Mag trouble

[FunkyMonkey]

My A-65 is running rough on the left mag. Its not so much skipping as just missing but rather significantly. its most noticeable above 1200rpm. the plugs were cleaned and switched with the right mag, the mag "fingers" (not sure what they're called) were pulled and cleaned as well as the contacts. The result was no change. im looking at disassembling the left mag. Any ideas as to the cause? Any advice could help thanks.

[Robert Lees]

Re: Mag trouble

What type mag, FM?

[FunkyMonkey]

Re: Mag trouble

hmmm, not sure ill find out. it looks like a brick.

[Guest]

Re: Mag trouble

My A-65 is running rough on the left mag. Its not so much skipping as just missing but rather significantly. its most noticeable above 1200rpm. the plugs were cleaned and switched with the right mag, the mag "fingers" (not sure what they're called) were pulled and cleaned as well as the contacts. The result was no change. im looking at disassembling the left mag. Any ideas as to the cause? Any advice could help thanks.

Do you have an A&P looking it at? That may be a big help.

He might check the harness, test the plugs, look for water/condensation in the high voltage area...

We had 14 to 20" of rain in the last few weeks that may be related

Dave

[crispy critter]

Re: Mag trouble

Yes,I must agree with Dave.Check the harness(spark plug wires) before you break open a mag.

[lfries]

Re: Mag trouble

I'm not a mechanic but once I had rough running mags. In the end the intake just needed to be tighten up some and it smoothed right out.

[zero.one.victor]

Re: Mag trouble

Couple years ago, I had what I thought was mag trouble. My 170 had been getting hard to start occasionally, then one day it just plain wouldn't start,period. After a lot of fooling around, I finally got it started the next day and it ran like crap. Seemed like mag trouble, so I pulled the mags (3 year old Slicks) & had a shop bench-test /troubleshoot them. Everything was just fine. He recommended I replace the plugs- Champions, with only about 500 hours on them. I was sure that wasn't it, they were in real good shape, but finally let him talk me into it. He explained that the "R" in REM40E stood for resistor, and that the resistor breaks down a little every time the plug fires. As time goes on, it takes more & more juice to fire them- pretty soon there just ain't enough. I think that maybe the Slicks put out a punier spark than my old Bendix mags too. Anyway, after new plugs it started right up & ran just fine.
About a year later, it started running rough on one mag-- not at run-up, but at cruise power, so it took a while to figure out. Just felt kinda funny at cruise, but nothing you could really put your finger on--finally tried running it in-flight on just one mag, that's how I figured it out.Turns out the points were burned, so I replaced the points & condensor in that mag & that cured it.

Eric

[crispy critter]

Re: Mag trouble

A leaky fuel or primer line could also make you think you've got an ignition problem.It may not leak any fuel but it can still suck air and this can create roughness similar to a fouled plug or bad mag.Primer O-rings are easy to check and cheap to replace.

[FunkyMonkey]

Re: Mag trouble

thanks for the ideas. ill check to see what kind of mag it is next time i get to the airport. i do have a a&p looking at it and he's very knowledgeable but doesnt yet know what the trouble is so i thought id get any ideas from the forum that i could.

[FunkyMonkey]

Re: Mag trouble

the mag is a bendix SF4R8. the airplane is hangered so i dont think its water. switching the plugs from the left mag to run off the right mag eliminated the plugs. My a&p and i replaced each spark plug wire one at a time and started it up with no change. we also disconected the left p- lead isolating the p-lead and mag switch. with the left mag still running rough we decided to pull it and take a look. every thing looked good as far as we could tell. having eliminated all other possiblities however, we're pretty sure its the mag. my understanding is that parts are no longer available for these old mags anymore. what options do i have other then switching to the slicks? is there anything else to try? old airplanes are mind boggling

[Doc]

Re: Mag trouble

Hello: I just had two SF4R8 Bendix Scintilla magnetos overhauled (rebuilt) by Navajo Accessories in San Antonio, TX for about $400.00 each...parts ARE available and the overhaul was less than the cost of new Slick Magnetos. One mag had a corroded race and cost a little more to overhaul than its mate. I am overhauling the A-65 on a 1946 BC12-D that will be parted out. If you contact Navajo, ask for Johnnie. Good luck. Doc

[Guest]

Re: Mag trouble

the mag is a bendix SF4R8. the airplane is hangered so i dont think its water. switching the plugs from the left mag to run off the right mag eliminated the plugs. My a&p and i replaced each spark plug wire one at a time and started it up with no change. we also disconected the left p- lead isolating the p-lead and mag switch. with the left mag still running rough we decided to pull it and take a look. every thing looked good as far as we could tell. having eliminated all other possiblities however, we're pretty sure its the mag. my understanding is that parts are no longer available for these old mags anymore. what options do i have other then switching to the slicks? is there anything else to try? old airplanes are mind boggling

Parts are available.

What do you need? Points? Condenser?

Fresno Airparts has many items for them.

Look in Trade-A-Plane and get their number.

If you can't find it get back to me.

Dave.

[AlvinMcIver]

Re: Mag trouble

I just had NOS coils put into my mags. They came in hermetically sealed tin cans with a tuna can type key opener. The label indicated 1939 and they looked all bright and shiney - and it's way easier to start now.

[flyguy]

Re: Mag trouble

FM: You can try this with the mag you have out. (IF it has a coupler on it.) Put a plug on one of the plug wires, or all of them if you want. If you don't have shielded wiring you will have to run a ground to the plug shell.
Turn the mag in the correct direction until the coupler snaps around. You should get a spark at one of the plugs or if using just one keep doing that uptil that plug comes up and fires. If one fires you can make a little rig with a screw adjustment to vary the spark gap. Adjust it way out to see how long a spark you can get. My eisemann's will make a little 3/4 inch lightning bolt, so you should get a pretty good one too. If it will do that there probably isn't anything wrong with the mag. You can also do it with two pieces of wire taped to a dry board with a gap between the ends of the wire. Show this to your A&P, he will know what I am getting at. Might save you some bucks.
Darryl

[matsuthunder200]

Re: Mag trouble

Dont know if this will help but ive only had a mag screw up two times and both times it was the condencer. one time the mag did not work at all. the other time it ran ruff on that mag.