Sorry to say but that mechanic does not know a thing, The tach reads off of the number 2 coil there is no wires running to your carburetor, as a matter of fact no tachs run off of the carbs!! It is more then likely a bad ground or dirty connections, if none of that after you have cleaned them up run the test thats in the tech forums. But if I was you I would not take this bike back to that shop just my opinion.