So I finally decided to replace my water pump in my 89 Johnson 150 OB this spring. First time doing one, went pretty smooth except getting the damn shift linkage bolt back in
I thought I was careful to not turn the shift shaft but when I got it hooked back up it wouldn't go into reverse... So took it back apart turned it one revolution taller and now it shifts into reverse, but it doesn't seem t like there is as much throttle area as before
. from neutral to WOT position on the helm the lever goes half way through the range of motion before moving the throttle cable at the engine.
So I am worried I got the cables out of adjustment when I had things apart...
Boat hasn't been in the water yet this season so it hasn't actually been powered up (repairing some beach rash tomorrow) but I would like to be able to get things as ready to go as I can before hitting the lake.
What's the procedure for adjusting the shift/ throttle cables?

Remove both shift and throttle cables from the linkage. Make sure the shift linkage (on the engine) will shift F,N and R properly (without cable attached) first. Place linkage in neutral position.
If this checks out, put the shift/throttle handle in the neutral position, slip the 'hole" in the end of the throttle cable over the stud on the linkage (without moving it) and adjust barrel (wheel) on the shift cable so it matches the recess where it's supposed to go. This is a good starting point. You may need to tweak this adjustment a bit to compensate for free play in the cable.

The throttle cable should have a bit of "pre-load" on it meaning the cable should push against the linkage a tiny bit to assure it returns to idle when throttle is at idle. Same procedure as above but make sure the throttle linkage on the motor is against the "stop". I use a bungee cord to hold the throttle linkage against the stop when I adjust throttle cables.

You didn't get the throttle cable and shift cable reversed did you?

ETA - why did you remove the cables to service the impeller? Was it an issue with access to the shift linkage bolt? I haven't done an OMC V6 in a while.

I didn't remove the cables, from the linkage, but I have to remove the (not sure what to call it, air intake box?) that the little wheels on the cables lock into to keep them from rotating in order to access the
shift linkage bolt that is under the carburetor.

I'm at work so not my engine (close but mine is the V6), but the little black wheels on the right side were disconnected, and probably spun a few times over the course of dickering with the shift linkage.
is there anyway to do an adjustment without a complete reset?

The little "wheels" should provide enough adjustment as long as the shifter rod that goes into the lower unit is at the correct height. Sometimes there's enough to "fudge" the adjustment if the shift rod height is off one turn either way.
The threaded brass sleeve the wheels thread onto are sometimes a give-away as to where they were adjusted originally.
The brass will be cleaner under the wheels so it may be possible to adjust the wheels so they cover the cleanest part of the threads. Again, this is just a ballpark adjustment. Final adjustment should be done after testing the shift/throttle operation when engine is running.

Roger will give that a shot.