Scooter with lawnmower engine will not stay started when tire comes off the block?

WE have a scooter with a lawnmower motor on it and whenever my son puts it up on a block it will run without a problem but as soon as he sits it off the ground and the back tire hits the pavement it will stop running and it takes a lot of pulling for him to get it started again but will start on first pull when sitting on block can u help with any suggestions on why this would happen or what is causing it to happen