Chain is loose when not pedaling

[reissistm]

Hi, this is my first post to the forum! I have an old Focus MTB, bought it just recently second hand and I am also new to biking so please excuse me for being naive.
The problem started after the first miles with the bicycle, hearing a noise from the back. It was getting louder and louder very quickly and now, when I am pedaling everything is normal but when I stop and keep the pedals still, while on the road, the chain turns really loose and even goes out of the ring. This happens all the time. Tried with some lube and also tried to see if there is an obvious problem with the chain but cant find anything. It is like the rear system, below the rings has some problem. Do not know the correct terminology so this may not help.
Any idea what this can be? Is there a way to adjust this system and make it work properly?
Thanks

[Painkiller]

Check to make sure your Idler pully's on the rear derailleur are both free moving. if not, take out of the cage and clean and lube.

[reissistm]

(07-19-2015, 01:52 PM)painkiller Wrote:  Check to make sure your Idler pully's on the rear derailleur are both free moving. if not, take out of the cage and clean and lube.

Seems to be free moving so the problem maybe caused by sth else

[Painkiller]

Try to post a couple pics, side view, rear view of derailleur and side view of the whole drive train to show the slack. google chain stretch and check that.

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With your chain still on the bike, place the ruler's '0' inch mark directly above the center of one of your chain pins. Now count 12 complete links. A complete link equals 1 inner and 1 outer. A rivet on a new chain should line up exactly with the 12 inch mark using this method

[DaveM]

If I understand you correctly, when you coast (rolling, but not pedaling) the chain goes slack?
If so, that probably means the gear cluster in the rear is not ratcheting well and needs to be lubed. If you lay the bike on it's side and spin the rear wheel, you want to find the gap on the gear cluster where the center is spinning, but the outer part stays still. Drip chain lube or light oil right at the gap slowly and let it work itself in by keeping the wheel spinning. Try to avoid getting the lube anywhere else, especially at the center where it would go in to the wheel bearings.

If that doesn't help, you may need to work on/replace the gear cluster - figure out if it is a freewheel or cassette, etc.

[reissistm]

Posting some pics...maybe can help
http://1drv.ms/1Kj5D2S