You don't state which operating system is fitted but this is the generic approach to adjustment of all front derailer systems.
If possible suspend rear of bike so that you can easily work your way through gears using hand rotation of pedals.
1. Put chain on to largest rear wheel cog.
2. Set front mechanism control to lowest gear position.
3. Make a written note of cable clamping to derailer mechanism. Disconnect cable at derailer - usually held by single clamp screw.
4. If cage of mechanism not over smallest chainwheel locate the low adjustment screw. This will probably be at the top front of the mechanism and is sometimes indicated by the letter "L" stamped nearby. If adjusting screws are unmarked you will have to find it by trial and error. Usually the high and low adjustment screws are about three quarters of an inch apart and are quite small so you should be able to identify them. Rotate the low adjustment screw until the cage of the mech is over the smallest chainwheel and the chain just clears the inside face of the cage as the pedals are rotated.
5. Move the rear wheel mechanisim throughout its full range to make sure the chain clears the front mechanisim cage for all of the rear gears. Tweek adjustment of low screw as necessary just a fraction of a turn at a time.
6. Put chain on to smallest rear wheel cog.
7. Now pull or lever the mechanism outward whilst rotating cranks to move chain on to the largest chainwheel. If it won't move don't force it over. Look for the high adjusting screw, perhaps "H", and adjust to give extra travel to the mechanisim.
8. Check that cage is parallel to chainwheel and it just clears chainwheel as it rises up and over. If the clearance gap is more than about one eighth of an inch it will have to be lowered.
If the mechanism is held on the bike with a clamp make a note of how much it has to come down to give a clearance of just under one eighth. Mark a line below the clamp for this adjustment and move the whole mechanisim down to the line. Now view from above the chainwheel and make sure that the cage is aligned parallel to the chainwheel. Take care not to align with chain as this is probably running at an angle to the chainwheel.
If mechanisim has fixed mount then there will probably be an adjustment screw. This should be the one remaining after you have eliminated the low and high adjustment screws.
If mech has been moved hand operate through gear range to make sure it clears all three chainwheels.
9. Now with chain on smallest cog at rear and largest chainwheel at front make sure that the chain just clears the cage as the chainwheel is rotated. Tweek with a fraction of a turn of high adjuster screw if neccessary and run rear gears through whole range to make sure that the chain clears the cage without rubbing anywhere.
10. Take the chain back down on to the smallest chainwheel.
11. Make sure front gear shift is still at low setting.
12. There is probably a barrel adjuster for the cable where it comes out of the gearshift. Screw this fully in. You may have to release a locking ring first. Check the other end of the cable sleeve for another barrel adjuster and screw this in if you find one. Check also for an adjuster at the derailer.
13. Take up all the slack on the cable and clamp it in position on the derailer mechanisim. This is when you need the note you made at 3 will save you hassle.
14. Put chain on nearest to middle rear cog.
15. Unscrew cable adjuster until mechanisim moves correctly up and down through all three gears. Check that chain clears cage throughout complete range of rear cogs.
16. If you get problems check lubrication of cable and mechanisim pivots. A very small tweek on the limit screws may be neccessary in difficult cases.
You can figure it out just by looking at it. Put the gear change lever in a position, and look at the front Derailleur (the metal framework that moves the chain from side to side and forces the chain onto a different sprocket). Adjust the Derailleur to the position it should be for that position of the shift lever. Most bikes have an adjustment at the shift lever, and an adjustable set screw on the cable where it is attached to the Derailleur. The adjustment at the shift lever is the round receptacle the end of the cable housing plugs into. Screw it in to loosen tension (make the cable longer) and unscrew it to tighten tension (make the cable shorter). At the other end of the cable you can loosen the set-screw at the Derailleur and pull more cable through to tighten tension or let cable out to loosen tension. The bigger the front sprocket the higher the gear, and the slower the pedals turn. It's just the opposite for the rear sprockets, the smallest sprocket is the highest gear. You may have to adjust the Derailleurs two or three times, riding the bike to test it, before you get it just right. You can look down to observe the Derailleur as you shift gears to get an idea of what adjustment is needed.
There should be one or two limiting screws for the front derailleur that you can use to adjust how far out/in the derailleur moves to shift the chain over. You can ask a friend to lift the back end of the bike off the ground while you do the adjusting of these screws and then try shifting the gear while moving the peddles with your other hand.
Answers & Comments
You don't state which operating system is fitted but this is the generic approach to adjustment of all front derailer systems.
If possible suspend rear of bike so that you can easily work your way through gears using hand rotation of pedals.
1. Put chain on to largest rear wheel cog.
2. Set front mechanism control to lowest gear position.
3. Make a written note of cable clamping to derailer mechanism. Disconnect cable at derailer - usually held by single clamp screw.
4. If cage of mechanism not over smallest chainwheel locate the low adjustment screw. This will probably be at the top front of the mechanism and is sometimes indicated by the letter "L" stamped nearby. If adjusting screws are unmarked you will have to find it by trial and error. Usually the high and low adjustment screws are about three quarters of an inch apart and are quite small so you should be able to identify them. Rotate the low adjustment screw until the cage of the mech is over the smallest chainwheel and the chain just clears the inside face of the cage as the pedals are rotated.
5. Move the rear wheel mechanisim throughout its full range to make sure the chain clears the front mechanisim cage for all of the rear gears. Tweek adjustment of low screw as necessary just a fraction of a turn at a time.
6. Put chain on to smallest rear wheel cog.
7. Now pull or lever the mechanism outward whilst rotating cranks to move chain on to the largest chainwheel. If it won't move don't force it over. Look for the high adjusting screw, perhaps "H", and adjust to give extra travel to the mechanisim.
8. Check that cage is parallel to chainwheel and it just clears chainwheel as it rises up and over. If the clearance gap is more than about one eighth of an inch it will have to be lowered.
If the mechanism is held on the bike with a clamp make a note of how much it has to come down to give a clearance of just under one eighth. Mark a line below the clamp for this adjustment and move the whole mechanisim down to the line. Now view from above the chainwheel and make sure that the cage is aligned parallel to the chainwheel. Take care not to align with chain as this is probably running at an angle to the chainwheel.
If mechanisim has fixed mount then there will probably be an adjustment screw. This should be the one remaining after you have eliminated the low and high adjustment screws.
If mech has been moved hand operate through gear range to make sure it clears all three chainwheels.
9. Now with chain on smallest cog at rear and largest chainwheel at front make sure that the chain just clears the cage as the chainwheel is rotated. Tweek with a fraction of a turn of high adjuster screw if neccessary and run rear gears through whole range to make sure that the chain clears the cage without rubbing anywhere.
10. Take the chain back down on to the smallest chainwheel.
11. Make sure front gear shift is still at low setting.
12. There is probably a barrel adjuster for the cable where it comes out of the gearshift. Screw this fully in. You may have to release a locking ring first. Check the other end of the cable sleeve for another barrel adjuster and screw this in if you find one. Check also for an adjuster at the derailer.
13. Take up all the slack on the cable and clamp it in position on the derailer mechanisim. This is when you need the note you made at 3 will save you hassle.
14. Put chain on nearest to middle rear cog.
15. Unscrew cable adjuster until mechanisim moves correctly up and down through all three gears. Check that chain clears cage throughout complete range of rear cogs.
16. If you get problems check lubrication of cable and mechanisim pivots. A very small tweek on the limit screws may be neccessary in difficult cases.
Happy riding.
You can figure it out just by looking at it. Put the gear change lever in a position, and look at the front Derailleur (the metal framework that moves the chain from side to side and forces the chain onto a different sprocket).
Adjust the Derailleur to the position it should be for that position of the shift lever. Most bikes have an adjustment at the shift lever, and an adjustable set screw on the cable where it is attached to the Derailleur. The adjustment at the shift lever is the round receptacle the end of the cable housing plugs into. Screw it in to loosen tension (make the cable longer) and unscrew it to tighten tension (make the cable shorter). At the other end of the cable you can loosen the set-screw at the Derailleur and pull more cable through to tighten tension or let cable out to loosen tension.
The bigger the front sprocket the higher the gear, and the slower the pedals turn. It's just the opposite for the rear sprockets, the smallest sprocket is the highest gear.
You may have to adjust the Derailleurs two or three times, riding the bike to test it, before you get it just right. You can look down to observe the Derailleur as you shift gears to get an idea of what adjustment is needed.
There should be one or two limiting screws for the front derailleur that you can use to adjust how far out/in the derailleur moves to shift the chain over. You can ask a friend to lift the back end of the bike off the ground while you do the adjusting of these screws and then try shifting the gear while moving the peddles with your other hand.