Re: 1991 evinrude 175xp timing
OMC made two types of performance 175 engines that year. The 175GT/XP was a 90 degree crossflow block. The other 175 was a 60 degree looper-GL model. I don't believe the 60 degree looper was ever badged as an "XP/GT." So, my comment is for the 90 degree crossflow engine: I wouldn't worry about the idle timing. It's different on every engine-just set it so the engine idles at the correct rpm's. That's the correct idle timing for your engine on your boat. (they are all different, depending on engine/boat combinations.) The max timing is set by the adjusting screw on top the intake manifold. The long screw has a rubber bumper on it to stop the timer base. That screw can be turned in or out to adjust the max timing. If you are planning on adjusting the WOT timing with the factory method-you will need to precisely follow the genuine factory service manual procedure. This is not easily done and if done wrong can have serious consequences if the timing becomes too far advanced. The max timing degrees is usually on a decal on the airbox cover on that engine-and in the manual. It's a delicate procedure, since the spark timing advances (creeps) internal to the ignition system as the rpm's increase. As much as 4 degrees.
