This is why I suggested, in another thread, that you should get a manual. It contains precise instructions for checking the regulator, which almost certainly has gone to that great electrical shop in the sky. 15V at 2000 RPM is WAY too much--it will overcharge the battery, as a minimum.
The ignition lamp is connected between the generator output and (in effect) the battery. When the generator is not putting out power, it is a zero volts and you get ~12V across the light, turning it on. When everything is working right, the generator and battery voltage are about the same, so there is no voltage across the lamp, and it goes out. You have too-high generator voltage, and that's turning on the lamp. As you rev it up, the voltage increases and the lamp gets brighter.
It might just need adjustment, but probably the regulator coil in the regulator box is burned out. With the instructions in the manual, you could try readjusting it, but it might be simpler just to get a new one. They're not very expensive.