Loose chain on high gears

[IamTT]

Hi,

I have been doing some work on my bike to get it back in shape. Everything is working perfectly except when im using the high gears the chain become loose.

I also notice that it occurs when im only resting my feets on the pedals. But when I remove my feets from the pedal I notice that the slack from the chain goes away but my pedals are moving forward of it's own. I think that is the problem. Because the whole pedal section should be still and not moving forward of it's own when im in motion.

What could be wrong? The casette? The chain?

It is a 24 gears bike mostly with Shimano parts.

Thank you in advanced

[cny-man]

I assume you mean that the chain sags on the top when you coast. If so that is most likely because the freewheel/cassette has excess friction, so that when the wheel is turning it moves the rear gears slightly clockwise. That's also why the pedals turn if you take your feet off - the rear cogs are turning because of the friction between them and the hub, and therefor rotating the chain, which turns the pedals. It may be that your freewheel just needs lube, but there are other possibilities as well. If by "high gears" you mean the smaller cogs in the back the sagging probably happens more there because the chain is already under low tension, especially if you are on the small chainwheel in the front.