Yamaha blaster power loss?

running idle it sounds fine, when in gear it splutters and there is random power loss in all gears, it then ran fine for about 10 minutes but the problem soon returned also there is a little bit of white smoke from the exhaust. i have used the correct oil as advised and mixed it to the ratio given. i have never owned a bike/quad before so have no clue as to what the problem could be or how expensive?