During today’s engine run up the engine started to back fire when switching to the right magneto. The explosions were quite scary.
I put the magneto back to both and ran the engine up to full power leaning it for a while in the hope to clean the plugs but this did nothing, when I did the magneto check the explosions where there again.
I tried the magnetos about 3 to 4 times and always had strong explosions. Now I am a bit concerned whether I risked breaking something in the engine or the exhaust system.
I am sure I have a bad magneto and not a problem with my plugs but should I be concerned that the exhaust explosion have damaged something else also?
I’d appreciate your views.
Thanks Placido
Yes, there is a chance that you damaged something. You have a TB21, correct? That means Lycoming with turbo and a dual magneto. That also means that the magneto is probably broken inside. Your shop will inspect the exhaust system and the magneto will need fixing or replacement.
Yes, I have a TB21. What else do I think I could have damaged? The exhaust system or also the turbo?
You had a fuel explosion inside the cylinder or in the exhaust system (in any way, the 2 valves were not closed), that can cause damage along the whole exhaust system including the turbo. I would do a visual inspection first, then clean everything, pressurize the exhaust (using either a cabin pressurizer or a professional vacuum cleaner that can be operated in reverse mode) and spray the whole exhaust system with leakage spray. I suspect the magneto has substantial internal damage but the exhaust system and turbo are most likely OK.
Dual magnetos are the worst POS every invented. I have them as well :-(
what I don’t understand is that there was not the slightest sign of something going bad. The magnetos checked perfectly well two days ago. The drop has been the same at every check. how can these things basically break down over-night?
That always happens. Everything runs just fine and then the problems start.
Obviously you will need to have the problem checked out and rectified before further flight. I agree with Achim, probably it’s a magnetoe problem and the exhaust and turbo should be ok, but they need to be checked.
A nut could have loosened, teeth could have broken off on the plastic magneto gear, etc. There are many ways. Chances are good that upon opening it, you will say “I am so glad this happened on the ground, I could have died”. Make sure to report back what the findings are!
The FAA inspector that certified dual magnetos should be in Guantanamo if you ask me…
You could have fouled spark plugs since last flight. When operating on one mag during magcheck, raw fuel may find its way into the exhaust manifold which is then ignited by hot gases coming from other cylinders. Check cheap things first.
I would send the mag to a reputable overhauler (I use QAA in Oklahoma) and while at it change the whole ignition harness unless it is fairly new.
They charge c. $800, plus shipping.
Dirkj has a point too. An inspection (only) of the magneto is a good starting point.
These single shaft dual mags are not a great idea but in reality forced landing due to failures are very very rare, and anything that stops the drive shaft rotating means your accessory gearbox is shredded so the camshaft isn’t going to be going anywhere either 
Just check it’s not fouled plugs. When I had my first, and only, experience of fouled plugs (see other thread) I had a really awful long/ rough shutdown where the engine ran on. It was very worrying.
