It sure does not seem like you have the full battery voltage on the starter, some where there has to be resistance between the battery and the starter motor. Bad ground, bad connection, partialy broken wire under the insulation/ Even the solenoid can have high resistance in its contact points./ could try jumping the two large connectors on the solenoid with a pliers to see if then the motor turns over at the correct speed.
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1983 Yamaha Riva 180cc
2007 Yamaha Majesty 400cc
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