Coil conversion

[opposedtwin]

Some time ago there was discussion (and a few good videos) about using OMC coils as a substitute for other type of NLA coils. And lately there has been talk of a military surplus coil being used as another type of substitute. Could we have that conversation again? I have used the search function on this site with no luck.

I need to find a decent coil substitute for my 1938 Thor Twin. I cleaned everything up and I am getting spark on both cyls but it’s a very weak white spark. I don’t believe it’s enough under compression.

If you know of good videos (you tube or otherwise) please share them here in this thread.

Speaking of good videos, where is Tubs? Haven’t seen his valuable posts lately and I know he has been down this road before. Tubs, are you out there?

Scott

[crosbyman]

may not be faulty coils… maybe the magnetic field is weak… maybe condensers are bad… have you tested them ??

Joining AOMCI has priviledges 🙂

[opposedtwin]

crosbyman wrote:

may not be faulty coils… maybe the magnetic field is weak… maybe condensers are bad… have you tested them ??

I guess that’s possible. I don’t know how to test a condenser. On mid 50s motors I usually just install new ones since they’re so readily available.

I tested continuity in the coil. The primary windings tested fine, but the secondary was very week. The needle on my meter barely moved. And so I figured that is why the spark looks so weak and white.

I will search for a good way to test a condenser. I don’t have any testing equipment. Where would I find the microfarrad requirements for this old magneto? Any ideas?

Thanks!
Scott

[crosbyman]

I tested continuity in the coil. The primary windings tested fine, but the secondary was very week. The needle on my meter barely moved. And so I figured that is why the spark looks so weak and white.

Are you using the proper scale on the meter … coils are not weak.…

they can only be

open...(broken internally)
have low resistance (internal short in the windings)
grounded (wires internally touching the laminates )
or…. good with 4-5-6-7-8 7 KOHM depending on specs using a 10-20 Kohm scale

testing caps is now easy if you build yourself the test box describded in the Outboarder magazine . If you work on oldies yes you can swap in new caps but even new caps can be bad … W.Mohat’s test box works great, not expensive to build if you are handy with a soldering iron or have a friend who is handy .

If you drop an e-mail to mr. Mohat he will send you the 3 diagrams but the Basic model works just fine shooting 270 volts in the caps.

The lamp tells you if good or no good period ! it is a fun project and stops any guessing.

Joining AOMCI has priviledges 🙂

• This reply was modified 4 years, 7 months ago by crosbyman.

[lyks2tinkr]

Generally white spark is good spark. When spark is weak its been my experience that it’s thin and purple.
You need to set your meter on the Rx1000 or K ohm scale to test the secondary. 3000 to 7000 ohms is generally a good coil.
As far as the condenser (capacitor)goes you can use a modern replacement of the correct value and type.
I use high dv/dt (volts per micro second) capacitors available from electronic supply houses.
I don’t know what the value was in your magneto but a common range is .15 uf to .4 uf.

[opposedtwin]

lyks2tinkr wrote:

Generally white spark is good spark. When spark is weak its been my experience that it’s thin and purple.
You need to set your meter on the Rx1000 or K ohm scale to test the secondary. 3000 to 7000 ohms is generally a good coil.
As far as the condenser (capacitor)goes you can use a modern replacement of the correct value and type.
I use high dv/dt (volts per micro second) capacitors available from electronic supply houses.
I don’t know what the value was in your magneto but a common range is .15 uf to .4 uf.

I will get a picture of my meter setting tonight.

I believe it is set as you recmmend. I’ve tested many omc and scintilla coils at this setting. As I said above, the primary winding tests good, burys the needle. The secondary only bumps the needle anlittle bit so I would say it measures less than 500 ohms.

So if the secondary winding is toast, how am I getting any spark at all?

I will get the condenser tested.

[crosbyman]

do you zero out the meter by shorting the probes and setting the needle on 0 before testing ??

I doubt they are toast if you get sparks. try good caps and touch up the points and wipe cleanwith carb cleaner

Joining AOMCI has priviledges 🙂

[opposedtwin]

I did all that save for the condenser. I will check it when able. Was told to set points at .020. Maybe too wide?

• This reply was modified 4 years, 7 months ago by opposedtwin.
• This reply was modified 4 years, 7 months ago by opposedtwin.

[Tom]

There is something (an unused connector?) sticking out underneath the wire connector for the condenser. That may be shorting to the inside of the flywheel. Also, verify that the kill switch is open. Consider removing the wires from the kill switch for further testing.

T

[Steve Martin]

Tom wrote:

There is something (an unused connector?) sticking out underneath the wire connector for the condenser. That may be shorting to the inside of the flywheel. Also, verify that the kill switch is open. Consider removing the wires from the kill switch for further testing.

T

I believe that’s the wire from the primary passing under the points.

[Author]

Posts