Ahooga Horn motor spins real slow

[Robbie Horn]

Original 6 volt hooked up straight to the battery. Motor spins slow & sometimes stops & won't start back on it's own unless you push start it. Got the adjustment screw backed all the way off so motor can free spin. Put some 3 & 1 oil on the felt pads & that didn't help. With adjustment screw turned it to make it ahooga it spins slow & won't ahooga but sounds like a sick duck. Brushes look good & I have a new set coming to see if that helps. Anyone have any ideas?