Erratic Mag Check
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Erratic Mag Check

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Joined: 6 years ago
Posts: 2
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I have an A3300 engine in a Europa XS monowheel, fitted with an Airmaster AC332 VP prop.  The mag check has been giving erratic readings, which I struggle to interpret, can any one help?  

 On ground run, I set 2000 RPM and turn off the left mag, RPM drop to 1840: I turn off the right mag, RPM drop to 1920.  Conclusion, the right mag's not performing correctly.  But because I've also been seeing unexpectedly low RPM on take off I also do a mag check at 3000 RPM: turn off left mag, RPM drops to 2940: turn off the right mag, RPM drops to 2650.  Conclusion: left mag's not performing correctly.

I've checked all the connections and spark plug gaps: I've checked resistance and continuity between P terminal and the coil base - 1.1 Ohms.  Mag coil gaps are spot on at 10 thou - just done it's annual and had a top end overhaul.

So I don't have a clear mag culprit: am I right in beginning to think this isn't  mag problem but a fuel one, related to the carb setting, jet sizes and the fact it's a tailwheel sitting on the ground with the float levels in question?

Any advice or experience welcome.


Member Admin
Joined: 6 years ago
Posts: 82

Hi Keith,

There's a long list of potential culprits for something like that, so I'll just take a couple of (hardly) educated guesses.

Of the two cases it's the change at 3,000 RPM that I'd be looking at; the sort of variation you've mentioned at 2,000 isn't all that unusual and generally doesn't mean anything sinister.

Sounds like you've done the basic troubleshooting so the next step would be to acquire a new coil, swap it for the one causing the big RPM drop at 3,000 RPM and see if anything changes.  Just be sure to trace the wires and make sure you're actually swapping the correct coil: It's very common in my experience for the left ignition switch to be wired to the right coil and vice-versa.

That's assuming that the plugs were changed at the recent service: we've had several weird issues over the years that were solved by replacing the plugs. And also that the distributor caps, rotors, contacts, seals etc are all in good working order.

Beyond that we start getting into less likely stuff...  Is the aircraft fitted with an EMS and do the EGT numbers look ok at 3,000 RPM?  Is the engine equipped with cooling tubes directing cooling air over the coils?

Best Regards,