This loosens the hub enough that it comes off easily with a puller. These are commonly called rollers, but it is obvious that they do not roll. They just rub back and forth at very high speeds, under considerable load, pressure and heat. The lube must go along the side links on each side of the chain, then allow 8 to 10 minutes for penetration and evaporation before starting the engine to ensure the centrifugal force will not sling it away. The clutch sprocket and the main drive sprocket should be perfectly aligned so that they don't rub against the side links.
All chains stretch over time.
When the chain starts falling off the sprocket then it is time to move the engine forward or backward to take up the slack in the stretched chain. There are four bolts that hold the engine on the motor mount plate. Tighten the bolts back again but make sure the two sprockets are perfectly aligned with each other. You don't want the sprockets out of alignment because this will cause the chain to come off or wear the sprockets unevenly. A tight chain can cause the kart to move in neutral with no one in the drivers seat.