EI/Mag Runup Question

[mmarmol]

I just bought my airplane. It has an electronic ignition on the "right mag" hooked up to the top plugs and a slick magneto on the left mag hooked up to the bottom plugs.
During the runup, at 1,700 RPM, when I turn off the magneto I get the usual 50 RPM drop and the engine runs smoothly. I then turn the mag back on and then turn off the electronic ignition. When I do that the engine RPM drops quite a bit but is still running fairly smoothly. I then turn the electronic ignition back on and everything comes back to normal.
Is this normal? Is my electronic ignition not working properly? I have always been a dual-mag guy so am a little scared by that behavior.

Thanks for all the expertise!

[Bevan]

Miguel

From your description, the performance of your mag and elec ignition would seem to be normal provided that the rpm drop on mag only isn't too much. How much rpm drop do you get when turning off the electronic ignition?

A larger rpm drop when turning off the electronic ignition (EI) could be expected as the EI outperforms the mag.

The fix (if required) for this would be to replace the mag with a second electronic ignition.

Bevan

[Toobuilder]

Agreed. Often, you will experience zero RPM drop when running on a single EI. When you say "quite a bit" of a drop on the mag only, that's the behavior you need to characterize. The EI sounds fine.

[mmarmol]

Hey guys,
Thanks for the response. I flew the airplane today. As far as quantifying the RPM drop there is an issue...my RPM readout is part of the electronic ignition. When I turn off he electronic ignition it turns off the RPM display so I'm not sure what the actual RPM drop might be. What I tried to do was to set the engine RPM at a number, then take it up to 1700 RPM and turn off the EI. I then compared this sound to the sound of the engine when I had set it at a lower RPM.
I think the results indicate that, when I turn off the EI, the engine RPM goes from 1,700 RPM to 1,200 or 1,100 RPM. Is this 500-600 RPM drop still in the acceptable range?

Thanks for the insight guys!
~Miguel

[Toobuilder]

A 500 RPM drop is excessive! You should be looking into anything over about 150. A "normal" mag drop is 50-75 for a magneto, and 0-25 for an EI.

Make sure you are in fact dropping that much though. A 500 RPM drop would be extremely noticeable, even without a tach display. I can't see it dropping that much without running very rough.

[DanH]

Originally Posted by mmarmol

As far as quantifying the RPM drop there is an issue...my RPM readout is part of the electronic ignition. When I turn off he electronic ignition it turns off the RPM display so I'm not sure what the actual RPM drop might be.

Head over to the model airplane store and buy an handheld optical prop tach (about $30). Quantify the RPM drop and report back.

[rv9av8tr]

I would strongly suggest you install a tach generator on your mechanical drive point such as this: http://www.aircraftspruce.com/catalo...clickkey=11114
I got one from Van's, but can't find it on their website right now.

Then wire your indication system to this. IMHO it's rediculous to be without RPM indication....

[RVbySDI]

+1 for Dan's suggestion. Get a prop tach that optically monitors the prop. Then tell us what you find.

As for how much RPM drop, my EI only drop is ~25RPM, the Slick mag only drop is ~75RPM.

[jjconstant]

One other consideration is that a number of people choose to put the electronic ignition on the bottom plugs to help minimize the rpm differential during mag checks. Bottom plugs can get more fouling and the electronic ignition deals with it better...

[Rupester]

Originally Posted by RVbySDI

As for how much RPM drop, my EI only drop is ~25RPM, the Slick mag only drop is ~75RPM.

Same for me. (Plasma II on the R, Slick on the L.)