Author Topic: 750 with old 3 ohm coils  (Read 963 times)

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Offline jakec

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750 with old 3 ohm coils
« on: August 04, 2020, 10:40:11 PM »
I have this 750 I want to get running which has these old crusty Dyna coils which are marked 3 ohm. The plug caps on the wires looked really poor and weren't fitting to the new plugs well, so I installed an old set of caps left over from my 400 rebuild.

I tried to start the bike and wasn't getting anywhere. Just wondering if mixing the 3 ohm coils with the caps off the 400f (5 ohm resistance type, I think). I am also using resistorless plugs (D8ea).
1970 CB750 K0
1977 CB750 Chop
1997 XR650L

Offline gtmdriver

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Re: 750 with old 3 ohm coils
« Reply #1 on: August 04, 2020, 10:59:12 PM »
3 ohms is the resistance in the primary coil windings and doesn't affect the resistance in the secondary high tension circuit. 3 ohms is correct for ignition systems using contact breakers. Are you using resistor plugs along with suppressed (resistor) plug caps? That might be a bit much if the rest of the system is marginal.

Offline jakec

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Re: 750 with old 3 ohm coils
« Reply #2 on: August 05, 2020, 08:29:50 AM »
I am using resistor plugs and non-resistor caps. The type of setup that has been on my 400 and 550. I just have never seen or used 3 ohm coils so I was wondering if the combination was going to cause a problem.

I haven’t done anything besides what’s mentioned in the original post, so there are many other factors to why it wouldn’t start. Just  wanted to rule this one out!
1970 CB750 K0
1977 CB750 Chop
1997 XR650L

Offline madmtnmotors

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Re: 750 with old 3 ohm coils
« Reply #3 on: August 05, 2020, 08:33:50 AM »
I am also using resistorless plugs (D8ea).

I am using resistor plugs.

 ???
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Offline jakec

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Re: 750 with old 3 ohm coils
« Reply #4 on: August 05, 2020, 09:34:30 AM »
Sorry, I was on my phone typing that. to clarify I am using RESISTOR type CAPS and NON RESISTOR type PLUGS (D8ea).
1970 CB750 K0
1977 CB750 Chop
1997 XR650L

Offline Don R

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Re: 750 with old 3 ohm coils
« Reply #5 on: August 05, 2020, 09:57:08 AM »
 Stock coils are 5 ohms, these has been discussion whether 3 ohm coils tax the charging system. I wasn't able to check the coil. wire and caps together because my meter wouldn't measure it until I took each part off and checked that way.
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