ignition works like this:
separate coil in alternator (cables red - brown) produces AC current which loads a capacitor in cdi unit. pickup coil in
alternator gives information about revs and position of piston to cdi unit. cdi unit determines ignition point ->
capacitor discharges. this short burst of current through primary coil of ignition coil induces the high ignition current in
secondary coil.
this is the reason why spark is not depending on load (there is simply no indicator for this in cdi-unit), but only on revs
(the revs originally produce the AC current). so the spark is always the same - weak or strong.
if the alternator coil does not produce enough current the result will be a weak spark. this leads usually to bad
starting behavior of cold engine. indeed, THIS is the condition where you really need a strong spark, not at high revs
when engine is hot. here even a weak spark is enough to ignite the fuel mix (infact, sometimes it can explode even
without spark). critical point on high revs is the ignition time (position of crankshaft degrees where ignition takes
place).
an indicator for a strong spark is the length between electrodes which he can overcome.
there are special devices to measure this.
usual method to check coil in alternator is resistance measurement. but unfortunatly even a coil with good values can
produce weak sparks when engine is running (as described in the link which maarten has placed above).
so if you don't have such device only way is to change alternator against a functioning one. but as stated before, i
don't think it's an ignition problem.
one other thing came to my mind: check/change ignition lock or disconnect black/white cable from cdi to lock. if your
lock is faulty (black/white cable is connected to ground), there is no spark.
hope this helps for the first,
klaus