CORRECT SPARK PLUG POLARITY 


			
from http://www.themagnetoguys.co.uk/Spark_Polarity.php
When current flow causes a spark to jump from a gaps cathode electrode to the anode electrode, the
cathode electrode eventually shows signs of wear. ["cathode" refers to the negative electrode which in
this example is the ground electrode.] So, on a well used spark plug, it can  usually be seen that one
electrode is worn more than the other. If the voltage polarity at the plug is negative, then wear will
occur on the centre electrode.

[This is why most spark plugs are made with a 'harder' metal on the center electrode than what is on the
ground electrode. That is so the center electrode will not wear down fast. If the center electrode has
positive voltage and the spark plug is not made with a 'harder' ground electrode then the ground
electrode will wear down too fast. This is the case with most engines. The standard ignition outputs
positive high voltage to the center electrode of common spark plugs not designed to work with a reverse
voltage system. Also the plug polarity affects the sparking voltage, with correct polarity causing a
spark at lower voltage.] read on...

from http://mgaguru.com/mgtech/ignition/ig104.htm
The spark plug has a thermally insulated center electrode (surrounded by ceramic). With engine running
the center electrode runs substantially hotter than the exposed end electrode. Design of the ceramic
insulator determines how hot the center electrode will run, leading to the designation of hotter or
colder spark plugs. As electrons go, they love to jump away from a hot surface and fly toward a colder
surface, so it is easier to drive them from hot to cold rather than from cold to hot. End result is a
difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug
depending on which way it is going. So the spark plug prefers to see a voltage potential that is
negative on the center electrode and positive on the end electrode for the very first hop of the spark.
Oddly enough, this has nothing to do with polarity of the vehicle electrical system, but it is
influenced by the common connection inside the ignition coil.

from http://firetrucksandequipment.tpub.com/TM-5-4210-230-14P-1/css/TM-5-4210-230-14P-1_545.htm
One of the major causes for hard starting or spark plug misfiring under load results when the ignition
coil lead wire to the distributor is installed on the wrong side of the coil.  This condition causes
reversed coil polarity. Voltage  at  the  spark  plug  terminals  should  always  be negative.
Whether  it  is  or  not  depends  on  how  the  primary leads are attached to the coil.  Remember,
primary lead hook-up   directly   affects   coil   polarity,   which   in   turn   determines whether
voltage  at  the  spark  plug  terminals  is  negative  or positive. What   difference   does   it
make whether   positive   or negative  voltage  is  supplied  to  the  spark  plug  terminal? It
directly affects the amount of voltage required to fire the spark plugs.  When polarity at the spark
plug terminals is positive, it's  harder  for  the  voltage  to  jump  across  the  air  gap  than when
polarity at the plug terminal is negative.  Just why this is so is related to a pair of electrical
theories--the electron theory and the theory of thermionic emission. According to the electron theory,
all current flows from negative to positive.  The theory of thermionic emission states essentially it's
easier for electrons to leave a hot surface than a cold surface. Combining  the  two  theories,  one
finds  that  electrons will  always  leave  a  negative  charged  surface  for  a  positive charged
surface,  and  they  will  leave  the  negatively  charged surface with more ease when the surface is
heated. Spark   plug   design   is   such   that   the   center   electrode almost   always   operates
at   a   higher   temperature   than   the ground electrode.  Since it's easier for electrons to leave
a hot surface,  it  is  preferred  to  have  the  electrons  "jump"  from  the hotter center electrode
to the cooler ground electrode. When    the    center    electrode    is    negatively    charged
(negative  voltage  at  the  spark  plug  terminal),  this  is  what happens.  Stated another way,
putting the negative charge on the  hotter  center  electrode  causes  the  gap  to  be  ionized  at
lower  voltage.    (Ionization  is  necessary  to  permit  passage  of the   spark   through   the
high   resistance   of   the   gases   in   the cylinder.) When positive voltage is supplied to the
plug terminals, which happens when coil polarity is accidentally reversed, the hotter center electrode
becomes positive charged. Consequently,   electrons   must   leave   the   negative   charged ground
electrode  and  move  to  the  positive  charged  center electrode.  But, since the ground electrode is
cooler than the center electrode (and remember, it's easier for the electrons to leave  a  hotter
surface),  it  takes  more  voltage  to  make  the current jump the gap-in fact, up to 45 percent more.

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Some motorcycles output positive voltage to the spark plug center. This can be corrected by reversing 
the wires going from the CDI to the high voltage coil and disconnecting the hv coils frame from the bike 
frame. I have done that on one of my bikes after noticing that the spark plug had a dished out ground electrode 
where the electrons were carrying the soft metal over the gap to the center electrode. Just make sure you 
coat the hv coils metal frame with silicone or something else to prevent shocks if you touch it and the engine 
metal at the same time. The coil can be connected to the frame with hose clamps. I have designed
a CDI for experimentation that has a reverse polarity voltage spike going to the high voltage coil but it won't
be made available for purchase. The two motorcycles that I can provide CDIs for both have the correct spark plug
polarity. 
 
From Graham Bells "Two Stroke Performance Tuning":
incorrect plug polarity

How to test for spark plug center electrode polarity:
testing spark plug polarity

Last time I did it I couldn't get the alligator clip to get a good bite on the center electrode so I jammed a kitchen knife into the spark plug gap and then connected the alligator clip to the knife blade. If you do this test then let me know what your results are.