One thing I've seen with Suzuki's is the vacuum line being connected and a breather connected to fuel outlet
(had hours of fun with one manager who refused to hear he was connecting it up wrong)
Its still pretty common, even today, to find Suzuki Bandits with an extra 3 or 4 litres of oil in crankcase when fuel is left on prime position for extended periods (even over weekend sometimes)
You probably have stuck shut of valves in carbs.
I like the GS series motors, easy to service and huge potential.
Cylinder heads are pretty restricted and need a lot of port work ( about 18~20 hrs
) as much as I hate to say it, its a far better motor than the CB550
A modified GS550 (620 kit, cams, exhaust etc) is about 20mph faster than a modified CB550, the benefits of a newer DOHC design (it starts with bigger valves than 550
)
The electrics suck big time though, so its not all good
all for now,
PJ