PROBLEM with changing gears on a bicycle?

[Big D]

I have a Mongoose 21 speed mountain bike that I've had for a few years, and it seems as though I've had this problem since I purchased it, but I haven't ridden it much in the last 3 years.

The problem is this: After changing into 6th or 7th gear, I pedal once or twice and then there's a loud sound, and the motion makes my feet fall off the pedals. I can't see what's going on, but it seems like the chain falls off the spokes when going into 6th or 7th gear, causing my feet to fall off the pedals since I'm pedaling hard. I don't hear any grinding noises, however, so I'm not sure.

I also have issues when changing from the 1st set to 2nd set and vice versa (i.e. switching from gears 1-7 to 8-14). Sometimes I need to downshift suddenly, say from 13th gear to 5th gear, but my chain sometimes gets stuck on the 2nd rung and won't go down to the first for gear 5.

Do I just have some sort of alignment issue? Should I just bring in my bike to the repair shop, or is there a quick fix like loosening a few screws or adding oil to the chain, etc.?

Thanks very much!

 

[edward w]

either your chain is stretched and taking links out wont fix it. Adding Oil to the chain is always a good thing but this IS NOT your problem.
The rear derailleur on this bike is the likely culprit and needs to be adjusted to correct the shifting problem and the shifters also need to be tweaked to ensure they are working properly. This is not uncommon on any bike ever but yours is a Dept store branded Mongoose (round or oval headbadge and NOT the inverted "M" ) thus the 6 or 7 speeds indicates it is. Your best bet would be to take it to your local shop and let them work on it so the chain NEVER actually ends up in the SPOKES....