Sunday, May 13, 2012

Fun with magnetos

Many years ago, when a lawnmower died on me, I bought the Briggs and Stratton service manual, some tools, and opened up the engine, determined to get it running on my own, even though I know almost nothing about mechanics.

Hours later, I finishing reassembling it, except for the O-ring or two that was left over. I proudly said to my wife, "look at me; I fixed the lawnmower," gave it a pull only to have the engine make a loud bang and fill the garage with smoke.

I rolled it to the end of the driveway, put a "free" sign on it and by the next morning, my troubles were over; someone had taken it.

That was the last time I've disassembled any part of an engine.

Until yesterday, when I took a magneto off the airplane's engine.

I'm learning -- very, very slowly -- the ins and outs of magnetos. I had an RPM problem on the TMX IO-360 and it was suggested that the mag timing was off.

A pal, using the buzz box, set the mag timing, but when we tried to fire it up on Wednesday night, the prop whirred a time or two, then hung up briefly -- apparently this is a sign of a kickback about to happen. I had whirred it a few times with the mag off with no problem.

So, it was suggested, the mag needed to be internally timed. The mag was off the engine once -- when I was trying to get access to put the AN fitting on the oil port for the cooler lines -- and I probably wasn't very careful putting it back on ... or at least as careful as I needed to be.

What's odd is that I don't remember this occurring on first start last September and it only seems to have been a problem in the few times I've started the engine this spring. (It sat all winter)

Anyway, I took the mag off yesterday and took the distributor cap off and played around a little bit. I don't have a timing pin -- I've ordered one -- but just using an LP-4 pop rivet, I turned the rotor until it dropped in. This is the orientation it got on the impulse coupling assembly. It does not appear to be anywhere near the point at which the impulse coupling begins to wind and I was under the assumption that the timing pin assures that it is.

Another question: I notice in this excellent article it says that the mag should be reinstalled like so:

First, remove the top spark plugs from all of the cylinders. Then, turn the propeller in the normal direction of rotation with your thumb over the spark plug hole on the No. 1 cylinder. When the air pressure on the No. 1 cylinder starts to build up and tries to blow your thumb off the hole, slowly continue to turn the prop until the timing mark listed on the engine data plate lines up exactly with the split line on the top of the crankcase for Lycoming engines or the split line on the bottom of the crankcase for Continental engines. Typically, Lycoming engine timing marks are located on the starter ring gear and Continental engine timing marks are located on the propeller flange.

I've been led to believe the proper spot is when the 25 degree mark on the flange is lined up with the reference hole on the starter. Which is it?

Any additional education you can contribute would be most appreciated.


  1. Look at the the starter ring. The marks on the front side are not in the same place as on the back side. The front are so you can use the starter for alignment and the back are so you can use the engine spine.


  2. We didn't depend on the starter marks at all. We determined TDC several times using the Rite Mag tools, eyeballs, dowels, and every other way of determining, and THEN referenced to the marks on the ring gear (front and back) to be sure they were accurate. They were (on this unit, the ring gear can go on any of six different ways so I changed it to be fully accurate).