Mag Check Backfire

[rakesb]

What could cause a single backfire when conducting a mag check on the right mag? How serious is this and what should I do before my next flight? Since my last annual last summer, I've been experiencing what I believe is a fouled plug when I conduct the the the before takeoff runup check on the right mag. So, to preclude putting the airplane away with a fouled spark plug, I added a preshutdown mag check after my mag ground check before engine shutdown. Ground check was good, left mag check was good with a 50 rpm or less drop, but right mag check at1700 rpm caused just over a 100 rpm drop and a single, loud back fire, followed by a return to normal rpm. Would appreciate any advice -- Bobby Rakes, 573.337.3447, rakes_bobby@hotmail.com, N6094Q, 1966 M20C Mark 21, Lycoming O-360-A1D.

[N201MKTurbo]

When you un-short a P lead it can be the same as the points opening, fireing the plug. Normally the combustion charge would be ignited at the proper time by the operating mag when you switch from one mag to the other leaving nothing to ignite for a random spark event, but if you have a fouled plug, that charge will not be ignited and the random spark event can ignite the fuel charge at the wrong time causing the backfire.

[kommers]

Well, I had such situation twice. Last time it was a bad plug, but the first time, the issue was caused by the "flooded" mag. Shop told me that one mag is plain bad, and has to be replaced. When we changed both mags, backfiring stopped.

[carusoam]

Nicely detailed post, Bobby.

Any idea which mags you have and how old they may be?

Are you leaning at all or is your mixture full in?

best regards,

-a-

[orionflt]

Definitely sounds like a Mag related problem, that being said it could be anything from a mis timed mag to a problem with the mag itself. My best recommendation is to get your mechanic to take a look at it before you go flying again.

 

Brian

[Mooneymite]

You might take a look at the muffler.  A backfire can cause havok with it.