When we bought our Piper Cherokee, I thought that I could smell avgas in the cabin when we went on the test flight.
So when we came back to fly it away, I stuck a "dead stop" carbon monoxide in an empty spot in the instrument panel.
When we had taxied to the run up area I glanced at it and the detector portion had turned black. I tried to shut off the cabin heat, but the hinge was rusted in the open position. I was able to force it closed by getting under the plane.
Fortunately, I had seen the previous owner leaning the mixture when he took off on the test flight, because we almost didn't clear the power lines at the end of the runway. I leaned and instantly got an extra 150 rpm which caused the plane to climb out well. You normally do not lean on takeoff at full power because it can melt your exhaust valves.
The muffler not only had cracks in it, but some of the baffles had broken off and were partially plugging the exhaust port. The obstruction in the muffler was causing the engine to run rich because it was restricting the air flow, so it did not cause a problem with the exhaust valves.
The guy we bought the plane from was selling it because his wife got sick every time they went flying. We had to buy a new muffler the next day. I called him and told him that his wife might probably was not getting air sick; she was probably getting carbon monoxide poisoning. He just acted like I was trying to get him to pay for the $400 muffler.
The plane turned out well for us after a scary beginning.