AM6 charging problem

Question

I have an Aprilia MX50 (2004) and -although the engine runs perfectly- the charging voltage is low (around 4-6 V and it should be 13-14 V). I tried with another generator (the flywheel was still the same), voltage regulator and CDI, but I got the same results. I cleared all the places where the wires are grounded and the ends of the wires, too. What else could cause this problem and how can I fix it?