Engine Problems Lycoming IO360 - Robin DR 500

From the data you provided :
16 GPH, wide open throtle, sea level is a bit low for the angle valve IO360. It should be arround 18 GPH.
Fuel pressure : 20 PSI seems a bit low too.
On a IO360 Mooney installation, fuel pressure is set to 25 PSI by default. I’ve read on Mooney forums that Lycoming now delivers new / reman / overhauled engines with a fuel pressure system set to 30 PSI.
But still, at 16 GPH, the engine should make (almost) full power.

Yes, 16 to 18 GPH is not much of a difference. That’s why I think that indication has something wrong with it.

An engine of this type has really no scope for not making the rated power provided that the fuel flow is right and it is leaned to a standard point (and peak EGT is the best efficiency).

Even if you assembled the engine with no piston rings, i.e. the compression was rubbish (nearly zero), it would probably make full rated power so long as you were able to start it (I vaguely recall somebody did that, and wrote it up). It would burn a LOT of oil though

fuel flow alone would not cause a rough engine, would it?

It could if it is very lean.

I’ve never seen mags being cooled by airflow. This is my IO540, dual mag, no airflow on it at all

Wrong mags / bodged mag overhauls?

Loss of magneto output due to overheating

There have been cases of magnetos overheating in service resulting in temporary magneto failure. This could be attributed to changes in the temperature of coefficient in copper wiring. Magnetos are constructed from many turns of copper wiring in the secondary coil. As a magneto heats, the resistance of the wiring in the coil will increase causing a decrease in output.
In several cases the engine could not be started when hot due to weak or no spark output from the magneto. Only when the engine magneto cooled down, could the engine be started. It was found that the coil in the particular magneto was susceptible to very high temperatures. Bench testing of magnetos by simulating in-flight conditions of heat, altitude and vibration can reveal issues not easily seen by a simple test.

Capacitors

The capacitor in breaker point circuit prevents points from burning and aids in the rapid collapse of the magnetic field thus increasing secondary out voltage to the spark plugs. A magneto exposed to very high temperatures, may also cause the dielectric in the capacitor to break down. This can cause a partial short between the plates which can lower the voltage in the primary coil. Signs of high temperature on the contact spring or severe breaker point erosion are signs of a failing capacitor. If the capacitor checks okay but the points are burnt then either the capacitor is being overheated or the capacitor has a poor ground.
Some capacitors use oil impregnated paper as the dielectric material. High temperature makes the paper brittle and then as mechanical shock from rapid temperature changes and engine vibrations takes its toll, the paper insulation
wears through, reducing the capacitor’s performance.

From page 13 of this here:
http://caa.gov.il/index.php?option=com_docman&view=download&category_slug=2014&alias=4762-magneto-defects-1&Itemid=669&lang=he

So, I repeat: check the electric system. When the engine runs rough when hot it must be the the electric system or the fuel system and it must be connected to heat.

So, it’s either a faulty component or, if all components are ok, the heat for some components is too high.

Check the mags (ignition coils), they may have faulty capacitors or they are running hot (see above for potential reasons). Check the spark plugs, one or more may have a crack that widens when cylinders are hot, increasing the resistance and inhibiting the spark.

But surely all this stuff “has” been changed.

From above: “The owner has spend almost 100 k on repair.” (more than the value of the plane?)

Something really weird is going on. Sounds like somebody (absent, or not involved) is being milked.

We had no report of a classic mag test after 15 minutes of engine runs ? All other ideas of fuel related defects do not seem conclusive to me as these are unlikely to be temperature critical. The engine is allright without cowling even with hot engine – BUT maybe not so hot mags in that state ? Sorry to say I am very familiar with many of those mag defects in the information links above – thank you for that ! So I´d be very interested to get feedback from an actual classic mag test AFTER 15 minutes hot engine with both mags — left — right mag. There should be a noticable difference between right and left mag drop, no likelyhood of same defects at same time on both mags. Vic