Bike won't kick start.. loosing power. HELP PLEASE!!

[ATK Rider 250]

Hey guys, My '03 will not turn over when i try to kick start it i have to jump it and even then it bogs then fires over.. when its running it idles fine and i can't really tell but it feels like it's loosing low end power.and for some reason the oil in my case keeps dissapearing from the eye glass even when its warm. I am not the original owner of this bike and i dont really know much about 4 strokes maintnence. Does any of this sound familiar to a certaint problem? please help

[ATK Rider 250]

Also I have a full yoshimura exhaust that came on the bike when i bought it and I'm pretty sure the carb was never jetted for it because it always back fires wheni let off but still it was alot easier to start before than it is now.. could the jetting be part of the problem too?