Flat spot below 4000rpm

[eidolon]

I've just bought a cheap winter hack (Suzuki GS500) which has been fitted with a stainless exhaust by the previous owner. After fitting the exhaust it apparently developed a flat spot at the lower end of the revs so he tried to mess with the carbs and the end result is that it won't idle properly and the only way you can pull away is if you rev to 4000 otherwise it cuts out.
It sounds like it needs rejetting and the carbs setting up but where should I start? (or should I just hand it over to my local bike shop and let them sort it?)

 

[eidolon]

The stock exhaust had rotted away which is why he fitted a stainless one. Dunno what he did with the carbs though, might be easiest to nurse it to my local bike shop and get them to sort it.

 

[eidolon]

I've stripped and rebuilt carbs on cars before so I don't imagine bike ones are too much worse but this week looks like nice weather so I might just use the last of the good weather to enjoy the ZX6R and drop the GS500 off at the garage to get them to sort it.