Re: 71, 18hp Rude Rough running at 1/2 Throttle
If you open the choke and look at the back of the carb., you will see the throttle plate. It should be closed when you turn the throttle all the way down. As you twist the throttle toward high speed very slowly, you should see the throttle cam on the armature plate start to turn to the right. That cam on the armature plate should have a couple of marks on it. When the cam roller hits the two marks on the throttle cam, the throttle plate in the back of the carb. should just begin to open. That means that the throttle plate and timing are synchronized. I can't remember how a '71 is set up on the throttle cam, but that should give you an idea what I mean. The throttle plate in the carb. should open at the right time as you advance armature plate. The tiller grip actually is connected by gears and such to the armature plate, and a little cam on the armature plate is what opens the throttle plate in the carb.
Motors become unsynchronized by having linkages bent, cam rollers getting grooves worn in them, and other ways. Even tiller gears jumping out of mesh and getting out of time can cause weird problems.
My low speed adjustment usually winds up between 1 1/2 turns and 1 turn out from gently seated. My high speed adjustments usually wind up about 3/4 of a turn from gently seated, but all motors are slightly different.
good luck,
JBJ
