How to test a coil?

[Happy Rob]

Project bike, 1983 Honda xr 100. Was working, now no spark as far as I can tell. I have checked u tube and thumper talk but can't figure out how to ensure this coil is providing spark? Anyone here to help me? I do have a multimeter but am not sure if I am testing correctly. See photos. Over two feet of snow here today so I figured this would be a good time to play with the project bike but am now stalled. Any guidance appreciated.

 

[dwalby]

I do have a multimeter but am not sure if I am testing correctly. See photos.

Since the multimeter is turned off, and the leads aren't connected to anything on the coil, I would guess that you are not testing correctly.

Are you able to google for a coil resistance spec? Without those numbers as a reference, its hard to say if its good or not, but you may be able to detect something like an open circuit to find a hard fault in either the plug side or the 12v side windings.

I'm more familiar with multi-cylinder coils where you have a primary and secondary resistance to measure winding resistance across two leads. The concept is similar for your single cylinder coil, and I think you measure from each wire to the ground plate, but I'm not sure about that. Try it anyway, and you should see a few ohms on the primary lead, and several thousand ohms on the secondary (spark plug) lead. That's where knowing the exact spec will help, because there's no standard values.

Hopefully someone else with single-cylinder knowledge can confirm or deny my suggestion about measuring between each wire and ground for the coil resistance.

 

[Paul st1300]

I am not familiar with the Honda XR 100, does it has points. Most of the time with older bikers if the points connection are not perfect the spark will run to ground before it should and it will make you think the coils are bad. No spark at the plug or the points. Electrical current is always seeking the quickest way to ground. Points cover bent and to close to points or the wire connected to the points turned the wrong way or has the plastic insulator worn, dislodged or missing.

 

[Brant]

If it has points, then it has a condenser also. I would first suspect condenser to be bad, as that is a cheap fix.