Stuck in Hamburg, engine problems...

What has worked for me, for 10 years now, is

• fine wire spark plugs (Tempest URHM38S for the TB20, currently, after all the Champion resistor problems)
• leaning deeply during all ground runs (just above the point where the engine doesn’t want to run at the normal 1200rpm idle)
• magneto harness in good condition (changed after 6 years and now 7 years old)
• magneto overhauled in the USA every 500hrs (I have the D3000 single shaft dual mag!)

On just 1 occasion I recall, I departed with one bunged-up plug, as indicated on the EDM. An in-flight mag test, done after a few mins, showed it was fine. Such a test needs to be done with great care and with a special procedure whereby if a cylinder produces no power (no EGT is very obvious) when the ignition switch is in one of the half-positions, you must close the throttle before returning the switch to BOTH, otherwise you can get a backfire and exhaust damage. I used to think departing with a bad plug was a dodgy procedure but now I think it is fine provided a multicylinder monitor shows everything else 100%.

Come to think of it, I have rarely succeeded in clearing bunged-up plugs using the normal high power lean method. In the early days of my TB20 ownership I got a number of them and the cowlings always had to come off…