Originally Posted by wildcat1

I have had one get partial power. I was flight instructing and as the student was doing the run up I noticed the mag drop was rather large. The student was far enough along that he should have known there was a problem. When he finished I gave him a hint..."Was that normal?"

"Yes!...Altus Tower, xyz ready for takeoff."

I rolled my eyes and let him take the runway. When he pushed up the power we go something like 2000 RPM (this was in a 152 btw). I pointed to the RPM and said "Is that normal."

"Yes....Rotate!"

So we where off the ground, and I was seriously worried that the a/c wouldn't clear the trees at the end of the runway. We did, but not by more than 50 ft. As we where climbing out I started probing the student about how much power he should be able to get. Finally, I had enough. At 2000 feet (I'm not sure how we got that high, we didn't have the power to get up there, but we did) I took the airplane and limped it back to the airport.

Turns out that a float in the carb had been signed off as being replaced in accordance with an AD. The old, larger, float was still in there and was causing the mixture to run extremely lean.

Like Duck said, aircraft will almost always tell you something is wrong long before it fails on you. We had 2 very clear clues that the engine was not working properly.

Lesson to learn: I let the student continue when I knew there was a problem. I let him continue because I thought a good instructor lets a student get to a problem before he "saves" the student. The fact is a good instructor teaches a student how to stop at the first sign of a problem, not the last.