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1990 120hp performance

H

hstega

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"Hello, I am looking for someo

"Hello, I am looking for someone that know how to get more performance from a Force 120 hp, I have had this motor since it was new. It has been very reliable... starts well, trolls well, plenty of power get'in out of the water. However on top end it performs like the Johnsons and Mercs 90 hp. Motor is rated at 5500 rpm and I can just barely get it up to 5000. Have even droped down to a 17 pitch prop and still can not reach rated rpm's. Anybody know anything that would help me out? Harry"
 
"Harry, older Merc 90's we

"Harry, older Merc 90's were actually up near 100 horses. Until the EPA came along and started regulating emissions, the builders used to "under-rate" their motors. That practice stopped and now most motors are within 3-5% of actual (posted) horsepower).

If your motor is running good, I really wouldn't mess with it. Even if you could say get an extra 10% out of it (make it a 135), you wouldn't notice the difference unless you lied to yourself. Sorry, the one difference you would notice is the loss of cash from your wallet.

A new set of plugs once a year goes a long way to performance - also accurate prop matching. Every motor should have 2 or even 3 different props to account for the seasonal changes which affect how the motor runs. You want to try and keep it at it's optimal wide open throttle all year long, that can only be achieved by switching out props. Max horsepower is developed at optimal wot - during the hottest part of the summer your motor may lose up to 15% of it's horsepower because of an incorrect prop...."
 
"Harry & Graham, That'

"Harry & Graham, That's GREAT info - thanks. I noticed the same thing happening at the end of last season. No Tac, but early in the season I ran between 37 & 38 mph and yet in September top spead was about 35mph. Can the difference be attributed to water temp, early/late season etc...? The motor ran great throughout the season, just a (very) slight difference in power/speed. Oh BTW - 85hp"
 
"David, here's a little pi

"David, here's a little piece right from Merc. Basically, you want to keep your motor running at it's optimum wot, which is usually about 200 rpm below it's max.

On my 15 horse I run an 11 pitch in the spring and fall and when I notice the heat/humidity having an effect, I drop down to a 9 pitch.

Anyhow, here's the diagram...

147326.jpg
"
 
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