Q: I own a 1963 PA24-250 Comanche. Over the past two years I have replaced all six cylinders, both mags, harnesses, primer lines, new fine wire plugs, fuel lines, engine fuel pump, both electric boost pumps, and I am baffled. During cruise settings at 2,400 rpm and 23 inches of manifold pressure, my engine still feels as if I’m flying in rough air. There’s a constant roughness that is minor in nature, but still there.
I’m supposed to be able to lean it back to 13.5 gallons per hour, but if I go below 14.5 the engine wants to cut off, so I’m normally left to fly it at 15 gallons per hour.
The shop I use did find a primer line that was leaking a few months back and that was replaced. I did notice a difference in the smoothness, but it did not take the minor vibration or miss out completely.
The compressions are excellent, all in the high 70s, and oil consumption is about one quart every 12 to 14 hours. I did have the shop replace the rubber intake hoses, but I’m not sure if the gaskets were replaced holding the intake pipe to the engine.
I would appreciate any suggestions you might have to solve this problem.
Larry Keener, North Myrtle Beach, S.C.
Q: Larry, I must say you’ve certainly invested some money in your Piper 250 Comanche recently, so let’s see if I can solve this mysterious minor roughness you are experiencing.
After reviewing the list of components you’ve replaced, I’d say there isn’t a whole lot more that I can think of.
When I look at your comments regarding your inability to lean the mixture to what you expect, that sounds like a fuel management issue to me. My concern here begins with the age of the aircraft.
Aging aircraft are notorious for having cockpit instrumentation that can be inaccurate. So, my question to you is have you had your tachometer calibrated recently? We may have a condition here where the tach is reading a couple hundred rpm low which, in turn, means the engine is drawing more horsepower and not getting the proper amount of fuel to support that extra power.
If your carburetor has a flow setting to support 250 hp at 2,575 rpm and your engine is actually turning 2,650 or 2,700, which is not indicated on your tach, the engine is not getting the proper amount of fuel to support the horsepower being taken out of the engine. This would explain why you are unable to lean because if the engine isn’t getting the fuel it’s asking for to support the power, and you try to take more fuel away by leaning, the result is the engine wants to quit because it’s starving for fuel.
Let me back up and mention one other thing. What is your manifold pressure at engine idle around 650-700 rpm? On a normally aspirated Lycoming engine the manifold pressure should be around 10 to 12 inches at most. If the manifold pressure is higher, say 13 to 14 inches at idle, this may be a result of an induction leak. This could have a negative impact on mixture leaning, too.
You mentioned having the rubber induction hoses replaced, but you weren’t certain if the gaskets on the intake pipe flange were replaced. I’m sure you’ve checked that by now, but I’d also encourage your maintenance facility to closely inspect the intake pipes at the flange end for any possible cracks. This could also cause an induction leak. Also, look for any fuel staining in the area, indicating a leak.
If we remove the leaning issue from the equation, there is another thing that may cause a constant minor roughness and that is the engine mount rubbers. Please check to see when they were last replaced. Since they live in a very hostile environment and are often overlooked, this also may lead to some strange things happening while the aircraft is in flight.