When I put away my Sportjon with the 200 Mercury Sportjet last Fall, it was performing well. I winterized it by running fuel stabilizer, quickleen and DFI oil through the engine while idling for ten minutes as I always have. Before I put it on the water this season, I replaced the wear ring with a SS relined one from Greatland welding along with a rebuilt impeller from them. Not that that has anything to do with the engine but just including it for completeness sake. I took it out and ran it for four hours this week. Right off the bat, I noticed that the engine seemed to idle a little rough. I could notice a little more vibration. I opened the doghouse and removed the engine cover to inspect it. The compressor belt seemed to whip more than usual. Increasing the throttle would smooth the engine out somewhat at certain RPMs, but still a subtle vibration to it and continued belt whip- more pronounced at some RPMs. I can hear the compressor chirping away but maybe I'm more awaare of it with the doghouse open while it's idling. The motor revs up fine and moves up and down the river OK but my top speed is diminished at WOT- from the low 40s mph to around 33-34 mph. My fuel usage was up too. I wasn't loaded heavy (two men and a boy with fishing poles) and could plane the boat at 3900 RPMs but ran it mostly at 4200. My GPS showed about 25mph when it used to move about 29-30 mph at this RPM. I returned and changed the spark plugs. The former plugs had 75 hours on them but all looked good. The new plugs didn't make any difference when I tested it again. The engine almost seems a little unbalanced.
Could my impeller be imbalanced? Would that cause my problems?
What else should I check besides the spark plugs that could be making the engine rough/vibrate more at certain RPMs?
Any other ideas?
My boat is on its 5th season and has 151 hours on the engine. I don't have access to a Mercury mechanic. I live off the road system.
Ideas and advice are appreciated. Thanks.