Symptoms only while pedaling

Chain jams (while pedaling, the chain stays stuck on chain ring’s underside and either locks up or jams over top of the chainstay):
     Check chainrings for wear (should have perfect semi-circle between symmetrical teeth)
             Replace ring with widened circle between teeth, low teeth, or “dolphin-fin” teeth, and chain also.
     Check chain for wear or bent links (12 links = exactly 12 inches). Remove bad link .
     Is chain rusty or stiff? Replace vs. lube-work-and-lube
     Is chain cruddy with mud or has dust built up within excess lube? If rings and chain seem OK, clean and apply light lube to chain

Sudden "scrunch" sound and chain slip as pedal jerks forward, usually in middle ring while pedaling hard:
      Usually caused by chain stretch plus worn front chainrings , even if you think they're not that worn
     Check chainrings for wear (widened circle between teeth, “dolphin-fin” teeth)
     Check chain for wear or bent links (12 links = exactly 12 inches). Remove bent link. Replace worn chain.
     Check cassette (cogs) on rear for wear. Replace worn cassette.
     If chain/rings/cogs seem fine, remove cassette and freewheel to check pins, springs, and slots (usually a bike shop job)

"Clunk" with each pedal stroke:
     Usually means the crank is loose (or worn) where it attaches to bottom bracket – tighten
     If cranks tightening fixes things temporarily but problem recurs...
          If crank attachment looks worn, replace cranks
          Try thread-locker on bolt
     Is pedal cage loose or cracked?
     Check pedal for stripped threads
     Check pedal for worn axle bearings, replace if loose

Skipping around in gears on rear (or "clunk" NOT at same spot in pedal stroke):
Quick look for weeds wrapped in cassette, around derailleur, or caught in cables
Make sure the rear wheel is correctly mounted in the dropouts! Loosen, re-tighten, and test again!
     Check derailleur hanger carefully – should be directly vertical to bike frame, with no twisting
     Check cogs of cassette for bent teeth (and weeds, pebbles, or wood chunks between teeth)
     Check chain for bent links
     Check the derailleur pulley wheels. Are they warped? Do they rotate easily?
     Check cable housing for bends, cracks, or floppiness where housing joins derailleur, frame ferule, or shifter
     Check limit screw settings, then
     Tune rear shifting carefully. Replace rear derailleur if damaged.
     If tuning doesn't solve problem, replace cables and cable housing. If supplies not available, try cleaning.
     If it still skips, get help from your local bike shop

Rear cassette won’t go into biggest cog:
     Shift all the way to smallest cog and try clicking down again. Now, does it shift up 1 cog with one click?
             If not, tighten cable until it shifts to 2nd cog with the first click, then tune shifting.
     Does it shift correctly until you get to the big cog? Adjust low-gear limit screw until it barely shifts into big cog.

Can't drop into smallest cog on rear:
     Check derailleur hanger to see if it's bent inward.
     Is the cable a little bit loose? Adjust the high-gear limit screw until the pulley falls under the smallest cog.
     Is cable quite tight? Loosen the cable and see if will go to smallest cog. After cable loosens, adjust limit screw.

Front won't shift onto biggest ring:
     Does the chain get physically jammed between derailleur cage and chainring?
             The front derailleur may have slipped down. Loosen, raise a tad, tighten, and try again.
     Go to tiny ring, click down again to be sure you're in 1st gear. Does it shift to middle ring with first click?
             If not, tighten cable until it shifts to middle ring with the first click, then tune the front shifting.
     Works correctly in small and middle, won't reach big ring?  Adjust high-gear limit screw until big ring takes up chain
             while you're turning pedal and pushing on shifter lever.