Normally if a setting is too rich, the engine will slow down and die with the nose down. Let the engine idle for 15 to 20 seconds on the ground. Advance the throttle and see how it acts. If you're familiar with nitro engines, you know the drill. The nose down thing has to do with something called, "tank head" Maybe you don't notice any changes in your car because of the different amounts of fuel a plane carries, and the size of the tank compared to a car. How is your plane idling etc?