2018, 05:18:34 PM
Sorry for the delay. When at their more 'natural' angle as they would be on the bike, the fuel seemed to be leaking from the middle of the carbs where the slide and needle jet are.

Then when directly upright, it also seemed to be leaking from the air holes on the lower airbox side (not sure which, or if both).

The float bowl gasket appeared to be fine.

Strangely I also noticed some fuel leaking from the hose circled in the image attached. I thought this was the air vent hose so I was very surprised to see this happen?

Looks like float seat O ring is bad.
That can caise the fuel to rise into that top vent hose, as well as leak from the slide and needle area.
Cool.
Buddha.

Hmm, I assumed this would be fine considering the seat and seat o-ring were replaced (or at least should have been), when I took it to the mechanic.

Is it worth purchasing some kind of hand vacuum pump to hook up to the gas inlet to test the seal using air pressure, or is there another step I should take first?

Thanks for the help so far!

Final update for the sake of closure.

I resorted to taking the carbs to another mechanic who tried to diagnose them.

As they were only leaking from the LHS, he swapped the parts between each side to verify it wasn't an issue with the new parts. The LHS still leaked and the RHS was still fine, suggesting that there was an underlying issue.

Rather than sink more money into labour, I bought a 2nd hand set of carbs on eBay. Bench tested these and checked the float bowls - no leaks and all was clean inside. Got them on the bike and it fired up with no issues.

Just need to replace my fuel lines (the 'on' line cracked during disassembly), which I've got a few questions about here: http://gstwins.com/gsboard/index.php?topic=72274.0

Thanks to all those that helped, I definitely learned a lot during this process!