Topic: Resistors

[upperlake04]

From http://www.sohc4.us/forums/index.php?topic=4051.0 by Cvillechopper on Sept 13 2005

5k resistance is good for supressing interference but you shouldn't use resistive plugs, caps and wires at the same time (?)   -SteveD CB500F

That's a good summary, I think.  And, I might add that that is exactly what Honda delivered in stock form.  I haven't found many extraneous, non-useful, parts on these bikes.  - TwoTired

This discussion of electrical theory is over my head  but I just looked over the 78 750F. Don't know what is stock - Coils TEC FL703-12V7.9G and unmarked wires except numbered 1-4.
                     Nichiwa caps SDO-57 (straight) and VDO-57(angled)  show 5000 ohms on the mm 
                     Spark plugs are resistor type, NGK DR8ES-L, which is correct  for the Canadian  model.

Questions - why Cdn. models with R plugs?  too many resistive components?  what to change?  will I  notice a difference?        Thanks,    Dave

[Ibsen]

The coil generates a certain amount of energy. This energy wants to go somewhere. At a voltage of about 7KV the plug fires (irrespective of resistance). Until that happens NO CURRENT FLOWS. Whether you have resistance or not does not matter. A paltry 5KOhms does not do anything when compared to the nearly infinite resistance of the gap itself (until the plug fires that is).

Once the plug fires the resistance comes into being. The coil cannot get rid of its energy in the shortest possible time due to the resistance.

This reduces current flow in the spark and it takes quite a bit longer until the energy in the coil has expelled itself via resistor and spark gap. This results in a longer spark. However the spark is weaker due to energy loss in the resistor.

A scope shows this very nicely and it also gives reason why the resistor helps to suppress RF. With the resistor changes in the rate of current flow are much less resulting in "flatter" edges on the voltage vs. time curve that the scope shows you. This implies less high frequency components of the signal in the cable feeding the spark plugs.

Conclusion: It is quite safe to insert a resistor. It will not stop the plug from firing at all. High values will however lead to a weaker spark -but it is going to spark, no matter what.

However, contaminate the plug with fuel (even just a little) and a high value resistor will cause the plug not to fire. 10K plugs as are used on motorcycles seems to be about maximum.

[upperlake04]

Why do you think the Canadian model calls for Resistor plugs?

All of my minuscule electrical system knowledge has come from this forum. While not having read precisely that Cdn. model 78 750Fs require resistor plugs, I have read that any plug with an 'R' in its number should be avoided because it signifies 'resistor'.  The owners manual specifies NGK DR8ES-L for here and NGK D8EA USA


Ibsen - thanks for the very understandable overview.    Learning should begin with a framework of theory and then to the details, general to the particular - hasn't been that way on this project.  I've looked inside an electrical theory book but haven't been able to make myself read.   Hopefully I now know all I need to for these components

[Bodi]

The resistors give a longer duration spark that is presumably not as "hot". Since we have a dual fire system sparking 2 plugs, the resistance gets doubled. I would avoid resistor plugs and caps, although they will work I find I needed new plugs more often. With 5K caps and D8EA plugs I get good performance and good plug life. With 5K caps and 5K plugs you have 20K total resistance in the coil secondaries.
I've tried non-resistor plugs and caps and I had some misfiring at high RPM WOT. Resistor caps installed immediately eliminated that.

[upperlake04]

good real life stuff Bodi - so using one resistive component works well, using both plugs and caps not so good.  I will change one or the other. 
  I'm still under the assumption that the Honda specified resistor plugs  and 5K caps for this market - why would that be?  Maybe to satisfy a regulation friendly to the users of HF signals?

[smashme33]

I have a 1980 CB650C Canadian model. I just use D8EA plugs and 5K caps. Runs great for me.

[dafrpa]

Why do you think the Canadian model calls for Resistor plugs?
Cheers,

The shop manual calls for them. Not sure if those Nichiwa caps are resistors on our sleds or not. I've got the NGK resistor caps on, running the DR8ES-L plugs. To my surprise, they're coming out black. I expect this has something to do with the exhaust being stuffed with an unknown amount of packing and other junk. (I pulled 4 washers with 1/2 inch holes from the end of an aftermarket 4 to 1 of unknown origin, there may be more...) I'll be trying the Nichiwas I still have and looking for the right plugs. Those coil numbers sound right, though, from memory.

Dan, Ottawa

[Ibsen]

I'm using DR8EA plugs in my CB650, which is the correct plugs for the European model.