What to do about a ‘slightly’ rough running engine

I always look forward to hearing what you have to say, so here is my question: I fly a 1954 Piper PA-22/20 with an O-320 (no suffix) Lycoming, with about 975 hours since major. I fly it 150 to 175 hours per year, and keep up with all the maintenance an old ship requires. The engine runs beautifully, takes about 1/2 quart of oil between changes, has no leaks and has good compression. However, when I pull back the power for a long descent (2,300 to 2,100 rpm), I get a “”slightly”” rough running engine. If I reduce further, to 1,500 or so, it smooths out. I had the prop statically balanced at a reputable shop and it seemed to help a little, but I was wondering if dynamic balancing would be the next place to look, or is there another direction?

JOE CASEY

Prior Lake, Minn.

I’ve got a couple of thoughts regarding your “”slightly”” rough engine when the power is pulled back on descent. It may be a simple thing and, then again, it may be more complicated than we’d expect.

I’m glad to hear that you attempt to keep up with all the maintenance required for an older aircraft, which is important and something many do not understand. This is so important when it comes to continuing good service life of the engine and airframe.

My first suggestion for attempting to eliminate your problem is complying with Lycoming Service Instruction 1294. I’m certain you can locate a copy at your local FBO. Basically, what it tells you is to reroute your entire ignition harness as it’s currently installed. I’ll give you an example: If your ignition routing now has the left magneto firing the top left spark plugs and the bottom right spark plugs, then I’d like to see you reverse that routing so the left magneto fires the bottom left and the top right plugs. This same procedure should be done with the right magneto so its present configuration changes as well. This reconfiguration should not require anything other than some man-hour time and no expense for parts. Maybe it’s something you’ll want to do at your next regularly schedule maintenance event when you’ll have the cowl open anyway.

I suppose you’re asking yourself, “”what the heck is this going to do?”” and it’s simple, really. You’ve got to remember we’re dealing with a carburetor on this engine. The distribution of the fuel/air mixture in an engine utilizing a carburetor is poor, at best. From my experience, when this alternate ignition routing is used, the roughness disappears. It apparently has an effect on the flame propagation of the fuel/air mixture and the way that mixture enters the combustion chamber. I’d strongly suggest you comply with this SI before doing anything else.

The other thing that comes to mind is the old Lycoming Service Bulletin 258 dealing with the oil sump and carburetor. I’d be surprised if this one would be applicable to your engine, but you may want to take a look at it while you’re looking up the SI mentioned above.

Quite honestly Joe, I don’t think you have a prop balance issue, but having yours balanced was not a waste of money. It may have smoothed out your engine a little and may prevent things like cracked baffles.

I hope this information gets your engine running smoothly again.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: [email protected].