Hallo,
I fly a Lancair 360 Mk1 with a Lycoming IO 360 B1F engine. Recently the right Slick Mag was overhauled as it was approaching 500 hrs. The left mag already had been overhauled earlier after a malfunction. The overhauled mag was installed together with a complete set of brand new spark plugs. Both mags have been adjusted to 25 degrees btdc and I was hoping that I would have a nicely running Lycoming again. However something strange happened.
Before the overhaul of the mag and replacement of the spark plugs I was running about 65 % power (2300 RPM at 23 Inch) and could lean (Rich of Peak) to about 9,2 GPH and saw an EGT short of 1.400 degrees. I normally always fly this EGT setting which I consider an healthy condition for my Lycoming. By the way this gives me about 175 Kts airspeed.
However after the ignition overhaul to my surprise I could not lean any further than about 10 GPH in order to keep the EGT below 1.400 degrees. In addition full power EGT at take-off indicating about 16 GPH also showed about a 50 degrees higher EGT than before.
I rechecked the engine timing as retarded timing will also raise EGT, but the timing was exactly 25 degrees. I also inspected the sparkplugs in cylinder 3, the only cylinder in which I have an EGT probe. In addition I should say that the whole Airmotive RSA fuel system has been overhauled two years ago and works fine after overhaul.
Does anyone have an idea what might explain these higher temperatures / fuel flow?
It could be the internal timing of the mag gone wrong.
Did you try an inflight mag check ?
How are the CHTs ?
You have only a single channel EGT, and I take from your description that you have no CHT.
That is not much information. The change may or may not have any significance at all.
Much of the literature on engine handling state that not to much significance should be given to absolute EGT readings, as they are very sensitive to the state of the sensor itself and its exact location in the stack.
If you have noted the fuel flow for peak EGT before the overhaul, you could see if that is the same now. That is because the fuel flow/mixture for peak EGT (on a specific cylinder) is fairly reliably identified even if the EGT reading is not 100% accurate.
If peak EGT is achieved on the same fuel flow as before, and nothing else is different except a little higher EGT reading, you get the same fuel flow on take-off, fuel flow vs airspeed is the same, the engine starts the same, runs the same, then I would think that the engine is probably just as happy as always and you should just run it at the same fuel flow as you have, and accept that you now have a reference for EGT readings that is a little higher than before.
Of course don’t take my word for it, something could indeed be wrong – I just don’t find it likely from what you write.
If you are concerned about this you really should get an engine analyzer with at least CHTs and EGTs for every cylinder.
Thank you for your comments,
It is the difference between the fuel flow and EGT before and after the overhaul of the right mag and new spark plugs that’s worrying me. I have an CHT probe as well but only on Cylinder 4, which does not reflect much change (about 350 degrees in cruise). The right mag has been overhauled and tested by and well respected and certified company, so one might assume it would be OK. Is there a way to check the internal timing as compared to the timing check of 25 degrees btdc that we performed a couple of times?
As soon as the weather permits I will try an in-flight mag check and see what comes out.
Another thing which might be suspicious is the change that I made from Champion REM 40 to REM 38 sparkplugs, although I do not see the relation. I also should mention that these new sparkplugs have had a considerable storage time of 10 – 15 year, however in good and dry condition. It is hardly conceivable that they would perform worse than the spark plugs that have been in de the Lycoming for more than 15 years.
Dbo wrote:
I also should mention that these new sparkplugs have had a considerable storage time of 10 – 15 year, however in good and dry condition
That is right from the period that Champion was starting to have problems with the resistors in the spark plugs.
You are sure that your timing is accurately 25 deg. But are you 100% sure that’s where it was before? ….just brainstorming ideas…
Yes I / we checked the timing twice and it is most definitively at 25 degrees. As a matter of fact the A&P mechanic that reinstalled the right mag and adjusted the left one, claimed that the left one was timed too late (somewhere around 20 degrees). As a matter of fact adjusting it to 25 degrees should have given a lower EGT at the same fuel flow.
I indeed suspect the spark plugs to be to blame. Before investing in a new set I will just install the old REM 40 plugs after cleaning and gapping. Although the old spark plugs are getting closer to the end of their useful live, they did a good job before I removed them. After removing the new REM 38 plugs I will compare the resistance with the old ones. If the test with the old plugs works out I will buy a new set of spark plugs.
Due to the weather conditions it may be somewhere next week before I can do the test.
See this search for some Champion iridium plug threads. I would not touch them again.
Is your full power fuel flow (all 3 levers fully forward) correct, at 16 USG/hr? On an IO540 it is about 25 USG/hr. If this is wrong, check the fuel flowmeter and then have the fuel servo readjusted.
Instrumentation is another possible problem area.
Recently I heard of a weird problem like this having been solved with a new ignition harness.
Fuel flow is indeed somewhere arond 16 USG/hr at full power take-off. The Fuel servo has been overhauled and adjusted two years ago. The only thing I can think of, are ignition and/or spark plugs, since the weird behaviour happened directly after the overhaul and exchange of these parts. I will keep you informed as soon as I find some nice weather to do further testing.
I suppose your ground mag check is normal, isn’t it ?
