'74 cb125 12v conversion, 3ohm dynatek coil?

picked up a 3ohm dynatek coil in a parts bin... didn't know what it was, i just need a 12v coil. Will this toast my bike? Does anyone know the rating of the stock coil?

Should work fine.

The coil will work, but will draw more current than the stock 4.5 - 5 ohm coil. You may have issues with the charging system trying to keep up especially at an idle or low RPM. These bikes run like crap with low voltage at the coil.....especially at low RPM's

rupaulpierce said:

picked up a 3ohm dynatek coil in a parts bin... didn't know what it was, i just need a 12v coil. Will this toast my bike? Does anyone know the rating of the stock coil?

Click to expand...

my stock 6v coil on the 72 cl100 is 1.8 ohms. Im just adding a 2ohm, 1%, 25w resistor (series). For the 12v conversion.

So the 5ohm model is preferable?


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I've read in a few places including below that Honda used 3ohm coils:

http://www.motorcycleclassics.com/classic-motorcycle-repair/replace-ignition-coils-honda-four.aspx

To be clear, I have a cb125 with a 12v cb350 stator and cm400 regulator/rectifier. I also ordered the smallest earthx lithium motorcycle battery. I didn't think you needed resistors unless you stuck with the stock coil (to prevent wear on the points).


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I forgot you have a dual output coil....The Honda FSM does not specify resistance for your coil, just continuity. This implies that the resistance is very low and hard to measure....So let me modify my recommendation...the 3 ohm, dual output coil should work.....

The CB350 has a 110W (about 9 amp max) alternator...So my warning about current still is valid. that 9 amps is a t full RPM, not at idle.

The CB350 had 5 ohm coils stock...a 3 ohm coil draws more current, but if you keep the loads down (LED lights) you should be OK.

Make sure you change your condenser to the 12V model too.....They actually absorb in the 300 volt range....

A small battery may not provide enough power to keep the bike running at a long stop light. Honda's use the battery for power below about 3000 RPM or so. The problem will be if you drag the volts too low on the battery, it will cease to work......Read the manufacturers site FAQ's on that.....

good luck.

Got a 12v condenser too. Bike is single coil.


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I use a pair of 3ohm Dynas on my CB350 and they worked perfectly. Stock charging system kept up fine, even at idle.

Started right up with the 6v coil, so hopefully this goes off without a hitch.


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DrJ said:

I use a pair of 3ohm Dynas on my CB350 and they worked perfectly. Stock charging system kept up fine, even at idle.

Click to expand...

Some do, some don't.....Not sure how yours keeps up at an idle....as the stock system can't....The generator output at 12oo RPM is less than 12V, so the bike runs off the battery at idle. You have something different if it can....

Here's the chart from the Honda Service Manual for the CB350.....At idle, the battery is not charging, the voltage is below 12V.... Extra load will degrade it slightly more.....

With normal light load, you are not breaking even on load until 2000 RPM..... So add the extra current of 3 0hm coils, and the RPM is slightly higher to break even on load....

Getting 13.9 at idle, 13.5 idle with headlight. Is this the best I'm gonna do?


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That sounds good 8)