Howdy everyone:
Hopefully, this is the latest and final problem with the 1969 CB750. Riding along the Kansas Highways at about 4,200rpm (65 mph), about 80 degrees and sunny, after about 50 miles, the engine began misfiring and backfiring. Nursed it home and started to work on it. Float bowls were full, spark plugs were OK, spark plug tester showed the same current on all four plugs, so I was baffled. Took off the points cover and saw the 1-4 sparking normally, but the 2-3 had no spark at all. The complete points system has about 1,000 miles on new condensers and new points. Did the condenser go bad? Can anybody give me some direction as to how to fix this? Thanks to all in advance.
Spark at the points explained.
When the points are closed, current flows through them and into the coils. The coils build up a magnetic field, which holds until the current is interrupted, as when the points open. This is where the major action takes place. When the magnetic field collapses, the lines of flux now crossing the coil's windings induces a voltage within them. This voltage will exist as long as the mag field is collapsing. As the coil consists of both primary and secondary windings, voltage is created in both of these windings as the field collapses.
In large part, what happens in the secondary is "reflected" in the primary on a smaller scale.
So, let's talk first about what happens in the secondary circuit. As stated, when the field collapses, voltage builds in the secondary. The secondary circuit consists of the secondary coil winding, the spark leads, the coil leads, two spark plug caps, the spark plug electrode gaps, and the electrical connection between the plug electrodes through the cylinder head. If the coil's stored magnetic field was large enough, there will eventually be enough voltage created (5-6KV) to ionize the atmosphere within the spark gaps, create a plasma channel conduit and allow current to flow in the secondary circuit and within the spark gaps of the spark plugs (spark). The current flow essentially stops the voltage build up in the secondary circuit (energy being bled off), and as long as the coils mag field is still collapsing, current and voltage will be maintained within the spark gap. Eventually, the coils stored energy will be expended, and the spark event will cease, as the voltage will become too low to sustain the plasme channel between the spark electrodes. All this takes place during a span of time in the 40 millisecond range.
Voltage in the primary circuit.
Normally, when the points open, the built up field in the coil collapses, as stated above. As the field collapses, voltage is created in the primary windings. With a very small gap at the point opening ramp, the 200V or so volts potential across the gap creates a small spark. During this spark there is current. The coil's voltage rise leads the current build up by about 90 degrees, however a capacitor causes current to rise faster than the voltage by about 90 degrees. Using both these devices in the same circuit during points spark mitigates the power factor and thus point contact heating. The capacitors (condensers) do not eliminate points sparking, just minimize its detrimental effects.
The capacitor also speeds the collapse of the coils primary field (and thus the secondary's, too), because of its current leading charge characteristics. This speeding of collapse intensifies and quickens the voltage build up in the secondary circuit, as more lines of flux cross the secondary wires in a given time period.
So what can go wrong?
Well, a spark difference noted at the points must also effect the quality of spark at the plug electrodes.. Even though there is still sparking occurring there, it's different than what it should be. The difference could be caused by either a primary or the secondary issue (or both).
Gotta start somewhere. So, let's go to the primary side. Ideal capacitors (condensers) have no direct current path between their two connection points. In reality, they do have a small amount (close to infinity) and failed capacitors can decrease that resistance toward zero. The effect in this circuit would be a smaller buildup of magnetic field in the coil. A 9KV capability coil can still produce the 5-6KV needed at the spark gap, even with diminished input. It's just the spark duration is shortened.
So, cheap and easy test is swap the condensers on the bike and see if the point's spark difference follows the condenser. If so, probably a faulty condenser needs replacing.
Do check the point contact surface for carbon build up. As carbon is resistive, it can also change the coil's mag field buildup and fall.
The secondary circuit can also cause spark visual changes at the points.
Dirty plugs can, in effect, shorten the gap and require less voltage to create the plasma channel. Less voltage at the plugs, is reflected back into the primary circuit as less voltage fed back toward the points.
Plug cap resistors can fail, either by changing their value, or actually parting (open). The plugs still fire, but the extra gap increases the voltage needed to jump all the gaps present in the secondary circuit. So, instead of needing only 5Kv to make spark in the secondary, perhaps 7KV is required. Again the higher voltage development in the secondary is reflected back to the primary and is seen as a brighter spark at the points.
Extra gaps may also be at the secondary lead connection points, too. All the gaps in the secondary circuit increase the voltage requirements to jump all the gaps present. The larger the sum of the gaps, the higher the voltage needed to arc at all of them. The higher the voltage developed in the secondary, the brighter and more severe the sparking at the points.
Poor secondary insulation.
Cracks and deterioration of the spark leads, can lead to shunting of voltage and power in the secondary circuit. This can lead to misfires if the coil energy is shunted off too quickly, in which case the primary would see less power/sparking, too. Or, it can raise the voltage needed to actually spark the plugs. Raise the secondary voltage, and the voltage seen at the points increases, as well.
Now that you understand the above, I'll mention the effect of "high voltage" coils.
The stock coils can make about 9-12KV in ideal circumstances. But, the spark gaps and the atmosphere composition between the electrodes determines that actual voltage developed in the secondary. Simply replacing the coils with ones that can develop more than 12KV, does NOT mean the plugs fire at that higher voltage. In fact, they fire at exactly the same voltage as the stock coils. They will increase the current while the plugs are sparking (more electrode erosion) and/or allow the plugs to be set at a higher gap distance, which will increase the actual voltage needed to jump the increased gaps. Higher voltage capability is needed if the atmosphere between the spark electrodes is changed, as it would be when the engine compression ratios are changed. So, realize that just because a high compression engine needs a higher output coil, does NOT mean a higher output coil will make more power in an unmodified engine. In fact, it will consume more electrical power without providing any benefit unless other changes are made to exploit the higher voltage capability.
So, be wary of salesmen or marketeers, that tout high voltage coils, particularly with "bench-proof". Unless you plan to install that bench into your engine cylinders, it will be a placebo for your mind, rather than a real benefit to you or your bike.
