Forum

[Reg]

Just backtracking to Nigelphotos comment. "NGK red plug cap is resistor type". Does this mean NGK black plug cap is non-resistor? According to this web site both are resistor type. Or am I mis-reading something?

[Dennis C]

Someone is trying to mislead us again, a resistor wether in the plug, lead or cap is there purely to reduce interference, the resistor will reduce current flow giving a weaker spark for a longer duration. Look here to find out why and how . http://www.ultralightnews.ca/articles/r ... dplugs.htm

[Tim NZ]

One plug may be at fault, two rarely, three and the problem is elsewhere.
The simplest way to test the ignition system is to reduced the plug gap to .020". Go for a decent run to get every thing to operating temp, (20 miles) then reset the gap to +.030"


If misfire commences almost immediately the issue is NOT with the plug




Have you done any oil consumption checks?


IF the oil ring is 'pinched' in the piston (all too common) and the combustion chamber is being contaminated with Oil graphite deposits can form on the plug shell and insulation...




One of the more common causes of 'weak' spark is from oxidation inside the top of the coil Out-put tower.


Moisture can sit inside the top of the coil tower and the alloy will corrode (rot).

[Rattlebattle]

Quite so. You beat me to it. I have a very useful booklet entitled "Spark Plugging The Classics" by Stan Dibben; it is well worth seeking out. It explains pretty much everything there is to know about spark plugs for older bikes. Relevant quote "(the advent of in-car radio)...resulted in radio suppression systems such as carbon leads, "resistor" plugs (those usually with an R in the number) etc that removed the radio interference caused by the sparking plug". This was always my understanding.

[Rattlebattle]

A few posts in between. My initial comment was agreeing with Beezabryan.

[Dennis C]

Just to correct Some former mis information, changing an NGK plug from 8 to 9 is in fact going to a colder plug not a hotter one as stated. Also as I said earlier which has been repeated by others, DO NOT use any form of resistor with a magneto it can and does cause it to over heat and will damage it eventually.

[Presto]

And Champion plugs go in the other direction - a softer [hotter] plug being a higher number.

[jefrs]

Battery-coil ignition. The coil is a transformer. By placing a resistor in series with the spark plug it limits the load current. As a transformer's load current increases so its output voltage drops. This is true of all transformers. Ohm's law, at a given voltage a resistor will pass a certain current, I=E/R. The bigger the resistance, the less current will pass. In other words the spark voltage will be higher with with a resistor than without.

The fuel is ignited by the heat of the spark which is proportional to the voltage of the spark - more volts, hotter spark.



Magneto ignition is somewhat different. The output voltage is proportional to the rate it is spun, a magneto produces low voltage at start up, which can be a problem. A magneto system usually incorporates a capacitor which acts as a snubber, which increases the spark voltage.

[Mark M]

I recently discovered (from that nice Paul at the very motorcycle friendly MOT station at Gardner and White, Stoke Goldington, Bucks, highly recommended,) upon looking into the NGK catalogue that resistor plug caps can be had in both red and black. The black resistor ones are less likely to be in stock though and there are only a limited number of shapes. The non-resistor black ones are usually available at garden machinery places and cheaper than through automotive outlets. Also, NGK's heat grading system is the reverse of most (?) other manufacturers, ie, higher number is colder.

REgards, Mark

[zonggong]

I think the spark plug air gap may also be responsible for some of the resistance that jefrs discusses. I recall in old cars mounting the spark plug lead a few mm away from the spark plug terminal to get a 'hotter' spark. I guess it was OK as long as the coil could handle it. I have been using a BPR6EIX iridium as a replacement for the OEM twin earth plug on my 2015 B5. It seems to start and run more happily with that change. A number of other folks I know with C5s are also using the iridium plug successfully.