Could a Vacuum Leak Cause Low WOT RPMs?

[kinggabbo]

I have a 1996 Force 120HP that is running about 4200-4400 RPMs WOT with the same prop that it used to run 5250+ with. I have had a very reputable shop check the motor and they said it is running great like it should. They checked compression, carbs, reeds, cylinder dropping, fuel system. It ran the proper RPMs on the dyno as well.

Could for some reason there be a vacuum leak while the boat was underway that might cause an RPM drop? I don't know if that would affect it at all anyway.

I suppose it might be waterlogged, but I really don't think it is. I am planning to take it and weigh it at a truck stop soon.

Thanks for any help you can give me.

 

[Frank Acampora]

Re: Could a Vacuum Leak Cause Low WOT RPMs?

If it ran well on a dyno--ASSUMING that the mechanic actually put it on a machine and under load or even with a test prop which will correctly load it--- then it should run equally well on the boat. Therefore based upon your input, the engine is OK and should have no vacuum leaks.

However: Check that the reed valve Vee block screws have not worked loose--you need to remove the carbs and manifolds to do this and will probably need new gaskets. You can NOT tell just by looking into the carb hole and eyeballing the reeds. Loose vee blocks will have almost the same effect as a broken reed petal and depending upon how many are loose and to what degree, performance will suffer.

Also check the balance hose between manifolds--this is a 3/8 id hose and if broken, idle speed may be higher or lower than normal but power will be down.

Now you must check the hull, and not just for weight. Check for deformities in the bottom and weight distribution. Check that trim is holding the engine and not bleeding down under thrust forces when running. Check the prop.