I have a gt montain bike and the chain slips when i aply to much power ? could you tell me why this is

Hi, Yes, If it is worn out. do so. Chain and gears do wear in time and will slip or skip over. So both gears and chain will have to be replaced with new...

If this is a coaster brake bike (single speed, no hand brakes) then the problem might be inside the rear wheel hub. If that is the case then you can only fix it by completely dis-assembling the rear wheel hub - or it might be simply that the chain is slipping, which would not happen unless the chain is too loose which would be indicated by the bottom of the chain hanging down too low. However, if this is a multi-speed bike, then it could be that the chain and rear-wheel cogs are worn - which can be fixed by replacing the freewheel and the chain - however, it takes a lot of use or rust and dirt before that level of wear happens.

Change the engine oil to the recomended 20/40W, oil the chain drive adjust the tension of the chain.
hope this will solve the problem

It sounds like your chain is loose. Flip the bike over and loosen up the nuts on the back tire and tighten the nuts as you are pulling the rim back making the chain as tight as you can. You might need someone to help you pull as you tighten the nuts.

Manuals here...

http://www.schwinnbikes.com/support/owners-manuals

The chain or gears teeth could be shot...

http://www.sheldonbrown.com/chains.html

Adjustments:

http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

http://www.parktool.com/blog/repair-help/categories/derailleur-systems

Or you might have chain, drive train wear.

http://bicycletutor.com/chain-wear/

This happens when the back wheel is not lined up properly with the bicycle frame. Because the gears are not in a straight line with the chain it can slip. If it slips to the right, away from the wheel, then loosen the wheel and move the end of the wheel next the pedals slightly to the right as you stand behind the bike. If it slips to the left, in towards the wheel, then move the end of the wheel nearest to the pedals to the left. Once the wheel and the chain are in line it should stop changing on it's own.

It is not slipping unless your sprockets are SHOT.

There should be about 3/4 inch play in the center bottom of your chain when the bike has a normal load on it (read YOU).

Now then, your clutch could be slipping ... have you adjusted it?

I hope this helps you

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The rachets in the rear gear cog have gone bad.  Buy a new gear cluster and have it installed on your rear wheel.  about $30

Go to any reputable shop/mechanic; you probably need a new chain and/or sprocket...OR Possibly just a chain adjustment. Don't wait around, do it asap, I wouldn' t play with this particular problem.