After not being used during the pandemic my wifes 86 650 wouldn't run. The carb has been rebuilt (by a shop) yet still doesn't run smoothly, it has been back but is little changed and we have lost faith in them.

The bike starts and runs but 'stumbles' and looses power - it wont exceed 50 mph on the flat. When we got it back it would idle for a short while and then stop; I have increased the idle speed and it now runs without stopping but there is a distinct stutter/stumble every 20 seconds or so. I have tried adjusting the fuel mixture on the carb but without much difference.

Same thing happens when riding. I suspect some sort of air leak.

Any ideas?

Thanks

My first place to investigate is fuel delivery.  Bad stuff in the tank or fuel filter.  Are you running a fuel filter?  The stock vacuum petcock sucks on these bikes.  An authentic Yamaha 660 Raptor petcock is the way to go, and cap off the vacuum nipple on the carb.  But check your fuel delivery first.