Shimming consists of adjusting small magnetic fields (shims) so that the total field across the sample is as homogeneous as possible. For a detailed discussion of shimming see http://www.acornnmr.com/sam/shimintro.htm. One can shim burger manually or automatically.
Gradient amplifier setting
You should be sure that the pulsed field gradient amplifier is on if you will be performing experiments which utilize this amplifier. Check the current state of the amplifier by entering:
pfgon?
If the spectrometer response is
nny
then the amplifier is on, if the response is
nnn
then the amplifier is off. Enter the following to turn the amplifier on:
pfgon='nny' su
The amplifier has a warmup time of 30 minutes to 1 hour. The optimum shim setting for Z1 may be unstable during amplifier warmup.
Lock adjustment
Adjust the lock power and lock gain so that the lock level is between 30 and 70. When the lock power is decreased by 6 units the lock level should decrease by a factor of 2. If a two fold lock level reduction is not observed reduce the lock power.
Manual shimming
Open the Acqi window after loading a shim parameter set known to give good magnetic field homogeneity. Click on the shim button and adjust Z1-Z4 until the lock level is maximized. Adjustment of each shim should be repeated at least once because the shims interact. Usually it is most convenient to use the ±16 or ±4 buttons when adjusting the shims. Manual adjustment of Z1-Z4 will often give a decent spectrum. To check the spectrum, go ahead and acquire one scan then look at your solvent peak.
Autoshimming
The autoshim is much easier, but requires more time. There are small programs for autoshimming. Type the following exactly as shown:
method='nsall' shim <enter>
spin=20 spin <enter>
method='hires' shim <enter>
These commands (in order) turn the spinner off, shim all of the non spinning shims, then turn the spinner on and shim all of the spinning shims. After entering all of the commands, the QUEUE will indicate 2 experiments queued. Shimming can take anywhere from 20-45 minutes. The machine usually does a pretty good job of getting it right if the starting shims were not a long way from optimal. If your spectrum looks bad, you should reload a shim set that was known to be optimal in the past, manually shim Z1-Z4, and try autoshimming again.

Start acquiring!!!