RPM and WOT questions

How can I find out what the WOT should be and the range of rpm's for my 1968 Mercruiser Sterncraft. It's 22' long with a 165 hp straight 6. It has a 14 1/2 x 17p prop?

Today we were out on the water and it only ever got up to about 3200 rmp. Figured it should do better than this.

All suggestions welcome.

Re: RPM and WOT questions

Unless it is diesel, 3200 is low. How much too low? Nobody here can tell you, until you identify the engine in it.

Re: RPM and WOT questions

It's a 165 hp straight 6.

Re: RPM and WOT questions

I expect WOT should be ~ 4200 - 4400.
Is the hull clean? Is the engine running smooth? How many seconds to get up on plane? Did it run higher rpm previously with that prop?
What does speedo say when at 3200 rpm?

Re: RPM and WOT questions

Does the engine sound like it is reved up good. I was warned that tachs sometimes go bad and can be 500 or so rpms off.
Is this a 22' fiberglass or tin boat.
Is the carb getting WOT, check to see, do you have good fuel.

Re: RPM and WOT questions

AT 3200 rpm is was about 14 mph according to the gps. Never timed the getting up on plane time. When we first got the boat before we replaced everything it would run about 22 mph according to the gps. We did not have a tach then. New engine and tach now.

The hull is clean and is fiberglass.

The carb is new and a new fuel tank as well. A 2300 holley 500 cfm is the carb with a new arrester. New water separator and fuel filter and gas lines. New plugs(gaped per service manual), plug wires, coil, dist, electronic ignition.

The first run we took it out the boat ran around 3900 rpm. We were in the river and hit something floating under the surface of the water and it bent the prop. Took it down to 3200 rpm. Took the prop and had it serviced professionally but the boat still runs only at 3200 rpm. We took the boat offshore about 20 miles one way and on the way back home the carb started messing up and then the rpms went down to around 2600 and the gps said we wee running about 7 to 12 mph at different times. It bogs down real bad in swells.

Re: RPM and WOT questions

I would get a different prop, the prop guy that fixed it could have changed the pitch. I think.
I like my adjustable composit prop. I would get a pirahnna prop, I have the one they sell out of westmarine.
Also are the plugs burning good, sooty or light tan. Look at all the plugs.
You could be running too rich.
How about timing, I would recheck timing.
I would make sure you are getting wot at the carb.

Re: RPM and WOT questions

Yes the plugs are the right color. Thinking about going up higher in pitch on the prop like to a 19 pitch. We advanced the timing a little bit. Don't know the exact degrees. It starts better after the advancement because it used to be so cold natured. Also, how do you make sure it's getting full wot at the carb?

Re: RPM and WOT questions

Thinking about going up higher in pitch on the prop like to a 19 pitch.

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For what purpose - as it stands right now, you can't even spin the 17"

We advanced the timing a little bit. Don't know the exact degrees.

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Stop guessing before you destroy your new engine.


Power comes and goes, is effected by the swells = it is NOT the prop. Look at both the ignition and fuel systems and systematicaly eliminate every single component and connection from the battery on and from the fuel vent to the exhaust flapper.

DAMN! got light bulb moment there - is your exhaust flapper still in place of has it come loose and is now clogging the passage?

Re: RPM and WOT questions

Sorry I don't the exact degrees. I am the wife just listening to what my husband says. I didn't ask him the exact degrees. He knows all about that or he wouldn't have been working on it. All of this info I have given is just what I have gathered here and there from him talking about it. I am learning a lot from helping him and listening to him.

When he replaced the points and put the electronic ignition system he didn't remove the resistor that was between the coil and dist. so that is going to be removed today then he will check the exhaust flapper (thanks for that suggestion). The vent is clear.

Re: RPM and WOT questions

1968 sterncraft said:

Sorry I don't the exact degrees. I am the wife just listening to what my husband says. I didn't ask him the exact degrees. He knows all about that or he wouldn't have been working on it. All of this info I have given is just what I have gathered here and there from him talking about it. I am learning a lot from helping him and listening to him.

When he replaced the points and put the electronic ignition system he didn't remove the resistor that was between the coil and dist. so that is going to be removed today then he will check the exhaust flapper (thanks for that suggestion). The vent is clear.

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That resistor is there to drop the voltage to the coil when the engine is running to avoid burning up the points.

Re: RPM and WOT questions

Sorry I don't the exact degrees. I am the wife just listening to what my husband says. I didn't ask him the exact degrees.

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Fair enough...I wish my wife had even a passing interest in this kind of thing.

Good catch, pulling the resistor.

Have either of you browsed through the "adults only section" yet? Factory manuals and some great troubleshooting guides in there.

Re: RPM and WOT questions

Yes that adult section is very informational. I got him a service manual last summer. Yesterday I got a vacuum gauge to check the pressure like the section said about checking the fuel side.

Now just to check the exhaust flapper. How hard is that? Will the engine still run if the flap is shut? I looked in the manual and seen where it is and how to remove it. We were headed out to work on it today but the rain will not go away. Florida weather in the summer has rain almost everyday this time of the year.