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 Grubee 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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Spark plug polarity needs to be tested since some high voltage coils are opposite in regard to polarity 
of the output in relation to the input. If the plug polarity is incorrectly positive then that can be 
corrected by reversing the wires going from the CDI to the high voltage coil. If you have a my CDI 
you can reverse those wires as long as the high voltage coil does not mechanically or electrically connect 
its metal core to the bicycle frame or engine metal. I have done that on my bike with success. I had noticed 
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 coils metal frame with silicone or something 
else to prevent shocks if you touch it and the engine metal at the same time.

From Graham Bells "Two Stroke Performance Tuning":

Here is how to test the 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 this test doesn't work out for you then do this one:

Latest News

I am now experimenting with a new design of CDI which reverses the voltage going to the high voltage coil without reversing the wires to it. This reverse voltage CDI produces blue sparks in my iridium spark plug. My "normal" CDI produces purple sparks.
The following quotes informed me that a purple (violet) spark indicates  more spark voltage.
To see if that last statement is correct about violet I looked up the wavelength/frequency of each color (see listing below). Blue is a higher frequency than yellow and red and since it indicates a higher spark voltage we can assume that higher frequencies of color indicate more spark voltage. And since blue is higher than yellow we can also assume that violet indicates a higher voltage than blue since it is of a higher frequency.

       wavelength  frequency
violet 380-450nm   668-789Thz
blue   450-495nm   606-668Thz
yellow 570-590nm   508-526Thz
red    620-750nm   400-484Thz

But now we have a mind-bender of sorts.
The reverse polarity blue spark indicates that the spark happens at a lower voltage. Is that correct and if so then doesn't that indicate it gives worse performance? Great question. The previous information told us that correcting the spark polarity will cause the spark to happen at a lower voltage. So what I've observed should of been expected. And since the power output of the high voltage coil is determined by the stator coil and CDI, and not by spark gap, then we need to remember that electrical power is figured by multiplying voltage by current. So if the spark happens at a lower voltage then the electrical current is obviously increased. Higher current basically means a "hotter" spark. And the fact that the spark happens at a lower voltage means there will be less misfires and more consistent engine power, all good. So even though a "reverse" polarity CDI causes a spark color that looks inferior to what it previously had, that is not bad at all but actually very good. Like I said, a mind-bender.

QUOTES

from http://www.ankn.uaf.edu/publications/vs/magneto.html
The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.

http://automateinfo.com/spark-plug-care-in-your-two-wheeler/
Notice the color of the spark, it should be blue/purple in color.

http://www.pwctoday.com/showthread.php?t=342678
A weak yellow spark is not very good and in most ignition systems means trouble. A bright blue is your most desireable and is usually a sign of a healthy ignition system. The hottest is violet .