What causes a formerly flawless engine to run rough?

QUESTION: I have a 2001 Maule with an O-360-C1F engine that had been running flawlessly until a few months ago. I have a JPI-700 and have noticed that the peak EGTs have been slowly climbing from the original low-1500°s to 1600° or so. Cylinders #1 and #3 are higher than #2 and #4. Also, #1 now will not reach peak EGT. While the other EGTs lean to peak and then drop, #1 will continue up past 1600°, then the engine runs very rough. CHTs are always under 400°, and pretty even across the board (#4 is low due to the spark plug CHT tc).

My A&P checked the intake cam follower lifts on #1 and #3 and they were the same. He changed the #1 hydraulic lifter, but no change. This is because one piece of advice I got was that the cam could be going flat on #1 intake.

Spark timing is correct. I checked the baffling and it looks good. I swapped #1 and #3 EGT probes and the problem moved to #3 on the JPI, so the probe is good.

Takeoff EGT on #1 is also high, so I’m not using full power on takeoff. #1 acts as though it is lean, but why won’t the EGT peak on #1?

My hangar neighbor is replacing his intake valve guides because his cylinders are all running lean: rich to lean EGT difference was 50° in his case. He thinks that the intake guide leakage will cause lean operation. Could this be my problem? But why won’t the EGT peak on #1?

We are stuck here, about ready to do a TOH on #1 because we can’t think of anything else to do. Any suggestions?

Larry Charneski
via email

ANSWER: Larry, you certainly have an interesting situation and I’ll do my best to see if I can’t offer some suggestions to sort things out for you.

I think the most important thing I should mention before we start is that we must never lose sight of the fact that the distribution of fuel/air to all cylinders on a float carburetor equipped engine is, shall we say, poor at best. It’s just the nature of the beast and is not as precise as the distribution found in fuel-injected engines. This certainly doesn’t mean it’s bad, but just means we have to have a basic understanding of it and learn to cope with its idiosyncrasies.

When did this problem actually begin? Was it following a regular scheduled maintenance event, or following the installation of the JPI system, or some other event that you can recall? Thinking back may give you a clue as to where to begin looking for a solution.

Now, my next comment will probably get me in trouble, but I’ll make it anyway. In order to operate a normally aspirated carbureted engine it is not necessary to have an EGT gauge, but they do serve as a reference tool for the operation of the engine. I hope you haven’t forgotten that you never lean, under normal circumstances, when you are above 75% power.

What if you put tape over the JPI gauge and flew the aircraft and leaned per the Lycoming Service Instruction 1094D? When you mentioned that while leaning the engine goes rough, this is what we’d expect if we were leaning for “”best power”” at a given cruise power setting. There is one thing that may be causing the number 1 cylinder to misbehave and that is an induction leak. If you have a manifold pressure gauge, at idle (650-700 rpm) we’d expect to see around 10″” of MAP. If it’s higher than that, say around 12″”, this could indicate an induction leak, which would cause problems when leaning. Have your maintenance staff check the intake pipe gasket at the cylinder head flange, as well as tightness of the hose clamps where the intake pipe attaches to the sump. Also check the intake tube in the sump for any looseness.

I’m not sure I understand the situation of your hangar neighbor, but I’d certainly question any maintenance facility that would suggest replacing intake valve guides to cure a lean condition on any engine.

Larry, I’m sure there may be other things that may have an impact on solving your problem, but these are the ones that come to mind as I write today. I hope these will serve as a starting point.

QUESTION: Who usually does an engine conversion (i.e., 150 hp to 160 hp)? Lycoming or a service station?

Mohammed Selim
via email

ANSWER: This conversion may be made under an FAA STC. I’d suggest you contact any one of the reputable engine overhaul shops that are capable of performing this conversion.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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