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200 rpm Mag drop Bendix left mag

F

floatpilot

Registered User
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
 
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
 
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.
 
By sparking at the mag you don't check the ing. Lead/ wire to the plug. IMG_2669.PNG
 

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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.
 
floatpilot said:
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
Click to expand...


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:
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
 
It should be close; but don't worry about it so much as when they start to open. The "E gap" is critical.
 
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.
 
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
 
floatpilot said:
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
Click to expand...

How long since they were overhauled? Harness condition?
 
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
 
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.
 
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.
 
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.
 
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
 
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
 
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.
 
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
 
mike mcs repair said:
thats not true....(in the sense that mags either work or are dead)
Click to expand...
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.
 
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