Test for bad coils

[Groover]

Is there an easy way to test for bad coils? I'm still having intermittent issues, thought it was the key/switch but that does not seem to be the issue. Bike will misfire abruptly, mostly at low RPM, say making a slow turn or at a stop sign and it will then shut off (not always) as if I just hit the kill switch. Luckily it fires right back up. Visually inspecting, everything looks good. The symptom is just like a bad coil on a single coil bike, but this has dual and seems if one failed, the engine should still run? Right or no? What could it be? I have the original configuration with the Magneti Marelli oil filled coils, points, condensers and all that. This on a 1981 G5 - No side stand switch on the circuit anymore (but the symptoms are the same as when that started to fail)

[ozarquebus]

$9 DIY COIL TESTER (2008 prices)

https://model-a-ford.org/top-page-1/resources/technical-information-indexes/electrical-information/coil-tester/

[nc43bsa]

Doesn't sound like a coil problem to me;  sounds more like a intermittent power problem.  Does the bike still have its original 40-year-old wiring harness?  Stranded wires age and fracture internally with no outward signs.

[Groover]

Thanks for the replies. Need to check things a little closer then. The wiring harness is all new (installed 7 years ago) and all that I can see looks clean and and good. May need to clean all the main connection. Cool coil tester there! Maybe someday I'll make one.

[moto]

This sounds a lot like the problem that I had with the electrical connector between headlight shell and the main harness, on my T3. Turning the handlebars would slightly dislodge the connector, killing the motor as I recall. But putting the handlebars back straight ahead would often temporarily cure the condition. This was an intermittent problem and so was hard for me to diagnose.

[Groover]

Good tip, I will check those areas.

[lucian]

When 's the last time you checked the point gaps ? Or  replaced the point's and condenser's? 

[moto-uno]

  ^ +1 . FWIW , I still have an old Snap-On coil tester , it simply tested for the maximum spark gap it could consistently make
and then battery power was passed though it for 5 minutes to get it hot and the spark gap was rechecked to see if it stopped
all together or dropped significantly . Not particularly high tech , but great for points ignition systems .  Peter

[bigbikerrick]

have you checked the sensor, on the right side of the bellhousing to see if  it picked up any metallic swarf from the flywheel ring gear? Im not sure that would cause the engine to shut down, but it can cause an intermittent misfire.
Rick.

[Scout63]

I think it’s unlikely both coils would fail at once Groover. I’m guessing power to the coils is compromised somewhere or maybe a ground somewhere. Start with the main ground cable and the positive battery connection.  Put a test light on the coil + terminals. Also check the switched fuse to the ignition switch.  My 860 died mid ride today and it had blown the switched fuse to the ignition. You could install a relay to run power directly from the battery or fuse panel to the coils and just use the kill switch to trigger the relay.  Or just wire the coils to the switched fuse box and cut out the kill switch entirely like I do but don’t really recommend.

[Groover]

Thank you for the additional replies and suggestions. I'll be checking some of those items in more detail this weekend I hope. Will follow-up with what ends up being the fix. Thanks again!

[berniebee]

As mentioned by others, it's highly unlikely that both coils are failing. And yes, the engine should not die if only one coil fails.
 (BTW, the only 100% reliable test for a bad coil is substitution with a known good one. I've seen coils that test good act up only when warm or when the moon is waxing.)

Ideally, have the bike parked, the engine idling, and a fan to blow on the engine so that you can take your time. Attempt to reproduce the problem. Swing the handlebars back and forth several times. Wiggle/pull on all the wires you can reach. If you can set the bike up to run with the tank off and the headlight removed, that will give you better access. Jiggle the key in the ignition. Wiggle the wires going to the ignition switch.

With the engine off, pull every connector apart and reconnect, two or three times to scrape the corrosion off. Pull lightly on the wires coming out of the molex connectors to see if one is barely hanging on. And if you have the original fuse box, those tabs that hold the bullet fuses get brittle and weak. My SP1000 came with rubber bands wrapped around to hold the fuses tight. I replaced the box.

Intermittent problems can be a pain. Be glad you aren't paying a tech!

[Groover]

It was the switch. Posted more info in this thread. Will post photos later. Thanks again for the help.


https://wildguzzi.com/forum/index.php?topic=116237.0