just bought a74 merc 40 4hp. above the bracket where you mount it to the boat is has a black switch that says "TROLL" what is this for? also i've looked the motor over and i cant find an off switch any ideas? also is the throttle supposed to stay when you turn it or is it supposed to spring back? and finally its got a remote fuel setup how many times do i need to squeeze the bulb? oh wait i got more..lol do i have to take the whole lower unit off to get to the impeller? and what is a shear pin?