2Slow,

A 4.8v pack shoud run the motor very well. The pump has 4 screws on the black plastic side. You may need to loosen them a bit.

BUT, it's a carefull adjustment - if you loosen them too much, the pump will suck air and won't pump.

The best way to do it is to back all 4 of the screws out one full turn, then retighten them 7/8 turn (if possible, do this while the pump is runnng and pumping water) Repeat if necessary.

You want it to be as tight as possible but still running smoothly. Eventually you will find the spot where the pump works well (not sucking air) and runs smoothly.