[Bay Kid]

I had my 2012.5 4 seat Yamaha, w/5,000 miles, serviced at TV Golf Cars in Sumter Landing last week. They replaced the belt with a severe duty belt. I drove home, then back because it wasn't right on takeoff. From 7 mph on it was great, better than ever. From 0 to 6 mph you have to give 75-100% throttle or the engine reeves high, but moves very slow. This is very concerning, especially when crossing roads, either wide open or very slow. The lady at service took it for a quick spin and said it was fine, but she only would push it to the floor to go. What to do because 150 miles later and I still feel it isn't running right. I don't want to upset her but I feel it isn't running right. Can this be easily adjusted? Thank you.