Excellent news :-) May your exhaust system remain true for many years!
Good to know!
How nice that your daughter enjoyed the flight so much.
Just thought I would close the loop and thank you all for your responses.
It turns out that the issue was as @alioth suggested – slightly warped exhaust mating faces. They were smoothed out and appear to have solved the problem.
This is good as I was finally able to take my 9 year old daughter up on her first flight on Father’s Day Sunday. I filmed the take off and the look on her face when we got airborne really made me – and obviously her – happy that I was able to share my passion for flight with her. I let her fly and she did a surprisingly good job, despite not being able to reach the rudder bar. She liked the steep turns 
and asked when we can go flying again…
Thank you all for your responses. I really like how one answer built on another in terms of a progressive checklist!
We are still working through the problem, but strongly suspect that @alioth has identified the issue. There may be another issue related to this, but bottom line is the gap which is intermittently causing this popping, despite new gaskets. Thanks for your experience on gaskets as we might otherwise have worked through every size in the book! We are also checking the screw threads…
@Euroflyer – EuroFlyer wrote:
Does leaning the engine change the popping in any way ? If it goes away when leaning you might have a stuck valve where unburnt fuel ignites.
Do you have an EDM that gives you a readout of EGT and CHT ?
Do you have a clue which cylinder it is ?
What does the mag check say ?
Did you check the plugs ?
And you might have a look at the intake manifold bolts and then the rubber couplings. The problem can be on the way in, not only the one out
@Silvaire – operating on avgas now. I am told that the engine could take mogas, but AFAIK, has been flying with avgas for a number of years.
@aviathor – we grounded the aircraft as a precaution against the type of incident you describe.
Frustrating these problems, but on the upside, having taken part in the annual, I have learnt a few things (and got my hands dirty). Still so much to learn… but good one to learn ones aircraft.
Cobalt wrote:
I thought oxygenation is what happens when you turn fuel into exhaust?
You’re thinking of oxidization.
Silvaire wrote:
I have heard the same on a C90 running oxygenated auto fuel. On Avgas the issue isn’t present.
On our Cub (C90) I often fill up with auto fuel that has MTBE (or at least had, when Statoil still sold fuel). I can’t remember hearing a popping sound, but I think the engine sounded a bit “harder” on autofuel (could be imagination of course).
I thought oxygenation is what happens when you turn fuel into exhaust?
I have heard the same on a C90 running oxygenated auto fuel. On Avgas the issue isn’t present.
One day a friend at the aeroclub who knew I was supposed to fly a M20C from Hamar ENHA to Helsinki EFHM in the next few days called me to warn me that he thought there was a popping sound from the engine. So I decided to take the plane to the shop before my trip. I drove to ENHA, very carefully pre-flighted the airplane, made a long run-up, even to high power and did not find anything. So I took off for the flight to ENKJ where the shop was located. The flight was completely uneventful. Once on the ground I did another run-up and could hear a very distinct popping sound.
I walked in to the shop. The mechanics had heard the run-up and told me the engine sounded perfectly normal, but they would take a look. Then I went to work.
In the late afternoon I was back. The mechanic gave me a strange look and told me to follow him to a corner of the hangar where there was a pile of tubing. He said that when they removed the cowling, the pieces of the exhaust that were not bolted to the engine fell out. The cowling had been keeping it all together, but metal expansion when hot, would open up gaps between pieces and that was where the popping sound came from.
No trip to EFHM for me.
Don’t take potential exhaust issues lightly.
