new engine starting trouble....help!

[tellor]

hi all,
i've just got a new engine and am struggling to get it started to run it in

my smd starter box is ok but it doesn't take many tries before the starter box won't turn the flywheel.
the engine turned over for 5 secs but didn't fire up a couple of times

the battery in the starter box is a 12v 7.2Ah Yuasa battery...but i have now connected it to my new 12v 120Ah battery but that hasn't the umphh to turn the engine over, it's now on charge

what am i doing wrong?

thanks