since 1940

Excessive rpm drop on left/right aircraft magneto

1. Shorted shower of sparks vibrator, causing it to stay in the retarded position on the left magneto. Excessive magneto drop when checking magnetos 1. Fouled spark plug(s).

2. Incorrect spark plug gap(s). Fouled spark plugs. Broken spark plug ceramic.

3. Incorrectly timed magnetos. The magneto that gives the greatest rpm drop (or the greatest rise in EGT) is retarded compared to the other magneto. Incorrect timing or an inability to time the magneto may be due to a worn out magneto. Incorrect internal timing "E" gap.

Worn woodruff key slot allows the engine to rotate 15-20 degrees without the Slick magneto rotating. 

4. Incorrect fuel/air ratio.

5. Incorrect fuel

6. Continental C85 through O200 engine. Bendix and Slick magnetos are wired so that the left magneto fires the lower spark plugs. If is not uncommon to get a larger magneto drop on the left magneto since the lower spark plugs may be dirty or fouled.

7. Induction leak. A little induction leak causes a lean mixture that may ignite when both plugs fire but may not ignite when only one plug fires.

8.Improper internal timing on Slick magneto. On Slick 4200/6200 and 4300/6300 series magnetos check to see that the timing pin (Slick T-118) was inserted into the proper holes in the distributor block. If the magneto is pinned incorrectly the contact points can still be synchronized to the engine firing point, but the distributor finger is positioned over the wrong tower. This can lead to a confusing condition where the contact points seem to be synchronized, but the engine runs poorly on the subject magneto.

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Many mechanics estimate E gap  when setting the points on Slick magnetos. This results in each magneto set slightly different and causes a slight difference in RPM drop between magnetos when the pilot does the mag check.

 

	Setting E Gap on Slick Magnetos Newer Slick 4300 and 6300 magnetos use a  T-150 tool to set E gap. The tool locks the rotor in the exact position that the points should open.
	Slick magneto showing points and plastic cam follower
	Slick E gap timing tool inserted into slot in rotor and against poll shoes. Now adjust the points to be just opening and you have the optimum point of magneto energy. Note: many mechanics do not use this simple tool. They estimate E gap. Comparing this method with the tool shown above you can be off considerably. This may account for differences in rpm drop between magnetos when doing the magneto check.

9. Shorted "P" lead or magneto capacitor. 

For engines using Bendix D2000./D3000 magnetos check Continental Service Bulletin CSB 662A dated 01/24/00. Subject concerns magneto capacitors that may internally ground.

AB-382681 magneto capacitor

10. Burnt magneto points. Burnt magneto points are caused by insufficient capacitance in the primary system i.e. failed capacitor.

11. Failed magneto capacitor

12. Failed magneto coil. See Testing the Magneto Coil

13. Broken impulse coupling spring. Also causes a large increase in EGT

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.