Re: Fuel system?
You are already way low on engine rpm if 4000 is what you get at WOT (Wide Open Throttle). You want to see between 4400 and 4800 rpm (closer to 4800 is where you want) at WOT. Going to a 21 pitch prop is going to make the problem worse, not better.
One other thing. You only have 115 hp on a 19' boat. have you checked the weight of your boat lately?
Here is a list of possible causes of low
WOT (
Wide
Open
Throttle) rpm.
In no particular order
Engine Won't Reach Operating RPM. Check
1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve
Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression
Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.
