1999 90 hp Force -Low RPM's at top-end

[Polar Craft]

I am the original owner of a 1999 90 hp Force. It is mounted to the original boat that has not been modified. About 6 years ago during a dealer tune up the mechanic advised that I should reduce the size of the prop slightly to achieve favorable top end RPM's. It was running about 4500 (5000 target). I brought the prop to the local shop and had it shaved. After installing the engine ran at about 5025. This year I did my standard seasonal prep. (Changed spark plugs, lower unit Oil and checked ideal). This past weekend I put about 8 hours on the motor and it is only turning up 4600 with minimal load. I also noticed that from about 85% throttle to 100% RPM?s do not change only engine tone gets deeper.

Can anyone suggest items to check?

 

[imported_bowfisher]

Re: 1999 90 hp Force -Low RPM's at top-end

How old is your fuel? Is there any stabilizer in the fuel?

 

[Polar Craft]

Re: 1999 90 hp Force -Low RPM's at top-end

The original fuel was from last season, but i put a fuel treatment in at the end of the season. I have burnt 40 gallons of fuel this year so far. I also added ethonal treatment this year.

 

[imported_bowfisher]

Re: 1999 90 hp Force -Low RPM's at top-end

What are the chances maybe one of your plugs is fouling. I have seen new plugs not work properly. Pull all plugs out and do a comparison. See if any one of them look substantially different. This might help narrow it down. Also, you have not added any more weight to your boat this year have you?