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Battery like everyone else said. An an 8.4v 1100mah is not powerful enough to pull that heavy-ass spring with stock gears and stock motor. It's not the motor, and as you've repeatedly been told (but seem to ignore), spring length has nothing to do with it.
You'll find that upgrades (particularly strong springs) aren't as simple as just dropping in a new spring. More often than not, you'll need other parts like larger batteries, possibly torque-up gears (highly recommended with a spring that powerful), and possibly a high torque motor. And you WILL blow that non-reinforced mechbox shell all to shit with that spring.
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