Force 15hp impeller

[gmsmeaton]

Hi, I've got 1998 mercury force 15hp that I am trying to drop lower gear casing to change impeller , have undone lower casing bolts and disconnected gear shift rod, casing drops about 10mm and wont come any more, dont want to force it, is there a shift shaft retainer in the upper part of the engine and where is it located ? any help would be appreciated. thanks