OT re Fairbanks Morse Magneto

[wtrueman]

Hi all: Haven't been here too often lately. I have a Wisconsin Single Clylinder Air Cooled gas engine about 9 horse I believe, with no spark. It was rebuilt, I believe, about 40 years ago and has sat since then under cover but still in the elements. What is the breaker gap supposed to be with the Fairbanks Morse magneto? Does this magneto have to be "charged" and if so how? Thanks for your time, Wayne.

[portlandRon]

Think the people at this site could answer your questions better: http://www.smokstak.com/forum/

[rws]

I don't think it has to be charged. The only FM magneto I've dealt with was on an Allis-Chalmers 4 cylinder tractor. For some reason it would jump time every now and then. This mag had a spring loaded "something" that would spin the mag fast at each wire. (I'm talking back in my teens years, so it's been a while). Point gap is probably not that critical, maybe .020" or so? It's possible the coil has shorted out.

[J Tiers]

The magneto has a magnet, usually the rotor (that one should be a rotary), and if that has lost it's magnetism, it needs to be "charged". Sitting doesn't lose magnetic field particularly (not like a battery) but sitting does corrode coils. Taking it apart can lose field, though, so be careful, and use a "keeper" if you need to take apart.

There are magneto repair places, they advertise in Gas Engine magazine, or can be found on the web. the folks at Smokstak will know them.

Might just have corroded terminals that are "leaking" to ground. The F-M magnetos (rotary) I have seen are sealed cases, and might not be as affected by sitting as an open Wico.

[Lu47Dan]

One thing you need to replace is the condenser, they do go bad just setting. If the magneto has set for years the no spark problem could be the condenser. I had problems with the OEM condensers before in my TJD's, I replaced them with condensers from a 1970 Chevrolet C-20 truck with a 292CI straight six and they hold up well.

The point gap on my F-M mags on my TJD is 0.020" if I remember correctly, the manual is out in my shop right now.

I have two TJD's on equipment here one is on my Logsplitter and the other on my Buzzrig. I do not think I have replace a condenser since switching them out for the Chevy ones.
If you can bench test the mag, it will save a lot of screwing around. Bend up a piece of stiff wire. bolt it to one of the cap mounting holes and bend it so you have about an 1/16 -1/8" gap between the coil contact point and the end of the wire, mount the mag in a vise and turn the gear while watching the gap.
I have been bit off on one of my mag's and it is not pleasant.
Dan.

[jhe.1973]

Hi Wayne,

Lots of good tips already!

These mags are the same basic one that Harley had used for ages. I want to point out a couple of details in case you get into it. Here is a photo of one probably like yours:



The coil is held in place by 2 setscrews (early slotted, later socket head) that stick out a bit on the bottom of the case in this view. They push the central laminations of the coil up against the casting to make contact with the field laminations that are cast into the case. (tail end of the red arrow). Corrosion can isolate these laminations from each other. On the coil out of the case there is a brass strip that can be seen on the right lamination. This is to help insure contact & not all of these coils have it so don't be alarmed if yours doesn't.

The rotor can lose magnetism & I think it is more a matter of where it has stopped in relation to the field. I have never seen a keeper used during disassembly or storage but I wouldn't store the rotor on a steel bench or similar. Taking it apart & reassembly should not give you any problems.

The can full of parts is the impulse mechanism (spring something ) that that transfers drive to the rotor. It functions by storing energy from the gear and releasing it to the rotor in a sudden snap to give a hotter spark during hand cranking. Yours may not have one, but I suspect it does.

The tip of the red arrow is pointing to a CRUCIAL area that I wanted you to be aware of it you are going to try to remove the rotor.

The snap ring in the right foreground is under the rotor and is VERY easy to miss. I have seen countless cases destroyed because the rotor was pressed out w/o removing the snap ring first. The aluminum is very thin above the snap ring and shears easily so a person does not even 'feel' it break until it is too late. The ring has to be removed with a needle nose pliers w/good jaws 'cuz there isn't much to grab.

If you do get into your mag & it turns out to be a bad coil, you are welcome to have both of these (for shipping cost only) as I have no use for them. I don't know if they are good or not, I picked up these mags to convert them to a Harley application.

PM me if interested.

[gt2ride]

I have found that you need to clean the points as well as setting them. On my old eng. the ck valve
in the fuel tank will gum up. On a 9 hp there may not be any. That thing has to weigh over
1000 lb

Leon

[wtrueman]

Hi all: Thanks for the quick replies. I'll try out the simple things first. Wayne.

[bruto]

I've been running a FM magneto equipped Wisconsin on my Kut Kwick lawn mower for ages. Probably about the same horsepower as the OP. It's an AENL, the same engine used on the King Midget car once upon a time. They're very reliable, and except for the condenser and points, little else can go wrong. .020 as mentioned will do it for gap, or you can just use a piece of a matchbook and eyeball it. Just make sure that when the cam closes the points really close. If you're not sure about corrosion, use an ohmmeter. They corrode over time, and need to be very clean. Hard to find replacements and expensive, so it's worthwhile dressing them with a diamond file or the like. The impulse mechanism mentioned above in the Harley post makes a clanging noise while starting, which you should not worry about. Originally these magnetos came with a thin paper gasket between the cover and the body, and that is sometimes lost. It does more than you think to preserve the points, so make sure it's there and make one if you need to.

When shopping for points, if you do, be aware that these magnetos were made for many applications, and the points are specific to the direction of rotation.