I bought my ´81 CB650C a month ago. It had been sitting all winter and there was a significant amount of rust in the tank. It wouldn´t start, so I had the carbs cleaned and the tank Kreem coated. The petcock was leaking, so I replaced that. Also installed a fuel filter. The mechanic said the vacuum fuel shut off was not working, so bypassed it. Now it stalls out at about 5000 rpm under a load. Any ideas why?