No spark, Speedfight 2 50cc?

Yes, shear muppetry! hold my hand up to that one. CDI testing should be left to a scooter shop, if you get one connection wrong that'll be the end of the unit, besides that, they can store enough current to give a nasty shock.
Coil can be tested on the bike, disconnect high tension lead off plug and low tension lead at the connector block, to test the primary windings connect one of the meter probes to earth, the other to the low tension lead at the connector block. Now connect a meter probe to the HT lead to test the secondary windings. Sorry do not know the resistance values of your bike but should be in the manual, for example the first reading could be 0.3 ohms, the second could be 3.5 kilo ohms. Check the manual to colour ID the correct wires from the source coil or alternator and to follow the correct testing procedure.

Ted
May 2011

TED - I'm afraid I can't do that since my problem is no spark, hence, engine doesn't start.
The alternator is connected to the CDI, which in turn is connected to the coil generating the spark. How can I measure if there is the alternator, CDI or Coil that causes the problem?

Tobbe
May 2011

set your meter to 0-20 volts DC connect red positive probe to battery + terminal and the black negative probe to - (negative) battery terminal, start engine and check reading of around 14.5 volts, if not it could be faulty battery or alternator. Engine must not be run with battery leads disconnected as alternator or regulator/rectifier may be damaged.

Ted
May 2011

Addon to the question: Measured again and now I got 1.7V DC and 0.7V AC, why does it change?

Tobbe
May 2011