Topic: Cb350f Spark Issue

[b_mick]

Hi! I recently bought a 1972 cb350f that had sat for about a year. The previous owner had owned it for 4 years and it sat this last year. I have it running but it did seem to sound strange so I started checking some stuff out. Cylinder one has no spark and by the plug it looks like it never did. I thought it might be the coil but I don’t know if it is because it has spark in the other terminal of the coil. It is an after market 5ohm coil that the previous owner said he got for cheap online. Could the coil cause one terminal to give spark but not the other, or could it be something else like? Attached is a photo of the coil and labeling of what I’m talking about.
Thanks

[Quattrocilindri]

Hi! I recently bought a 1972 cb350f that had sat for about a year. The previous owner had owned it for 4 years and it sat this last year. I have it running but it did seem to sound strange so I started checking some stuff out. Cylinder one has no spark and by the plug it looks like it never did. I thought it might be the coil but I don’t know if it is because it has spark in the other terminal of the coil. It is an after market 5ohm coil that the previous owner said he got for cheap online. Could the coil cause one terminal to give spark but not the other, or could it be something else like? Attached is a photo of the coil and labeling of what I’m talking about.
Thanks

That coil appears to be a Dynatek coil; but it is not mounted in an ideal position. If you turn it around front to back, the spark plug leads would be routed better and not forced against the top of the engine as they are now.
 Chances are the one not giving spark is improperly seated in the coil. Remove it, strip 1/2" at the end and reinsert in the coil by screwing it in. Then check again for spark.

[b_mick]

Okay. You mean like just flip the coil around to have the letters going the right way and to strip the cable that isn’t getting spark?

[HondaMan]

Also check the plug cap resistances. In both plug caps there shuld be a resistor, usually either 7.5k ohms (if from the 1970s) or 5k ohms if more recently replaced. Both caps need to have the same resistance on a coil, or else the spark will not be equal on both plugs.

Also: since the caps screw onto/into the cables, check to make sure they are well screwed-in at the cap's end, after you check the coil end per Quattro's tip above.

[Quattrocilindri]

Okay. You mean like just flip the coil around to have the letters going the right way and to strip the cable that isn’t getting spark?

Yes, so that the spark plug outputs point toward the front of the bike, not toward the carburetors. And yes, do an Ohmmeter check to verify leads and cap resistance. Both should be about the same.

[b_mick]

I’m getting no resistance in the caps. Also I tried using a plug and plug wire that was already getting spark in the terminal of the coil that wasn’t giving spark and had the same result. I also put the plug and wire from the terminal with no spark to one with spark and I got spark. I figure this shows that it’s not the plug or wire that’s the issue, but rather something else. There are two wires (one black, one yellow) going to the coil. The black is getting about 12v as soon as the key is on and the yellow gets 1v and when the start button is pushed it jumps to 2.8 and falls back to 1 with the rotation of the motor. Is that as it’s supposed to or is something wrong? I’m just trying to provide as much info as possible

[Quattrocilindri]

If you are getting a spark out of one coil wire, instead of both, the coil is getting voltage, and the problem is either that wire or cap. Very unlikely your coil has one damaged output.
By elimination, take both wires and caps off the coil and do continuity checks with your Ohmmeter individually on each. Each wire, without the cap attached, should have 0.0 Ohm resistance, or no more than 0.1-0.2.Ohms.
Each cap, not attached to the wire, should show around 5.5 K Ohms or 7.5 K Ohms, depending on the cap model.
If your meter shows OL on any readings, that wire or cap is defective. In this case, before throwing out the wire, strip 1/2" off each end so that the inner wire is exposed, and measure again.
If a cap shows OL resistance, it's time for a new cap.

If all readings of the ohmmeter are good on both wires and caps, inspect the coil wire cavities to ensure the copper spike is present and centered, not bent to one side, or corroded. Any of these conditions will require a new coil.

[Robbo]

I also put the plug and wire from the terminal with no spark to one with spark and I got spark. I figure this shows that it’s not the plug or wire that’s the issue, but rather something else.

Correct…that’s what I would have done.

Replace the coil.


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