High EGT after cylinder replacement

One should make a list of what has not been changed. The ignition harness, the plugs?

There is also a residual possibility that the mag was “overhauled” by some crook. There is an engine shop in middle-ish Europe which does engine overhauls by fitting new cylinders and painting the crankcase. They are popular; good value for money… One of their jobs recently ended in a field.

What happens to the EGTs when you bring the engine up to run-up speed (e.g. 2000 RPM), and then start leaning the mixture? If you’ve checked the ignition system and it looks good, then you have to consider fuel.

Placido

You should be able to compare EGTs during mag check on one mag, then the other then both.
High EGT occurs when only one plug is firing, slightly delaying combustion.
If it is a fuel mixture issue (be it induction- or injection- related) then the mag check would not run rough on one and not the other, so you do have an ignition problem (additional to the cylinder&valve problem addressed with the new cylinder). The former may have been cause to the latter but that is secondary now.
I you have a rough mag check on one mag and not the other , then you do have an ignition problem.
First, by checking your egts and mag switch position you should confirm the ignition problem is either lh or rh mag and one cyl or another and I am assuming you will find the faulty ignition is on the offending #5. LH or RH tells you either top plug or bottom
Having done plugs (not necessarily new, but they are easy to swap and repeat the check, don’t trust the looks, insulator may be cracked, resistor broken/high resistance, I assume they have been swapped as a minimum and checked whether the problem follows a certain plug) and magneto, next culprit is harness.
A good shop can replace the single wire or you can replace the whole harness.
Failing that, magneto may have had a bad repair, but more easily the fault could be with the unreplaced mag cover which is part of the harness, not the magneto.

Let me know if any of my assumptions on your symptoms and actions above is wrong.

Antonio

They told me they checked all plugs with a special device and they were all found good.

Plugs with high resistance will pass the “bomb test”. My engine failed mag tests because cylinders shut down on one mag even though all plugs produced sparks during the bomb test. I once lost a cylinder during taxing just because I leaned the mixture. After I changed all plugs the problems were gone.

So again, look at the EGTs during mag test. Rev up the engine to whatever rpm you use, set the EGT monitor to “nornalise” once the temperatures have stabilised, then turn the key to RIGHT mag and watch the EGTs. They should all rise by a similar amount. Go back to BOTH and let the temperatures stabilise before you switch to LEFT. It is common to see different rises on left and right cylinders. On one mag you may see pair cylinder temps rise by 80℉ and the odd cylinder temps rise by 120℉.for example. But they should all rise.

It would be surprising if you got roughness during mag test without at least one EGT falling.

I have a feeling somebody is saying such and such has been done but actually has not. I don’t mean Placido. I mean some mechanic.

Troubleshooting “technical things” has to be done very carefully and systematically.

Many years ago I had a customer who was having a problem with some product of mine and was absolutely adamant he had checked a particular setting which was the obvious explanation. Eventually I drove to him (UK Ministry of Defence) for about 5 hours, walked in, and 10 secs later fixed the problem…

Yes, but you still have the same ignition harness. And same instrument.

What about A&C’s point about the hydraulic tappets?