Well, if its as you say and the problem definitely only started after they did the work then you should definitely give it back to them. And you should not expect to pay a single penny extra if that is the problem.

For them to say it was the carb is cheeky unless they were so certain that the timing is correct.

It's tricky as a mechanic when customers come back to you after a job as there are sooooooo many people who try to rip workshops off with the "it was fine before you touched it routine"

When I was working for BMW we had one guy who would bring his bike in to have very minor work done (tyres checked, valet, handlebars adjusted). Very cheap jobs. He'd then come in saying his ABS didn't work anymore and we'd have to work our lunchtime to diagnose it etc. His ABS pump (£1000) was shot and he blamed us for it. Kicking and screaming. Only when I printed him the fault reports of the recorded dates of error codes (before he came to us) did he know his game was over and stormed out..

Loads of similar stories...

Anyway, back to the point..

Be 100% sure the bike didn't have the problem first.

Sometimes when you do a job on an older bike, you can upset things when you disassemble it. The cam chain might be fine but they've knocked or broken something else. If thats the case then its very difficult to place blame. They should have mentioned it (if they realised).. And on the other hand, a workshop/mechanic shouldn't be at fault when they break a heavily corroded bolt or a previous bodge etc.

First though... Have that filter checked. When they removed the tank a lot of muck might have been displaced and now is partially blocking the filter.

Your symptoms are very very similar to a blocked carb filter. Enough fuel gets through for low speed running but not enough for higher rpms so the bike stutters.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.