This tip of the month deals with a problem which showed up in a Twin Comanche.

Owner complained of the left engine surging every now and then during climb power settings. The problem would show up only during climb power settings (25" MP and 2500 RPM) and normally only do it 2 or 3 times for a second or two and then you could catch fuel flow reading extremely high and EGT extremely cold and then it would all go back to normal readings and be okay. Hardly ever noticed it happening in cruise or let down configuration. Next flight it would start it all over again.

We ended up reworking fuel injection system, replaced magnetos, spark plugs, checked for sticking valves, replaced hydraulic lifters, reset dry-tappet clearances, replaced engine driven fuel pump, checked valve springs, checked valve lift and duration, cleaned all fuel screens, looked in fuel tanks for obstructions to pick up tubes, replaced electric boost pump; you name it and we had done it.

Finally, we decided to check fuel selector valve. We found out that one spring which loads ball and seat on fuel selector valve was rusted and broken near the center. We pulled the selector and found that it was internally leaking through that port. The owner’s only comment was "why didn’t you check that $5,000 to $6,000 earlier? He had a good point. We had chased this problem almost a full year before we caught up to it. Once we changed out the selector valve, the airplane performed flawlessly.