[LlamaZorz]

When I ride my bike which is a older GT MTB that has a Shimano 21 speed., and apply a lot of force to the pedal, sometimes the entire system gives out and my foot on the pedal just flys to the 6 O'Clock position. Its not very fun, and ya kinda never know when it is going to happen, except it only happens when I put a lot of force on the pedal.

any possibilities on what this can be. I still trying to learn the mechanics of everything.

 

[Steamer19]

It's possible that you have a sticky link in your chain. This could give the feeling that your chain is skipping and your crank feels like it's slipping. Try checking your chain link by link to see if you find a sticky one. You can also pedal slowly backwards watching the links wrap through the cage to see if one doesn't flex around the pulleys like the rest.

 

[4slomo]

From your description, if your chain isn't skipping, it may be the internal pawls in your freehub (assuming you have a freehub and not a freewheel) losing their engagement, a sign of a worn freehub maybe needing to be replaced.

 

[rockyuphill]

A bike as old as you're describing likely has worn chain, chainrings and cogs so the chain will skip off the teeth. A drivetrain replacement (cassette, chainrings and chain will likely cost more than the bike is worth.

 

[LlamaZorz]

I cleaned the gears and all the sprockets like the guy at the LBS,suggested and alot of grease came off. But it didnt help or solve the problem.

I checked the chain, for the sticky link before and after the cleaning and didn't see anything.

So I still have the problem. Would some pictures at all help?

 

[MartinS]

If your chain, chainrings and cluster are old and worn then I agree with Rockyuphill and you are looking at a drivetrain replacement, and cleaning it won't help.