Your helicopter may not had been properly tweaked while in the factory. If you just bought it, you should ask for an exchange. Your vendor suggested you shouldn't try to adjust the proportion screw because he assumed that everything rolls out of the factory is 100% tuned within specification. But he may not realize that the factory workers who tune the helicopters in China don't tend to stay long in the same factory. They move from one factory to the next factory for better pay or some just quit and never come back after a holiday. That's why factories in China have problem keeping good and skillful workers and they constantly have to hire and train the new workers almost every season to replace those workers they've lost in order to keep the factory's operation going. So a newly trained factory worker with little experience of tunning rc helicopters will less likely tune your helicopter properly.