jackhammer rotary machine

I just purchased the jackhammer and when I hooked it up to stable 8volts and pressed the pedal it dropped to almost 5volts and from there no matter where I turned voltage it would not run faster or slower..cant figure the problem..need help, Not real familar with rotarys yet

Personally I love rotaries----but honestly have no idea why your machine is running as it is-I would contact the mfg.