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200 rpm Mag drop Bendix left mag

From AMS's troubleshooting page, which I linked earlier:

"Most “bad mag checks” are spark plug related. The spark plug is fouled and shorted to ground or is open and the magneto, which is functioning normally, is unable to fire it. A typical bad plug will cause an immediate drop of 250 or more RPM, at the mag check. The key indicator is the suddenness of the drop."

and re checking individual plugs:

"Pick out lead deposit clinkers, inspect the barrel for cracked insulator, inspect the nose core for cracks.Clean and inspect the plug (correct gap for most plugs is .015” to .019” consult your plug specifications).

Take an OHM Meter and measure the resistance value from the connection in the bottom of the barrel to the clean center electrode at the firing end, electrode must be bare metal.

A new Champion plug will have a value of 800 to 1200 OHMS. New Tempest (formerly Unison-Autolite) will measure 1000 OHMS. Replace any plug above 5000 OHMS.

A spark plug bomb tester can test a bad plug and lead you to conclude it is serviceable. The OHM Meter check is simple, readily available, and amazingly accurate in finding misfiring plugs."
 
Years ago I had rough-running / hard-starting issues with my old C170 that I was SURE were caused by a bad mag.
Pulled the mags, took them to a shop which had a mag-testing bench-- both tested OK.
The shop suggested it was the plugs causing the problem, I refused to believe them,.
Although they had around 500 hours on them, which some say is about all you can expect,
the center electrodes were still nice and round and they looked perfect.
I did reluctantly buy 6 new plugs and installed them --
what a miracle, the problems with that mag went away.
6 more new plugs, and the problem was solved.
At that time, I didn't know about the ohms test for plugs,
but I'm sure they would have failed it
 
I have heard of people measuring "point gap" but that isn't accurate; you adjust the "E gap". Using your Buzz box, check to see that the points just open when the timing gear tooth, (beveled end and usually red) is exactly in the center of the timing plug hole. (While rotating in the proper direction, or Left hand)

THANKS Ed
I have tried all the other suggestions so I will give it a try,but if that doesn't work I will send it of to a shop that overhauls mags.
Thanks to everyone for all the help.
Since you've already sent your mags out, a discussion of internal timing can wait for another day. The description above of locating "E-Gap" is erroneous. All that beveled tooth is telling you is that #1 is firing and it can be anywhere in the opening. E-gap is a particular position of the magnet which is totally separate from the distributor gear.
 
Since you've already sent your mags out, a discussion of internal timing can wait for another day. The description above of locating "E-Gap" is erroneous. All that beveled tooth is telling you is that #1 is firing and it can be anywhere in the opening. E-gap is a particular position of the magnet which is totally separate from the distributor gear.
]

You are correct in that I errored. (Thank you for catching it). I left out the word "to" (Adjust to the E-Gap)
I shall try again even though it is 2 am here in Alaska.... (dog snoring woke me up)

The description was not to be a method of locating "E Gap". It is only to ensure that you are "firing" the mag within the "E-Gap". (This is on the simple 20 series Bendix mags. Some others can vary)

The preference is to have that happen when the "tooth" is centered in the hole. If the points are set to open at that instant then you will find (If you check with the Bendix timing kit) that you are within specifications (if the internal gears were assembled correctly). 10 degrees past neutral +/_ 4 degrees.

The point gap at its maximum lift is to be within a spec of .018 +/_ .006. (You are only checking for wear or damage of components at this point and not setting any opening position by means of a measured gap.)

This procedure that I am trying to describe is what I use for easy field checking and for making slight adjustments without having to remove the mag and check with the timing scale and pointer. You would want to use those tools if you have dismantled it for major maintenance.

I set these centered to make installation to the engine easier, and to give a future reference of point deterioration. I can not recall any new Bendix mag not having the points set to open with the timing tooth centered.

There was a time when few people had "buzz boxes" and points were set with a continuity meter, but only after removing the coil and condenser leads. (Pain in butt). Installing to the engine was by simply aligning the tooth to center and engine on the timing marks. As I recall any "excessive drop" was often addressed by a slight advance of the mag. We have come a long way since then.

It sounds as if the Mags in question have other issues going on. Overhaul is the best medicine with 700 hours on em...

Now back to bed......(and I'm not sleeping in the dog house.....yet)
 
I recently pulled all the plugs on my O-360 and noticed the bottom ones had lead buildup down below the electrodes. Wondering if that is common and what would be the reason for the bottom ones the have built up but not the top.
 
It is very common, and one good solution is to lean the engine aggressively while taxiing / idling on the ground. I solved it on my engine by leaning to the point where any further leaning would shut down the engine.

When you start taxiing, you will have to enrich lightly, or it will shut down when you add power. The key is to lean so far that you cannot possibly take off with it leaned that much - the engine will just flame out. Remember to go to full rich (unless your DA is above approximately 5000 ft) before takeoff. But if you do forget, you will be reminded quickly by the silence up front...

good luck!
 
That is why some use fine wire plugs in the bottom, I don't know why but the bottom is where the lead usually builds up.
 
That makes since. When I first started flying a friend and I bought a PA16 with an O-235-C1 engine and I was cleaning the spark plugs at every oil change (25 hrs) until I started leaning it all the time except climbing. These work great for breaking up the lead balls.
12-00714.jpg
 
We should be thankful that we now have 100LL (blue) which has half the amount of lead that the old 100/115 (green) avgas did. The plugs were always needing de-leading in those days.
 
A good fix for lead balls or "clinkers" is unleaded mogas.
Only when and as approved, of course.
I burn 80% regular grade mogas and 20% 100LL,'
and my plugs are always clean as a whistle.
 
When they opened it up my mag had so many problems I just had them do an exchange for an updated overhauled mag.
 
When they opened it up my mag had so many problems I just had them do an exchange for an updated overhauled mag.

Okay,...so did your mag drop issue go away? When you get on here and ask others to troubleshoot a problem, it's kind of expected for to you to let everyone know if/how the problem was resolved. That way we all might learn something.

Jim
 
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