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Magnetos, Ignition Coils and Spark Plugs Discussion about magnetos, buzz coils, spark plugs, ignitors and low tension coils.

Magnetos, Ignition Coils and Spark Plugs

Fairbanks Morse RV4 magneto woes


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  #1  
Old 01-22-2019, 05:51:49 PM
Merv Walde Merv Walde is offline
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Default Fairbanks Morse RV4 magneto woes

We are working on a Massey Pacemaker with a rebuilt Fairbanks Morse RV4 magneto. The magneto is throwing a beautiful fat blue spark. The two aft terminals receive a spark every fourth impulse but the forward two spark at every impulse. Any ideas as to why? The forward two are nearest the coil.

Thanks in advance,

Merv
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Old 01-23-2019, 05:15:19 PM
mmcdonald mmcdonald is offline
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Default Re: Fairbanks Morse RV4 magneto woes

Glad to hear you have a good spark. Some of those RV mags are hard to get going. From a million miles away, it sounds like you have a conductive surface on the rotor such as carbon tracking. If not, the rotor may be shorted internally. I don't think the coil would be arcing to the towers since it seems it would be easier for it to arc to something metal nearer but I guess that's a possibility.

After I posted I thought about it. I guess it's plausible the rotor could be out of time enough to cause something like that if the conductive brass insert is in between the two sides and it arcs to nearest one? I don't know just another thought that came to mind if you haven't checked that. Good luck.
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Last edited by mmcdonald; 01-23-2019 at 05:17:26 PM. Reason: additional info
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Old 01-23-2019, 08:44:10 PM
Merv Walde Merv Walde is offline
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Default Re: Fairbanks Morse RV4 magneto woes

Thanks for your thoughts. The mag was rebuilt professionally and we are awaiting a response from the shop that did the work. The mag has just been installed for the first time and it was upon attempting to fire the old Massey that we discovered the problem. Wiped the rotor with a rag and acetone although the interior is pristine. I am not familiar with the construction of the rotor so can not tell if there could be an arc path through the rotor to both of the towers. There is a very consistent spark at each of the two forward terminals at every impulse. Arcing from coil to forward towers seems a long shot but would explain how they were receiving spark at every impulse. As I am writing this I wonder if the aft two terminals receive no spark when forward terminals are connected. I will check that next time I am at the tractor.
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Old 01-25-2019, 03:22:22 PM
Merv Walde Merv Walde is offline
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Default Re: Fairbanks Morse RV4 magneto woes

FYI, it would appear that we made a real rookie mistake in testing out this mag. We were testing spark with only one lead attached. Today we hooked up all four leads and sparking occurs as designed with each terminal receiving it's spark in correct sequence. It would appear that the spark was following the path of least resistance, which was through the air from the coil to the forward tower. So, we are now ready to fire up our 1936 Massey Harris Pacemaker!

Thanks for your interest and input.

Merv
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Old 01-26-2019, 10:18:04 AM
mmcdonald mmcdonald is offline
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Default Re: Fairbanks Morse RV4 magneto woes

Just for future reference, a spark will always follow the path of least resistance. I had no idea you only had one wire connected. That didn't cross my mind. One other thing, on any magneto it is a bad idea to operate it without an outlet for the spark. This causes that electricity to be discharged internally and it will short out a coil. Even if you're not checking an outlet for spark, put a wire in the terminal and be sure it has a conductive path back to the mag body. This will prevent the coil from having to discharge internally. Good luck and thanks for letting us know the problem.
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