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Magneto drop question

2009-06-29 14:03:24

Magneto drop question

Hello,
1960 Cessana 150A Continetal 0-200
During run up I do not get a mag drop that is noticable on the right mag.Left has OK drop about 50 rpm.
I checked all wires and verified that the mags were being grounded with the switch.I can start on right mag only and started on left mag only.
The question is.Is the timing of the right mag so close to TDC that it will run the engine without a noticable RPM drop? I believe the left mag shows the drop based on it having advanced timing.

I apprecaite any feedback before I take it over to the AP.

Thank you.

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2009-06-29 22:29:46

Re: Magneto drop question

Hello Mike;

The magneto drop is a result of each cylider firing off of only one spark plug instead of both plugs when you switch from Both to either Left or Right magneto. Combustion is not as efficient, and it should be close to the same RPM drop on either magneto. There should be no difference in the spark timing between one mag or the other. I believe the O-200 timing is about 24 to 27 degrees BTDC.

Usually, the cause of no mag drop is a broken magneto ground wire (p-lead). If this is the case it is also dangerous to move the prop by hand even with the ignition switch off and the master off. The problem could also be internal to the magneto, so I would recommend having your mechanic check it out ASAP.

Jon

jplaxton00: jplaxton00; 186 Posts

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2009-07-02 17:22:38

Re: Magneto drop question

The timing for this engine (O-200-A) should be either 24 or 28 degrees before top center. This is based on whether your cylinders are affected by TCM AD 96-12-06. Your mechanic should be able to verify the AD. The difference in the mag drop between magneto's should be around 50 but not more than 150. At IDLE, quickly sweep the ignition switch from BOTH to OFF and back to BOTH to verify the P leads are ground the magneto's. This is a safety check that I have been using for many years and thankfully have never been bitten by an engine with hot mags...