How did you connect the batteries each time? In series or in parallel?
If you connect your RC battery in parallel with the 12V bike battery, the RC battery will not increase the voltage to your starter, and it won't contribute any current unless the voltage on the bike battery drops to 7.4 volts.
If you put another 12 volt battery in parallel with the stock battery, you will increase the current available to to the starter motor. when you go to start it, the voltage is less likely to "sag," and the additional current will allow the motor to spin closer to its max speed for that voltage.
If you put the batteries in series, then you will have the voltage increase, and the starter motor will tend to spin faster (speed = Kv*voltage). Higher speeds will demand more current, but the additional batter can supply that as well.
w/o knowing how you wired the battery in, it's hard to say what's up.
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