Hi, without flash, if the result is too dark, it may seem obvious, but there is not enough light falling on the sensor. So effectively, for your very fast shutter speed selected, you're amera is not capable of selecting an appropreate aperture.
Few things you an try. Improve the lighting. Make the sensor more sensitive to light (increase the ISO) or gradually increase the shutter speed at the widest aperture until you find the fastest you can use for your camera & ISO setting.
I've found, like you, that the flash can sync at increadably high speeds with CHDK. If the result is washed out, the flash is too powerful & direct at the subject. Depending on your camera, you may be able to decrease th flash power. try diffusing by putting some white paper over the flash head, or best of all if you can, don't point it direct at the subject. Bounce the light off a reflector (white card?) onto your subject. mostly experementation & patience will get you good results. best of luck, let us know your results
