Venom

Just looked on the J perkins site, they have a video of the Electrafun doing quite a fast take off, so it looks like it should have plenty of power to fly well.

Don't take offence if I assume your a beginner.
Have you flown it successfully before?
Is the prop on the right way round?
To discharge the battery pack, leave it connected to the model and run the motor until you hear it slow down, (it may cut out automatically), though you shouldn't need to do this, normal flying will 'condition', (improve), the battery.
If your charging the pack off a car battery, it may not be fully charged, as a fully charged 8 cell pack is about the same volts as a car battery.
If you mains charge it, do it just before you go flying, not the day(s) before.
A full change, should just start to get the pack warm, just noticeable warm, not hot.

Hope this helps.