In-line resistors don't effect the firing voltage at the spark plug tips. The actual gap and atmospheric conditions between the electrodes determines this. Resistors don't drop a voltage across them until current is flowing (when the arc is established). Then it has a limiting effect on the current and voltage peak development. The resistors remove some of the high frequency components (slows the rise time of spark current event) in the wire core which can be radiated to nearby receptors. The energy isn't lost, it is spread out over a longer time period. So, after the spark is established the spark voltage and current event is spread out over a longer period of time, making the spark event at the plug tips longer and able to more completely initiate the combustion cycle.
If a little is good, a lot is better, right?
No. After the arc is established, the voltage requirement to maintain the arc is less than what it is to initiate the arc and plasma channel that's been created between the electrodes. However, too much resistance will cause the spark maintenance level to be lost sooner, as it will drop the voltage coming from the coils during spark sustenance and therefore shorten the spark event.
How much is too much?
For the stock coils, probably more than about 10-15K on each lead. The stock plug caps vary between 5K and 10K on production examples, among SOHC4s with point type ignitions. All the 750's I've encountered had 5K plug caps. 550's seems to get 10K, usually. But, I have found 550 examples with 5K (I don't know that these were as delivered from Honda though).
I couldn't find a spec for the NGK plug internal resistor, and I don't have any to measure.
I did measured one of the spark plugs from my chevy, which had an inline 5K resistor inside it.
I would guess that Canadian models that specified resistor plugs, used a 5K plug cap with a 5K internal resistance plug cap resulting in a 10K total resistance between spark plug and coil. Something to note, is that since two spark plugs fire simultaneously, the total circuit resistance presented to the coil is quantified by the resistors in both legs of the circuit. So, I don't think I'd want to use 10K plug caps and 5K resistors in the plug leads.
The effect of too much resistance:
When new parts are first employed, and on a nice warm day, nothing much.
However, these machines have a tuneup interval spec for a reason. Part of which is, as the engine is used, the ignition components become less efficient. Spark plug tips erode and require higher voltages to fire and maintain the arc event. Points lose their crisp on/off behavior as well as proper dwell angle, and condensers can change value and/or increase their internal resistance. It is when all these things happen that more resistance in the ignition circuit becomes a factor in firing and maintaining the arc across the spark plug tips.
In short, too high a resistance leads to more frequent components replacement, i.e. spark plugs, points, and condensers, particularly when the the bike is cold, causing the electric start to draw down battery voltage, or when the battery is low or is becoming marginal in power delivery. In addition, other causes of low voltage getting to the coils can have an interactive effect on too much resistance in the ignition circuit, like power distribution within the bike, such as connector and switch contact voltage losses.
Your results may vary.
Cheers,
