Looping recorded samples

Hey, I recently bought an SP-808 but cannot work out how to loop samples after I have recorded them from my keyboard.
When I record samples from my keyboard (which is attached via the line-in)I can not stop the recording at the perfect time so they are never in sync with eachother, which means when I play 2 or more recorded samples together they all eventually fall out of rhythm into a jumbled mess.

Is there anyway of assigning an automatic stop point after 4 beats or at least an easy way to align loops at the same BPM and rhythm, there is nothing about it in the manual except for the manual syncing which is virtually impossible to use.

Any help appreciated!