I was thrilled the other day when my chainsaw started right up after winter storage. If you know 2-cycle engines, this small success raises an eyebrow. No carb adjustment or anything - it just started.

My happiness was short-lived. Ten minutes later the saw died and even my typical trick of pouring a little gas into the carb couldn't get it going. It appears to have no spark anymore.

This saved me a lot of work that afternoon but also pissed me off big time.

My magneto is "type AM 32" very much like this one but nearly 30 yrs older. I'm wondering if there are any useful bench tests I can do on this thing.

Here's mine. The wire was grounded to the mounting bolts. The other tab connects to a wire that goes to the kill switch, the opposite end of which has a wire to ground. I believe the switch kills the engine by connecting the magneto tab to ground, making both sides grounded. It's a crude switch but I'm fairly certain it was not the problem.

There are some gashes in the spark plug wire that have been there a long time. Maybe they finally corroded enough to cause a problem? They don't look that awful but I can see the conductors inside.

These magnetos, if you can find one, sell for about 3/4 the price of new chain saw. A used one is $25-30, which is also a problem.

 

I usually hit the ends with some emory paper, then reset the magneto's distance from the flywheel using a sheet of typing paper. Did you measure the continuity of the main coil?

It might be time for a new chainsaw.

 

I usually hit the ends with some emory paper, then reset the magneto's distance from the flywheel using a sheet of typing paper. Did you measure the continuity of the main coil?

It might be time for a new chainsaw.

Continuity between the two terminals? Or spark plug to...what? Not yet. And yeah, plenty of chainsaws on Craigslist.

 

Continuity between the two terminals? Or spark plug to...what? Not yet. And yeah, plenty of chainsaws on Craigslist.

There should be continuity between the plug terminal and one (or both) of the terminals on the side of the coil (it's normally plug terminal to ground). If not, then your high voltage winding has opened and you'll need a new magneto.

 

There should be continuity between the plug terminal and one (or both) of the terminals on the side of the coil (it's normally plug terminal to ground). If not, then your high voltage winding has opened and you'll need a new magneto.

There is 184Ω between the ground wire and the terminal lug, and 2.2KΩ from the spark plug wire to the ground wire. The spark plug I have in the photo adds 40K, from the tip of the plug to the ground wire.

Just off hand, these sound reasonable?

 

Those coils have electronic switching inside them & are usualy pretty reliable. But if you can see the conductor through the cuts in the insulation then the spark may jump to ground from there rather than the plug.

 

Those coils have electronic switching inside them & are usualy pretty reliable. But if you can see the conductor through the cuts in the insulation then the spark may jump to ground from there rather than the plug.

Any idea if that wire could be replaced? I have some spare (car) plug wires on hand but I'm afraid to pull the old one out of the socket on the magneto. I've found a number of posts suggesting that the wire is screwed and glued into the magneto. So not impossible to replace, merely difficult.

 

Most 'regular' car ignition wire today is not metallic, but graphite impregnated Kevlar, or at least GM ignition wire is. The older mower and chainsaw ignition coils the wire could be removed and replaced.

 

Most 'regular' car ignition wire today is not metallic, but graphite impregnated Kevlar, or at least GM ignition wire is. The older mower and chainsaw ignition coils the wire could be removed and replaced.

Yeah, the wire I have on hand would likely not work. It apparently needs 7mm copper core.

 

Today I reinstalled the magneto and used my power screwdriver to turn the flywheel. It will spark but just barely. Not enough to fire a plug but enough to feel by hand and to spark across a tiny gap.

I used two sheets of notepad paper to set the air gap to the flywheel. Maybe that's too much but surely that's not the problem?

So I guess I may as well rip the spark plug wire out and see if replacing it would help.

 

Today I reinstalled the magneto and used my power screwdriver to turn the flywheel. It will spark but just barely. Not enough to fire a plug but enough to feel by hand and to spark across a tiny gap.

I used two sheets of notepad paper to set the air gap to the flywheel. Maybe that's too much but surely that's not the problem?

So I guess I may as well rip the spark plug wire out and see if replacing it would help.

Two sheets of paper is approximately 0.006", which is too much. One sheet is just about right. If you're getting voltage to the plug, the wiring is okay. DO NOT REMOVE THE PLUG WIRE FROM THE MAGNETO! If you do, you will most likely destroy your mag. If the cracks in the wire bother you, then either use some electrical tape to wrap it, or use some RTV silicone to cover the cracks. The silicone has a high enough of a breakdown voltage to fix the cracks.

 

Perhaps the weak spark is due to the flywheel magnets losing their strength?

 

It literally quit while running, so that seems unlikely to me. The flywheel magnet is definitely still a strong magnet but I have no way to judge if it used to be stronger.

Perhaps the weak spark is due to the flywheel magnets losing their strength?

 

Two sheets of paper is approximately 0.006", which is too much. One sheet is just about right. If you're getting voltage to the plug, the wiring is okay. DO NOT REMOVE THE PLUG WIRE FROM THE MAGNETO! If you do, you will most likely destroy your mag. If the cracks in the wire bother you, then either use some electrical tape to wrap it, or use some RTV silicone to cover the cracks. The silicone has a high enough of a breakdown voltage to fix the cracks.

According to this, my spark gap is supposed to be 0.011-0.015". (I can't seem to find my manual despite carefully saving it for 30 years. Grrr.)

Anyway, today I set the flywheel-coil gap to 0.002" and repeated my spin test. Now it sparks the plug but only weakly. Maybe that's all you would expect? The screwdriver seems to spin it fairly quickly but maybe a pull on the rope is even faster?

While browsing for a replacement, I did run across this useful reference.
http://beru.federalmogul.com/sites/default/files/ti_07_ignition_coils_gb_2013_lowres.pdf

 

According to this, my spark gap is supposed to be 0.011-0.015".

Better look at your reference again. From your link.
"SPARK PLUG GAP:

0.025 in. (0.63 mm)"

 

For what it's worth. I just junked a couple of old Mac saws, they were running when I stored them but couldn't get them to start a couple of years ago. So having some time I started to work on them. They both had good spark. I cleaned and rebuilt the carbs. Still wouldn't start. Wouldn't even fire on starting fluid. Know what the problem was? The crankshaft bearing seals had dried out and weren't sealing the compression in the bottom end of the motor. On a 2 cycle the fuel charge is partially compressed in the crank case without that there isn't enough to fire in the cylinder.

 

Better look at your reference again. From your link.
"SPARK PLUG GAP:

0.025 in. (0.63 mm)"

I should not have called it the spark gap. I was referring to the flywheel-magneto gap.