3.0 merc low rpm's

[brentjmorgan]

I have a 1988 searay with a 3.0 merc and a 17 pitch prop. At full throttle I only turn 3400 to 3600 rpms @ about 30 mph. I know I should be around 4200 to 4600 rpms. Any ideas on what could be going on.

 

[Don S]

Re: 3.0 merc low rpm's

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.

 

[crazyinkc]

Re: 3.0 merc low rpm's

Water logged foam? Has the boat been left out in the weather or did a PO replace the carpet or patch the floor? What is the length of the boat? I have a 17ft EBCO with a 19p aluminum with a 140 and the Merc data sheet that came with it said 40.1 MPH at WOT (I forget the exact RPM) with 150lb driver and half a tank of fuel and 38.5 fully loaded. The wifes boat is water logged and it takes forever to plane and hits 25 with a 50hp Merc and a 13p prop. The trailer ripped a hole in the bottom we didn't see and it soaked the foam. Thats my next project. Yea for me!

 

[insaneasylm]

Re: 3.0 merc low rpm's

i though i would post my porblem here since it is the same thing. with the same type of engine. My engine rpms when out on the water are only getting up to around 3300 at wot. But when in netural the rpms will get up to 44-4600 fine. So could this still be exhaust flapper stuck or carb problem. I have a 15in 17 prop on it. 19.5 ft bayliner

 

[bomar76]

Re: 3.0 merc low rpm's

RPM's in neutral mean absolutely nothing.
And quit doing that unless you are trying to blow it up.

Follow Don's posted list of probable causes and fix whatever is wrong.

 

[insaneasylm]

Re: 3.0 merc low rpm's

ok thx. yeah i know about running it up that high in netrual. I only did it for a sec.