Steady decrease in RPM/speed

[Have Fun Today]

1997 Merc 5.7LX and Alpha One with 23' Chaparral

Thanks in advance for the help...

I lost a race with a pontoon boat today, a very fast pontoon boat, that was doing in excess of 50 MPH. I have noticed a steady decrease in full-throttle speed (from about 50 MPH in July to less than 45 MPH today) and a decline in full-throttle RPMs (from about 4400 to just under 4000). I think it's time to troubleshoot the problem and would like to fix as many things as possible now in the driveway since it's a drive to get to water.

The engine runs great from idle up to full-throttle, no stalling or hesitation, just the decline in RPMs during the past few weeks. I haven't changed anything on the boat. When I raced the pontoon boat, I was doing 45 with the bimini top up. By the end of the day, I was doing less than 45 with the top removed. I did pick up a small nick in the prop, but this was after being beat by the pontoon boat.

1. My flame arrestor has some grease and oil on it, hasn't been cleaned replaced for several years...would this cause such a rapid decline in performance?

2. How many fuel filters are there? (Carb filter, fuel pump, etc.) Fuel filter was replaced in June by my mechanic.

I used the following checklist from a previous post by DON S.:

Engine Won't Reach Operating RPM. Check

Fuel condition. Type and Octane possibly old fuel OK
Propeller pitch or diameter, damaged blades Very small nick in one blade
Restricted fuel pickup tube or anti siphon valve Fuel filter replaced in June
Crankcase oil volume, high oil level can cause aerated oil and lifter collapse OK
Marine growth on hull and outdrive OK
Wrong gear ratio in outdrive OK
Restricted carburetor air intake (clogged flame arrestor) Greasy air filter
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive OK
Poor cylinder compression Too complicated for now
Carburetor defective, or wrong type I think it's OK
Fuel pump pressure and vacuum Too complicated for now
Boat overloaded, improperly loaded, or improperly trimmed OK
Engine Overheating OK
Engine timing and ignition system operation Too complicated for now
Remote control cables and linkage for proper travel to open throttle plates fully I think it's OK

 

[Haut Medoc]

Re: Steady decrease in RPM/speed

Clean the air filter with brake cleaner or gas & let it dry thoroughly....
Replace the water separating fuel filter.....
Pull the plugs & check for any unusual signs......
When was the last time a full tuneup was done, IE cap rotor wires?....​

 

[Have Fun Today]

Re: Steady decrease in RPM/speed

Follow-up...I cleaned the flame arrestor with carb cleaner and replaced the fuel-water separator. One of these fixed the problem and I'm back to 4400 RPM and 48 MPH.

I'll continue down the list, hoping for 55 MPH by the end of the list.