Why am I getting no power when I pedal? Yesterday when I was biking my mountain bike on level paved path, I would pedal but get no power to the wheel.  To use a car analogy, it was as if I had the clutch in and was apply gas.  It seemed to happen most often after I coasted and then went back to pedalling.  I would pedal and realize that no power was going to the wheel.  The chain was definitely on the sprockets and moving. I could change gears no problem while I had no power.  Sometimes stopping the bike and starting again would work, or just trying again.  Needless to say, I'm taking it in as I'm not mechanically inclined, but I'm hoping to learn something.  Perhaps a link to a site that explains the problem?

So my questions.


What is going on?  I have heard of something called the "freehub".  I really don't know how bikes work though, so I'm hoping for an explanation of how one can pedal and give power to the wheel and then stop pedalling and the wheel just keeps moving (without the pedals moving).  
If you are on a ride, is there any emergency adjustment to make?


For what it's worth, it's an old bike.  Circa 1993 Performance Mtn Bike.  So it's not too surprising that it's falling apart.  I haven't used it a whole lot, and have tried to wash and oil it, but I probably haven't taken the care that a biking aficionado would.  Also, besides losing power after coasting, one time I was pedaling and suddenly the pedals weren't powering the wheel.  I almost went over the handlebars!  So, I definitely want to get it fixed.  I'd just like to know a little bit about what is going on.

Thanks,

Dave.

Bonus question:  If there is enough info to make a diagnosis and you have ideas on costs to fix, perhaps you could comment on whether it's worth fixing.  Like I said, it's circa 1993 and cost about $450 then.  No shocks or anything.  Just 21 speed Mtn. Bike.
This sounds like a issue in your freehub/freewheel (both are mechanically the same for this question).  This is what allows you to coast without pedaling.  Inside these bodies are a set of "pawls" which will flip down and engage a series of splines when rotated "forward", and propels the bike forward.  Then, if you move them in the other direction "backwards", the pawls are designed to be pushed out of the way of the spline.  Given how old your bike is, there is probably a build up of dirt and drying out of grease inside these that are causing the pawls to be stuck in the "up" position and not able to grab the splines.  In this case, your local shop should be able to fix that right up by replacing the part.