[Dick Boneske]

Power valve first--Stock spring pressure on the power valve actuating rod is 200-250 grams. With a lumpy cam, that should be lowered to about 180 grams, which normally is about 5 turns of the spring. I measure this with a postal scale graduated in ounces and grams. Just remove the carb top and place the bottom of the power valve rod on the scale and see how many grams it takes to see it begin to retract into the carb top.

Second, I've attached pictures of the venturi assy. The hole located by the #2 in the first photo is where the idle channel restrictors are located. The other photos show various styles of venturi clusters. As Kenth said, stock for a 389 is about .046", with the 421 larger. I would begin with .050-.052". Try a smaller drill first so you can see the angle of the hole so you don't ruin the cluster. I don't know how you'd remove and replace the brass restrictors.