RPMs Too High at WOT???-1996 FORCE 120

[kinggabbo]

I have recently purchased a 1996 FORCE 120HP engine. I went WOT this weekend and the RPMs were around 6000. The owner's manual says the WOT range is 4750-5250. Is there any danger to the engine if the RPMs are running that high? Why would it be running that high? (NOTE: I have yet to verify if the built in tach is accurate using a hand held tach) I read somewhere that it could be the wrong size/pitch prop causing the RPM increase. Maybe the trim too high?

 

[roscoe]

Re: RPMs Too High at WOT???-1996 FORCE 120

Yes, either the trim being too high or the wrong prop will cause the rpm to go higher.
I'm betting on prop, if the trim was too high, you would have been blowing out in any turns.

Will it do damage? Depends who you ask.
I always set my motors up to hit the high limit and beyond.
For example, 5300-5500 rpm with an empty boat and fuel tank.
Gives a great hole shot and ensures the engine will not be lugging even with a heavy load.

That said, once set up, I rarely would ever run it that high, because the fuel use is excessive at those rpm, and the speed difference is minimal. I usually run 3600-4600 rpm.

 

[kandil]

Re: RPMs Too High at WOT???-1996 FORCE 120

what size prop and pitch do you have? it should be on the prop

 

[kinggabbo]

Re: RPMs Too High at WOT???-1996 FORCE 120

Don't know that info off hand. I will find out tonight and post it.

 

[Frank Acampora]

Re: RPMs Too High at WOT???-1996 FORCE 120

Set your mind at ease! Since there are no valves to "float" like in a 4 cycle engine, and since the stroke is short (2.80 inches) You could probably rev the engine to 10,000 without doing any damage. I run my 125 on a small hull with a racing prop at 5,900 for hours on end.--but I can't get the prop I want to bring it to 5,500 which is where it is rated to run.

6,000 certainly will not hurt it BUT you are running it outside of the range and efficiency will decrease. And above 6000, reeds will float somewhat, also decreasing HP output and increasing blowback through the carbs. Re-propping to bring it within range will result in a higher speed with better fuel economy. USUALLY a 2 inch pitch increase drop RPM by 400 or so. You would want to experiment with a prop 4 pitches higher ie. If you have a 15 pitch prop now, try a 19 pitch.

Personally, FIRST I would try 2 pitches higher to bring RPM to 5600 or so, to see what happens. Then, maybe I would cup it for better holding in turns. Cupping usually drops RPM by a couple of hundred. All depends upon how the hull reacts.

You have a Mercury mid and lower unit. Go over to the prop forum and give them the numbers on the hub of the prop. They will tell you what you have now.

 

[pnwboat]

Re: RPMs Too High at WOT???-1996 FORCE 120

Just a side note. A few years ago I bought a Bayliner Cobra with a 125 HP Force. It was also was showing 6000 RPM WOT. The MPH was around 38. I didn't have much experience with outboards. I saw that the RPM's were above the recommended max of 5500 RPM. It had an aluminum 13 1/4 X 17 pitch prop on it. I bought a really nice stainless steel 13 X 19 prop. First time I tried it out, it showed 5500 RPM WOT, but it killed the hole-shot and went 3 MPH slower top end. I had to get a couple of people to move to the front just to get it up on plane. As it turned out, the tach was bad. it was reading 1000 RPM higher than it should!! Replaced the tach and had to go back to the aluminum prop. Make sure your tach is reading correctly before you buy a new prop.

There are a few prop calculators on various WEB sites. Had I know this and plugged my numbers in, it would have showed that the 6,000 RPM reading I was seeing was not consistent with the MPH I was seeing with the 17" prop with my type of boat.