Mercury Four Stroke slows at WOT

[dahilly]

I have a 1999 Mercury 50HP Big Foot on an 18ft Lowe Jon Boat. My problem is that I can't get the engine to run at the rated WOT RPMs. The engine will run at 5200 RPM maximum. I was running a 13 pitch prop so I purchased a 10 pitch SS prop. The boat slowed down about 3mph however the RPMs would never go above 5200 RPM even with the lower pitched prop. The one other symptom of this problem is that I must pull back on the throttle slightly to attain 5200 RPM. If I don't do that the engine feels like it is laboring, and the max RPM comes down to 4900. When I pull back it smooths out and the RPMs go to 5200. My dealer says that I probably have a set of carbs that need to be tuned. He also said that I may have a bad tach. I have also changed the plugs twice, and added fuel cleaner a number of times.

 

[millerpdt]

Re: Mercury Four Stroke slows at WOT

how are you checking your speed?? gps?? lowering the pitch of the prop could cause cavitation. the 13 should be right for that boat. 5200 is up there as far as max. i think you should rethink prop pitch. just an opinion.

 

[dahilly]

Re: Mercury Four Stroke slows at WOT

I have a paddle wheel and gps both on board they both register pretty much the same speed. The manual says that max rpm should be between 5500 and 6000. Do you believe that the Big Foot configuration just won't turn the max? I have changed back to the 13 pitch prop since I wasn't able to increase the rpms anyhow. This isn't a major issue it just bugs me. Thanks for the input.

 

[graddy]

Re: Mercury Four Stroke slows at WOT

This being a four stroke it is Ok to rev up in neutral. See what RPM you are able to reach when in neutral. I would still do this while the boat is in the water so there is back presure on the exhaust. I would not reccomend this when working on a two stroke.

 

[dahilly]

Re: Mercury Four Stroke slows at WOT

I have in fact tried what you suggest. The rpms in netural will go up past 6000 without a problem. That is why I don't believe I have a bad tach as my dealer suggested. I have also tried to trim the motor out of the water when running full speed to see if I could get the rpms to peak out, but even then they will only go up to about 5400 even though the prop doesn't have a bite in the water at all. On my other boat if I do this the rmps will go out of site when the prop looses its bite.