The shamino rear gear keeps changing back and forther between gears. It gets worse the faster you pedal. How do I adjust the gear in the read to keep it from slipping

will need more information as to does it get worse while going faster -do you feel it in steering--present in front ,middle or rear because these are some of the reasons tires out of balance -front suspension problem--hub/wheel bearing failing-c/v axles-u-joints--out of balance drive shaft-debris rear suspension problem

Left side is rear shifter pushed all the way forward is 1st the easiest gear shifting down (pulling toward u)changes it in to a lower harder,faster gear .,., as with the front with it on the smaller gear it will be the easiest to pedal in all rear gears., moving it up to larger gear will make it harder to pedal and go faster.,., hope this helps

Brakes: 1- Turn the adjuster barrel at the end of the brake lever counterclockwise, this tightens the brake cable. turn a little, test the brake, turn a little more till it's where you want it. If it requires more than the adjuster barrel will allow, screw the adjuster back in and tighten the cable at the brake, by loosening the anchor bolt, pull the cable tight, then retighten the anchor bolt.

Gears: 2- This is also an adjuster barrel adjustment. With the rear wheel propped up so you can turn the cranks and shift.

First shift your gear without pedaling.
Now pedal
Turn the adjuster barrel (where the cable comes into the derailleur) slightly counter clockwise until the shift completes. You must continue to pedal.
Once the shift has been made, shift through all the gears too make sure it works.

You can use this method of turning the adjuster barrel back and forth until it is shifting as desired.

In both cases if you can not get it to work then take the bike into your local bike shop so they can take a look and fix the problem, there may be something wrong you can not see.

OK, for a beginner in the world of 18 speed bikes, put the chain on the big sprocket up front, it should be about 46 teeth, make sure it is center and doesn't make noise, if it does, adjust it just a hair either way, now, the rear, put the chain on the big sprocket in the rear, make sure the front didn't start making noise, if it did, adjust it again. You are now ready to take off, in low gear, as you start out, it is easy to pedal, soon you will be peddling air, shift into second gear in the rear only, don't touch the front. Just a little push with the thumb will make it shift, now were going faster, easy peddling, shift into 3rd, then 4th, etc, coming to a stop, down shift the back sprocket back into low gear (big sprocket) and be ready to take off again. Soon you will be shifting that thing like a Ferrari! The front sprocket, will be used for moderate terrain, wheelies, and mountain climbing, that will come later, learn how to shift the rear first. As each gear climaxes, hit the shifter, it will get smoother as you go, and keep the chain quiet, no climbing on the next sprocket or falling out of gear. Hope this helps.

If it's UP shifting to a higher gear in the rear you probably have a little derailleur cable stretch. A little tweak of the rear cable barrel adjuster CCW about 1/4 turn at a time (no more than 1 turn).

Go here and read up on it.

http://www.parktool.com/repair/readhowto.asp?id=64

Derailleur adjustment takes time to master. Try this out:
First, wash your drive train out really thoroughly and use a degreaser (Simple Green works really well) and a chain brush on the derailleur and chain. Rinse it off and let it dry, then relubricate the chain (and ONLY the chain - never lubricate your derailleur). Now flip your bike onto the bars and saddle so you can crank while you shift manually. Use the middle chainring up front to adjust the rear gears. Okay, shift into your highest gear in the back and then release the derailleur cable. Use this opportunity to make certain your "H" setting is good; the derailleur's jockey wheel should be perfectly aligned over the small cog. Now manually push the derailleur to the lowest gear (while pedaling) and set the "L" so the jockey wheel is immediately outside the last cog. crank back to the high gear. Turn the barrel adjuster for the cable tension to its half-way point (figured by unscrewing it all the way and then counting revolutions as you screw it back in). Pull the cable taut and reattach it. Shift into your middle gear (or if it is an even number of gears, pick one of the two middle gears). Use the barrel adjuster to get the jockey wheel exactly over the correct cog. Pedal and shift through the gears. In each gear, back-pedal to make sure the chain is not skipping. You should be able to free-wheel without problem in every gear on a properly adjusted drive train. If the gears are requiring much adjustment on the barrel adjuster, then you may have a bent derailleur hanger. This can be straightened manually without tools if you have a good eye and patience. If it goe that far, come back and repost "bent derailleur hanger".

i hope this helps
-matthew

Wash, degrease and scrub the chain and derailleurs with a good degreaser (like Simple Green). Dry the chain and relubricate it with an appropriate bike chain lubricant (not oil or WD-40). Shift the derailleurs to the smallest chain ring and cog on the cassette (low front gear, high rear gear) and then loosen the cable at the derailleurs, pull the cables taut and reattach them. If the derailleurs still need adjustment, here is a procedure for it:
Flip the bike over (so you can pedal the bike manually while you shift) and shift the front to the middle chain ring. Shift the rear to the highest gear (small cog). Release the cable from the pinch bolt on the derailleur. Adjust the screw marked "H" on the derailleur until the jockey wheel on the derailleur is aligned perfectly over the small cassette cog. Set the cable adjuster on the derailleur to the middle of its range (find this by screwing it all the way in and then count how many revolutions it makes until it screws out completely. Screw it back in one half of the total revolutions). Reattach the cable and shift to the lowest gear (the biggest cog). Over-shift and hold on the shifter, then adjust the "L" screw on the derailleur until the jockey wheel sits just past the last cog. You have to hold the shifter to do this. Now shift into the middle gear (or one of the two middle gears if the bike has an even number of gears) and adjust the derailleur using the barrel adjuster you previously set to the middle of its range. Adjust it so that the derailleur is sitting perfectly over the appropriate gear. Shift up and down the cassette while pedaling and in every gear, reverse pedal (freewheel) to make sure the chain does not hop. Adjust as necessary until all gears are smooth.
Now the front:
Set the rear derailleur to the lowest gear (big cog) and the front to the lowest gear (small chain ring). Disconnect the derailleur cable. Screw in the adjuster for the cable tension all the way (this adjuster is on the shifter of a mountain bike or the down tube of a road bike). Pedal to make sure the chain is not rubbing on the derailleur cage. If it is, adjust the "L" setting on the derailleur until the chain moves by without touching the cage. Now pull taut and reattch the cable and shift the front to the big ring while pedaling. If it does not shift up well or completely, hold the shifter past its normal shift and adjust the "H" screw so that the derailleur allows the shift. Shift into the highest cassette gear and keep pedaling. The chain should not be touching the front derailleur's cage when you pedal. Adjust with the barrel adjuster until the chain moves freely without touching the derailleur cage.
That should be it, unless there is another mechanical problem causing the missed shifts, like a bent derailleur hanger or damaged teeth on the cassette cogs or a bad derailleur. If it is still problematic, come back for further information.

i hope this helps
-matthew

Front or rear? What is it doing?
Flip the bike over onto the handlebars and saddle and let us fix it.
Here is how to adjust the rear (first):
-Set your front gear to its middle position.
-Shift the rear into the highest gear.
-Disconnect the derailleur cable.
-Screw the barrel adjuster on the shifter (or down tube if it is a road bike) to fully turned in clockwise.
Screw the barrel adjuster on the rear derailleur to its halfway point.
-On the back of the rear derailleur you will see two screws. One will be marked "H" and the other "L". These mean high and low. Adjust high to get your jockey wheel on your derailleur perfectly centered over the smallest cog on the cassette. Manually shift by pushing the derailleur (while cranking the pedals) to the largest cog (1st gear) and set this stop on the derailleur "L" where the jockey wheel is over the cog, but past it where it would be touching the outside of the cog if they were side-by-side.
-Pull the derailleur cable taut and re-tighten it.
Shift the rear to the middle gear and adjust the derailleur using the barrel adjuster to center the jockey wheel directly over the cog it should be.
-Now shift up and down the cassette, cranking as you go and in every gear, back-pedal to make sure there is no problem with the gears hopping. If they do, adjust as necessary with the barrel adjuster.
Here are some tips for the front:
-Shift into low gear on the front and rear derailleurs. Make sure the chain is not rubbing on the front derailleur. If it is, adjust the screw marked "L" until the chain just clears the derailleur but does not touch it.
-Pull the derailleur cable taut and re-tighten it.
-Now shift the front into the middle gear and while it is still in the low gear in the back, crank to make sure the chain is not rubbing on the front derailleur. If it does, adjust the tension of the cable with its barrel adjuster.
-Shift the front into high gear and shift the rear to the highest gear, cranking as you shift. If the chain is not rubbing, you are all set.

There are several things that could be wrong:

Shifter out of adjustment

Chain worn

Chain and gear(s) worn

Shifter out of adjustment:
Try adjusting the cable tension by turning the barrel (the part between the cable and the shifter). Count the number of turns and reset it to its original position if this doesn't solve it.

Chain worn:
Chains wear out. hold a ruler up to it, with one end centered on a pin. 12" away, another pin should like up very very very close to the 12" mark. If not, it's probably worn out and needs to be replaced. No way around it. Some say chains are good for 500 miles only. Your mileage may vary. (keep reading...)

Chain worn and gear(s) worn:
If the bike has been ridden witha worn chain for a while, it will make the gears in the back wear faster too, and a new chain alone will not work. If this is the case, you need a new chain and new gears in the back. Often, gears in the back are replaced all at once in a 'cassette'. You'll have to make sure your front gears aren't worn out too!

this is because there is a switch on the brake pedal that is connected to a solinoid that lock the gear selector in park .
you need to A : check the switch on the brake pedal is working this may only need to adjust the distance between the pedal stop and the pedal its self.
B: The solinoid on the gearbox should click when the switch on the brake pedal is closed or opened
if not the solinoid on the gear box could be sticking or could be faulty

hope this helps