Ask Paul: What’s causing intermittent roughness?

Q: I am an European-based A&P with a customer who is experiencing intermittent roughness and loss of rpm (200) typically close to the top of climb on a long climb out. The roughness goes away once it is throttled back, and it does not occur all the time. The snag can’t be produced on the ground. The aircraft is just off annual check and had the 1,000 hours mag inspection carried out, apart from various other visual inspections, plus the routine other items.

I’ve thought about the possibility of a sticky valve, an issue in the carb venturi or some issue with the mags, but thought if it were any of these that the issue would be there all the time?

The aircraft principally operates on mogas although it has had a fill of avgas as recently as a month ago.

James Flynn

A: First of all, it’s good to hear from a European-based A&P who is challenged with one of those problems that probably has been a hurdle for many experienced technicians around the world. It’s one of those things of not if, but when, will it be your turn to be confronted with something like this. Usually this type of challenge is intermittent, which may cause such things as severe headaches, increases in vocabulary, etc., so let’s see if we can’t attack this by doing a little troubleshooting.

I’d suggest we do some easy things first before really getting to the more serious stuff. I’d recommend the first thing that you try is complying with Lycoming Service Instruction 1294 which covers the subject of “Alternate Ignition Cable Routing.” This publication mentions some simple things to check prior to actually making any changes in accordance with the SI, so it’s important to read the entire SI before beginning any work.

To give you some background on how this particular SI came about, it boils down to the fact that Lycoming has more than 50 different specific models of the O-320 series engines. While these could all basically perform fine using one carburetor setting, it just isn’t that simple. Each airframe manufacturer uses its own design of the air inlet and cowling, which in turn causes different air flow patterns in the induction system. The result is that maybe a Piper installation requires a slightly richer flow setting than the carburetor used on a Cessna. So, the end result is that Lycoming may have 12 or more different carburetor flow settings to meet the airframe manufacturers requirements for a particular air induction system utilized on a specific aircraft model.

Now, as you’ve probably heard me say before, fuel distribution in a carbureted engine is less than ideal, which is the nature of the beast because the fuel is deposited at a central point versus directly into each cylinder like on a fuel-injected engine. The length of the intake pipes impact the fuel/air mixture and all of the intake pipes are not the same length. The result is a slightly different fuel/air mixture within the cylinder.

By complying with SI 1294, you are actually changing the flame propagation somewhat within the cylinders. Hopefully this will be enough to accomplish a better burn pattern compared to the original ignition cable routing. Sounds simple, doesn’t it? But it works in a lot of cases and it’s easy to do.

The second area I’d be looking at is the carburetor heat door and its actuation. It’s quite common, especially on older aircraft, for the bushing on the shaft of the carburetor heat door to become worn with the possibility of the door being forced open a slight amount by ram air during flight at different attitudes. If this occurs, the result is a richer mixture, which may be the cause of engine roughness. You’ve got to make certain that the carburetor heat door is working properly and is not allowing any heated air to pass unless you pull carburetor heat on.

The other possibility that comes to mind requires a little more work, but I wouldn’t overlook it because of your description of the problem. I know from experience that a broken valve spring could give the same symptom that you described and there is only one way to check the springs, and that is to remove the rocker arms and valve spring keepers in order to remove and inspect both inner and outer valve springs.

The easiest way to remove the valve keepers is to remove both the top and bottom spark plugs and put the piston at the bottom center on the compression stroke. Then insert an 8-foot piece of 3/8 inch nylon rope (no hemp rope) through the top spark plug hole, except for about 1 foot of the rope (just common sense, right?).

Next, bring the piston up in order to squeeze the rope between the top of the piston and the valves. With your valve spring depressor tool remove the valve keepers, making certain to keep everything in order in the way it is removed. Now, you can remove the valve springs and complete your inspection of the springs.

When reinstalling the valve springs and seats, make certain they are installed in the same position as removed. At this point, I’d remove the pushrods and shroud tubes in order to remove the hydraulic plungers and clean them of all oil, in addition to cleaning the ID of the cam follower of oil. You are now ready to reinstall the entire valve action and check the dry tappet clearance, which is .028 to .080 inches on your engine.

The age of the engine, how frequent the oil was changed, if it has had extended periods of inactivity and where it has operated (climate wise) all have a impact on what you might find during your inspection. Any of these would certainly have a great deal to do with whether corrosion may have attacked the springs, causing failure over time.

Finally, if it were a sticking valve it would be more common for that to occur during a cold engine start-up with engine roughness — and if it were severe enough you would possibly see a bent shroud tube and pushrod.

Since you didn’t mention the total time on the engine and you have some reason to suspect a sticking valve situation, you may want to consider complying with Lycoming Service Bulletin 388C. Complying with this isn’t too much more intensive than what you do to inspect the valve springs.

In a follow-up, James notified me that he discovered the accelerator pump tube in the carburetor venturi was loose.

After being advised about the problem James discovered, the now slightly dim light came on in my old head dragging something to the surface that I vaguely remembered about a carburetor nozzle replacement.

Upon checking my Lycoming publications, I found Lycoming Service Instruction 1395A (dated Nov. 23, 1984). While this publication only covers specific engine models, it just happens to be models used on aircraft manufactured in France by Socata and Robin. I find this interesting since James is based in Europe, but he never mentioned what specific aircraft he was working on.

Regardless, it appears the problem is solved, and my troubleshooting suggestions, while not hitting the mark for James, may be of some assistance to some other individual who is experiencing an intermittent roughness on their engine.


  1. Steve Reinsch says

    My vote would be for one of the mag’s breaking down under heavy gap resistance during load. Just because it looked good yesterday doesn’t mean it is OK today. Also maybe new parts are defective, the adjustment wrong or not tightened down. The full climb out loading and ram effect will draw more air in than what can be reproduced on the ground. More air means more air is compressed. More compressed air means higher resistance across the plug gap. Higher gap resistance means the voltage will find an easier route back to ground if there is one. Also it is stated “at the top of a long climb”. More altitude, equals less air pressure in the mag. Less air pressure means less resistance to internal arcing. It could also be a defective cable, cap or plug that is breaking down. Which is also affected by altitude.

    What do I base this on? My exact experience.
    My 180 HP 172. At sea level and on ground everything OK. At take off, everything OK until about 2,500 feet. Then one mag started arcing. Just that little change caused problems. It was like doing a run up mag check on climb out. After level off everything OK even at 7,000 feet.
    One mag had noticeable carbon arc track. Fixed and problem went away.
    Also a few years ago Champion had some caps that the wires were not crimped in correctly and started coming of the mag cap. Another time I had that problem also.

    • Mack says

      If a mag fires internally, it will destroy itself.

      For Slicks, you can download the seventy page service manual, and one page gives a spec for the maximum ohms allowed from the block to the center spark plug electrode. Stay under the spec, and you won’t be arcing inside the mag.

  2. Rich Dugger says

    I am just testing for my own understanding but I am to take away from this that re-routing the ignition cables has something to do with the flame propagation?

    I like to know more about how that works.

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