NGC - Do Good Coils Go Bad?

[sign216]

I'm posting this here, because of the board's wealth of mechanical knowledge.

Bike is  a 1965 Gilera 106cc, with 615 miles.  Has a moped-style electrical system.

I think the high tension coil that feeds the ignition coil, may be bad. But when I pulled it, it looks as nice and un-scarred as any coil I might buy. Hate to replace it needlessly.

Symptoms: when bike is warmed up, after 20 min, it starts to idle poorly, or not at all. At this time a tachometer run off of this coil's signal, will be slightly erratic. The display jumps from 1,900 rpm (normal idle) to almost 4,000 rpm, even then engine rpm hasn't changed at all.

Do "good" coils go bad, and develop unseen breaks in the wire insulation?

[Triple Jim]

They sure do.  The type your photo shows doesn't have a lot of hair-fine wire, so they rarely fail, but they can.  Are your points clean and the condenser known to be good?  I've had condensers partially fail and become heat sensitive.

[sign216]

I soldered on a new condenser a while ago. Solder job was ugly, but should have conducted.

Does the condenser's case ground to the engine? That might be a weak point.

[John A]

Heat it up with a heat gun with the engine cold and see if you can reproduce the symptoms. It may be failing when it gets hot. I realize its under the flywheel so that's sort of a project.

[Antietam Classic Cycle]

Symptoms: when bike is warmed up, after 20 min, it starts to idle poorly, or not at all. At this time a tachometer run off of this coil's signal, will be slightly erratic. The display jumps from 1,900 rpm (normal idle) to almost 4,000 rpm, even then engine rpm hasn't changed at all.

Disconnect the tach and see if the symptoms go away. When the electronic tach on a Loopframe Guzzi starts to fail, it can cause all sorts of weirdness. Could be a similar situation.

[sign216]

I now see that the Woodruff key is missing.  The key that aligns the magnetized flywheel on the crankshaft, to keep timing correct.  Without the key, it's possible that as the engine heats up the flywheel shifts and timing drifts.  Let me fix this, and see if it's a solution.

Now to find an assortment of metric Woodruff keys.

[1Sourdough]

I now see that the Woodruff key is missing.  The key that aligns the magnetized flywheel on the crankshaft, to keep timing correct.  Without the key, it's possible that as the engine heats up the flywheel shifts and timing drifts.  Let me fix this, and see if it's a solution.

Now to find an assortment of metric Woodruff keys.

Did you get the timing sorted?

[sign216]

I did. It was the middle of winter, so I had to heat up the crankshaft for the epoxy to stick, when I glued the key into place.  In addition replacing the woodruff key, I also rewound the ignition coil. The "exciter" coil at the flywheel.  That may have been the real problem, but I'm not sure.

After that, it runs very well. Although I'm still sorting out some other issues. I re-cut the spark plug hole, since there was room for an extra length spark plug. With a 1 inch reach spark plug, the bike idles much lower now. Although I had to back off the spark plug by 2 millimeters, to keep them from hitting the top of the piston.

The bike starts and idles beautifully, although I still haven't taken it out for a high speed run to check that end of the range.

[Chuck in Indiana]

I did. It was the middle of winter, so I had to heat up the crankshaft for the epoxy to stick, when I glued the key into place.  In addition replacing the woodruff key, I also rewound the ignition coil. The "exciter" coil at the flywheel.  That may have been the real problem, but I'm not sure.

After that, it runs very well. Although I'm still sorting out some other issues. I re-cut the spark plug hole, since there was room for an extra length spark plug. With a 1 inch reach spark plug, the bike idles much lower now. Although I had to back off the spark plug by 2 millimeters, to keep them from hitting the top of the piston.

The bike starts and idles beautifully, although I still haven't taken it out for a high speed run to check that end of the range.

I hate to rain on your parade, but if the plug is sticking "out" beyond the casting, it'll build up carbon on the threads and the first time you remove it, it'll bring aluminum with it..  :o

[rodekyll]

another thought -- sometimes the depth of the plug is determined by the distance needed to not burn a hole in the piston.  I'm not saying what you did is wrong, I'm just suggesting that there might be a reason for the way it was.

$0.02

[sign216]

 It can idle at a much lower rpm, and more smoothly.


Don't be a hater.

[rodekyll]

You have an odd sense of hate.