IMO I think your engine is just running a bit rich. Or simply you have a engine that uses a bit more fuel. As a instructor myself, I agree with CARRELLH, about 10 minutes is all you want in a flight at one time when learning. Many fliers who are not a regular instructor trying to help other people to learning to fly tend to fly the trainers way too fast. Once a trainer reaches safe altitude you can usually throttle back to 1/2. When I plan to fly my glider (30 minutes+) or any aircraft over 15 minutes, I bring out a chair so I can sit in.