she's in, she's slow

put the boat in the drink for the first time ever (since she's been mine) friday and ran fine until it started cutting off at planing speed against the chop. would start right back up, and i couldn't figure out what was causing it. after looking up the problem on the net, i determined it was either old gas (at least a year and a half) or the ignition coil. then i remembered how sensitive (not to mention beat up) the key cylinder was, and how it would shut off with the slightest provocation. spent a few bucks and a few minutes and she hasn't cut out since, and that's been about 5 times out. she runs smooth and quiet (esp. for a 25 year old motor), but lacks power. i'm running at about 22 mph at "cruising rpm", 4200 from what i understand. this seems slow to me, but is it just normal for this old an i/o? should i prop up, or not risk stressing an old engine? currently running a 14 1/2" 15 pitch. any suggestions? anybody else have a merc 470 on theirs to compare?

oh yeah, got the garmin 178c gps/df, sweet little unit, easy to learn.