I am good at getting mini-turbo boosts from the Standard Bike, but when I choose the Mach Bike, it turns too much and hits the fence or goes into the grass. How can I reliably get the mini-turbo boosts with the Mach Bike?
I'm going to have to disagree with Andrew. The best part of the Mach bike is that it has an extremely tight turning radius, which means that you don't start your turns until much, much later than you would with the other karts and bikes. However, you do have to get used to turning the opposite direction a bit to control exactly how quickly you are turning.
Personally, I don't worry too much about the mini-turbo with the Mach bike. By waiting until very late to start your turn, you can spend more time doing a wheelie. Then make a quick sharp turn and immediately start another wheelie. If the turn is long enough, you can steer against the turn in order to get the boost, but it's the long wheelies that get you the most speed boost out of the Mach bike.
The Mach bike has a very high drift, so if you're rounding a tight corner, you'll need to be prepared for that. Your best bet is to initiate your turn early so that you slide out into the middle of the track in the drift. Then, when you get out of the drift, you're already facing down the middle of the track. And, of course, practice, practice, practice...