There's a couple of potential problems.
Check the pitch of the prop. It could be too small or too big.  You have
heard the term 15 pitch, 17 pitch etc.
Thats actually stands for 15"
pitch and so on.  In case you don't know, 15" pitch means that the prop
pushes the boat forward 15" for each revolution of the motor.
Depending on the size of the boat, (I'll assume that it's bigger than
14' and smaller than 17') a 15" pitch prop will allow higher RPM's than
say a 17" pitch prop and give the motor and greater ability to reach max
horsepower. This little change would buy you maybe 3MPH at the top but
give a little less punch out of the hole.
Also keep in mind that a Force motor is in fact  a Mercury Marine
engine with one exception.  The power head is still a Chrysler product.
It is in fact a cheaper (kinda stripped) version of a true Merc.
Generally a Force motor does not have the power of features of a
conventional outboard.
Also make certain that Only Quick Silver TCW-III Oil is used in the mix.
Believe me as silly as it sounds, it makes a difference.
Anyways, that's just my coupl-a-cents on the topic.
Happy boating.
Steve
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