Hi all, I've got a problem with my sons r50x, it's a 2011 which he has had on the road for two weeks, the other night he rang to say it wouldn't start when he came out of work, I drove up to him and had a quick fiddle didn't really do anything to it then kept the engine cranking till it fired. I told him to ride it home and when he got home he said it was fine for a minute then lost power and slowed down to 10/15 mph. The next day I took the bike out for a test ride and it was fine for a bit, then started bogging down again so I brought it home and removed and cleaned the air filter sponge, it seemed to of cured it but the next day he phoned to say it was doing it again and when he got home he said it bogged down so much it was virtually crawling back home! I've noticed now when you start it and hold the throttle open it will rev up, stay there for a second, then big down and cut out,when you hit the button it fires back up and sits there idling, any ideas off anyone what the problem is? Carb? Piston/rings? TIA