1987 9.9 Throttle/Power problem

My father-in-law has a 1987 9.9 hp manual start, manual tiller with VERY LOW hours. Before using it last week I changed the lower unit oil, cleaned the fuel filter, put in new spark plugs, cleaned the carb and put fresh gas in the tank. I was really excited when it started on the 3rd pull. It was a little slow out of the hole, and topped out at much lower RPM than I remember my Johnson 9.5 put out. After several stops/starts on a pleasure cruise the throttle handle turned an extra 30%, increased the fuel flow/RPM and doubled our speed. After slowing down to cross a large wake the throttle would only advance to about halfway again (the original max position). The following day I got the true full throttle most of the day. At the dock I removed the cowl and attempted to identify the issue. The gear and throttle linkage are not loose and appear to be working correctly. The throttle linkage appears to have two stages and gets "stuck" at the top end of the low speed stage (the throttle cable, NOT the linkage). I tried following the cable down into the lower unit, but, didn't remove the LU due to time constraints. With the motor off, I shifted between gears and advanced to WOT. The throttle grip on the tiller advances to WOT about 30% of the time, otherwise it maxes out at the top end of the low speed stage. I thought it might be related to the idle adjustment setting (combined wtih the choke knob) and repeated the throttle advancment exercise at both low and high idle settings ... same results. Any suggestions?