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Thread: 200 rpm Mag drop Bendix left mag

  1. #1

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    200 rpm Mag drop Bendix left mag

    Hi
    I have Bendix mags on an 0-320 b2b and on the left mag I'm getting a 200+ rpm mag drop when I do a mag check. I did a search on here and read a bunch of things it might be .
    I changed the plugs that go to that mag one at a time,no luck,I even got brave and while it was idling I pulled the plug wires off each plug from that mag one at a time and there was a NICE spark that was jumping across from the wire to the plug when I pulled each one, plus I could hear an RPM drop as I pulled them and an RPM increase when I put them back.
    I checked the timing with buzz box, it was out a couple degrees so I set it back to 25 degree same as the right mag. Still didn't help . So I pulled the mag looked at the points and checked them to see and they were .018 gap.Cleaned things up reinstalled and still the same .
    Any more thoughts??
    THANKS

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    www.SkupTech.com mike mcs repair's Avatar
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    Internal mag timing??

    burnt distributor or block?

    im not a mag fix person. I run them over to an overhaul shop

  3. #3

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    I looked at the distributor and the wire block and both were in good shape.
    Internal timing, could be I don't think I'm smart enough to check that myself.
    Is a guy better to send it to a shop for repair or just buy a rebuilt one??
    THANKS

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    aktango58's Avatar
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    Did you confirm that the right and left switches do indeed work on the right and left mag as labeled? Sometimes it is as simple as your checks, but you work on the wrong one.
    I don't know where you've been me lad, but I see you won first Prize!
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    algonquin's Avatar
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    By sparking at the mag you don't check the ing. Lead/ wire to the plug. IMG_2669.PNG
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    algonquin's Avatar
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    The above is from the Slick manual but it applies, good luck and be careful pulling wire while running lots of shock power.

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    Maybe the other mag is hot all the time and its not grounding.
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  8. #8
    cubpilot2's Avatar
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    Quote Originally Posted by floatpilot View Post
    Hi
    I have Bendix mags on an 0-320 b2b and on the left mag I'm getting a 200+ rpm mag drop when I do a mag check. I did a search on here and read a bunch of things it might be .
    I changed the plugs that go to that mag one at a time,no luck,I even got brave and while it was idling I pulled the plug wires off each plug from that mag one at a time and there was a NICE spark that was jumping across from the wire to the plug when I pulled each one, plus I could hear an RPM drop as I pulled them and an RPM increase when I put them back.
    I checked the timing with buzz box, it was out a couple degrees so I set it back to 25 degree same as the right mag. Still didn't help . So I pulled the mag looked at the points and checked them to see and they were .018 gap.Cleaned things up reinstalled and still the same .
    Any more thoughts??
    THANKS

    As mentioned above you may have distributor contacts or its brush that are getting worn or dirty but to check will require complete disassembly. Do this only if the following doesn't help.

    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)
    Point/contact wear is the common place where your "internal timing" can shift. The other "internal timing" is the internal gear timing; which is only out of position if assembled wrong, or if the gear strips. (Mag would be totally dead)

    If your timing had shifted two degrees and your getting a fairly smooth drop then expect that the point setting has shifted. Condensers can also cause trouble but usually shows up as "burnt contacts".

    If the points are not set properly you will have a weaker spark which often shows up in hard starting ( especially if the left mag) or by excessive drop.

    Good luck
    Last edited by cubpilot2; 07-24-2017 at 10:11 PM.
    Ed

  9. #9

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    THANKS For all the tips.
    Ed are you saying that if I use the buzz box as you say to set the e-gap so the the points are just starting to open ,that the points will end up at the correct .018 gap when open??
    Thanks

  10. #10
    cubpilot2's Avatar
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    It should be close; but don't worry about it so much as when they start to open. The "E gap" is critical.
    Ed

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    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.

  12. #12
    Weasel's Avatar
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    Not very common but I have had a weak coil. When removed from the mag you could see a burnt place on the outside. Check the resistance across the primary and secondary windings.

    Sent from my SAMSUNG-SM-G870A using Tapatalk

  13. #13
    behindpropellers's Avatar
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    Quote Originally Posted by floatpilot View Post
    Hi
    I have Bendix mags on an 0-320 b2b and on the left mag I'm getting a 200+ rpm mag drop when I do a mag check. I did a search on here and read a bunch of things it might be .
    I changed the plugs that go to that mag one at a time,no luck,I even got brave and while it was idling I pulled the plug wires off each plug from that mag one at a time and there was a NICE spark that was jumping across from the wire to the plug when I pulled each one, plus I could hear an RPM drop as I pulled them and an RPM increase when I put them back.
    I checked the timing with buzz box, it was out a couple degrees so I set it back to 25 degree same as the right mag. Still didn't help . So I pulled the mag looked at the points and checked them to see and they were .018 gap.Cleaned things up reinstalled and still the same .
    Any more thoughts??
    THANKS
    How long since they were overhauled? Harness condition?

  14. #14
    www.SkupTech.com mike mcs repair's Avatar
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    have you pulled back the nut/outer braided shield and inspected the Plead inner wire??? it can crack and partial short out

    also check switch/wire

  15. #15

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    P leads are good and it has new harness on it 80 hours ago.
    The mag itself according to log book has 700 hours on it.

  16. #16

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    If you do send it out I would use Aircraft Magneto Service.
    DENNY
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  17. #17
    hotrod180's Avatar
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    Quote Originally Posted by DENNY View Post
    If you do send it out I would use Aircraft Magneto Service.
    DENNY
    http://www.aircraftmagnetoservice.net/

    http://www.aircraftmagnetoservice.ne...shooting-guide
    Cessna Skywagon-- accept no substitute!
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  18. #18
    algonquin's Avatar
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    If there is 700 hours on the mag's your due for a 500 hr check on both of them. That mag drop is the way they are telling you " keep going and it's going to get quite ". The power will erode slowly as they wear, when you get then overhauled it will surprise you what you have been missing.
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  19. #19

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    Well I just finished setting the E-gap as Ed suggested but that didn't help so she is off to the mag shop for repairs.
    THANKS Again everyone for your help.

  20. #20
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by floatpilot View Post
    Well I just finished setting the E-gap as Ed suggested but that didn't help so she is off to the mag shop for repairs.
    THANKS Again everyone for your help.
    send both of them.... very few things make you come out of the sky.... but these are one...

  21. #21
    www.SkupTech.com mike mcs repair's Avatar
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    also swap plugs with other mag first and see if problem goes to other mag.....(plug issue)

  22. #22
    www.SkupTech.com mike mcs repair's Avatar
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    did you spit on a finger and touch exhausts after running ONLY on bad mag? find a colder pipe?? if one pipe cold, plug/lead problem, if all hot then mag problem.... or use a inferred thermometer

  23. #23

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    200 - 250rpm drop was always a plug. Mags either worked or they didn't.
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  24. #24
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by graham ron View Post
    ..Mags either worked or they didn't.
    thats not true....(in the sense that mags either work or are dead)

  25. #25

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    I would suggest you Contact Aircraft Magneto Service..... They will give you some tips on determining if it is the MAG or not. They are very very honest and do incredible work. Do a search on the mighty interweb for Aircraft Magneto Service and they will help you out.

    Dan
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  26. #26

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    Yeah Mike even did check for cold cylinder after running on bad mag only,swapped plugs with good cylinder plugs,even went one plug at a time on bad mag and put in new plug.I have done every suggestion you fellows have offered .Even called the mag shop and talked to them for 30 minutes and he gave all the same suggestions you guys did.So they are now boxed up to get shipped out. I did talk to a friend that twice over the last 30years had the same problem I have ,on 2 different planes and both times he took the whole plane to the shop and both times it ended up being the mag itself and they had to do a mag overhaul.
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  27. #27
    180Marty's Avatar
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    Make sure you find out what the problem is and let us know. Please and thanks

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    When the did mine I sent in my harness also, they checked it for free.
    DENNY

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    Was it a rough 200+ with like a cylinder out or smooth but larger drop than normal? Try leaning it a little then check it while leaned.
    Wal

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    Quote Originally Posted by mike mcs repair View Post
    thats not true....(in the sense that mags either work or are dead)
    From a mechanics (mechanical) perspective that's correct. From a pilot's perspective or someone that is limited to basic maintenance by either knowledge or legalities, it's either a plug, wire, P-lead or a sour mag. If it's not the first 3 then it's the mag. Unless you really know what going on in there, take it to someone that does otherwise you'll be chasing your tail.

  31. #31
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    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."
    Cessna Skywagon-- accept no substitute!

  32. #32
    hotrod180's Avatar
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    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
    Cessna Skywagon-- accept no substitute!

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    skywagon8a's Avatar
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    Quote Originally Posted by cubpilot2 View Post
    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)
    Quote Originally Posted by floatpilot View Post
    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.
    N1PA
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  34. #34
    cubpilot2's Avatar
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    Quote Originally Posted by skywagon8a View Post
    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)
    Ed

  35. #35
    Wag2+2's Avatar
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    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.

  36. #36

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    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!
    Jim Parker
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    ?? Bearhawk Patrol - Building
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  37. #37
    G44's Avatar
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    Quote Originally Posted by DENNY View Post
    If you do send it out I would use Aircraft Magneto Service.
    DENNY
    Yep! I second that.

  38. #38
    Steve Pierce's Avatar
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    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.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

  39. #39
    skywagon8a's Avatar
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    Quote Originally Posted by Steve Pierce View Post
    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.
    Gravity.
    N1PA
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  40. #40
    Steve Pierce's Avatar
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    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.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers
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