Will Mag Check and Full Power Harm Engine With 1 Bad Plug?

[CBogle]

Guys:

I have another thread out there on high EGT but this is a related but different question.

By doing a mag check at full rich, full power, in flight, I'll be able to tell through my EGT/CHT monitor if one of the plugs is not firing under high cylinder pressures.

My question is, will this procedure possible harm the engine? My concern is, assuming that one of the plugs is not firing during high cylinder pressure, will the short time that there is no ignition in the cylinder cause excess fuel to accumulate and thus ignite and cause damage to the exhaust system or something else?

If it is not recommended that I use this method to trouble shoot, is the only other option to replace both leads, wires, etc. to that cylinder?

Thanks,

Curt

[cessna170bdriver]

Curt,

The mixture from the cylinder not firing will just get pumped out the exhaust system and burn as it goes; it won't accumulate in the cylinder. It shouldn't cause a problem, especially for the short time it takes to do a mag check.

If however you accidently switch off both mags under high power, DO NOT switch them back on immediately. The mufflers will load up with unburned mixture and turning the switch back on will cause a potentially damaging after-fire (it will BANG similar to a back-fire, but it's in the exhaust, not in the intake). If it happens in the air I would pull the mixture to idle cut off, close the throttle, THEN turn the mags back on, richen the mixture, and go back in with the throttle. If on the ground, just let the engine stop, then reconfigure for a normal start.

Miles