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Bought a used bike, know nothing about bikes
Hey everyone.

I just bought a used Giant ATX 770 at a garage sale. I know nothing about bike maintenance and couldn't find anything on the web for a general maintenance routine, though I did find a good cleaning tutorial on this site.

Here are my questions:

1. The gears start to shift after a few seconds even if you haven't pressed the button. Is this a question of maintenance or do I need a new shifter?

2. The back of my heel touches the ground when the pedal is at its lowest, is there anything I can do to fix this?

3. The bike is filthy, so I will clean it based on the cleaning tutorial from this site. Is there a good guide on the basic maintenance stuff I should do after that (in terms of tightening, lube, etc.)?
1. Likely it is maintenance-related... sort of. Dirt, gunk etc in shifters, cables / housing, dérailleurs or frayed cables can make it impossible to set up shifting in a correct way. Look at that and replace them as needed (also the brake cables). But then, the chain might also be worn. There is a tool to measure that, but a ruler will also work (12 full links should be 12 inch long, I think "stretch" should not be more than 1/16th for 12 links, look at tutorial on this site). Then, you might have to replace chain, cassette, maybe chain rings...
2. Don't really understand that... is your saddle too low? Basically a good starting point is this one: sit on the bike (balance against a wall, let somebody help you , whatever) and place heel on one pedal at the lowest part of the stroke. The leg should be almost fully stretched. Start from there and ride, then move saddle up or down and see if it is better. Proceed until satisfied.
3. Cleaning, lubing, checking cables (brake, shifters), replace brake pads, maybe overhaul bearings in head set and wheel hubs (should be done annually, depending on use / abuse / weather conditions). Dial in shifting, brakes, check frame for cracks, check stem / handlebars... also the tyres / inner tubes...
That's it off the top of my head. The list is most likely not exhaustive. I work with a check list that I don't have here, so I cannot check what I forgot.
[color=#FF0000]1. The gears start to shift after a few seconds even if you haven't pressed the button. Is this a question of maintenance or do I need a new shifter?
In my case, I found a broken pawl (white plastic paperclip looking indexing part) inside the twist grip shifter. I didn't replace it, just ordered another shifter & greased it in the mean time. I followed the tutorial for adjusting the rear derail & found that even though it looked right, my lower limit (I mean the biggest cog) was wrong. Everything worked much better afterwards, even with the broken shifter part. It works so well I haven't put the new shifter on- sort of a cross between friction & index shifting!

Every used bike I get, the first thing I do is go thru the wheel bearings, clean, replace the balls (I have tons of spares) relube & re-adjust.
Then I check the bottom bracket for play & grinding. Then I go thru the headset- clean, replace bearings & relube.
Since I'm a huge heavy guy I inflate the tires to full pressure & inspect them for cracks etc...
Actually, Joe_W gave an excellent answer.

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