not reaching max rpm

boat is a 1997 wellcraft eclipse 2000 with a 4.3 v6 installed in 2006... its running an alpha 1 gen 2, flame arrestor says it should rev 4400-4800. at full throttle im stopping anywhere from 3900-4k flat. wont go any higher, its a project boat i bought, when i first bought it, it was stalling out at anything over idle. so i put new cap and rotor in, and pulled the carb off, took it apart and cleaned it up and put new gaskets in. that got it running decent, im going to install new plugs this weekend. the prop is a 14.5x19

is it possible the throttle needs adjusted and its just not opening all the way for some reason?
also thinking maybe the 14.5" prop is slightly too large and maybe it cant spin it fast enough?

looking for suggestions, im gonna be taking the boat out sat, sun and monday this weekend and would love to get it sorted out so i can see how fast it runs, trimming it up i was only able to get it to 37mph at 4k rpms this past weekend


made a similar thread on another boat forum, just looking to get as many opinions as possible

You could go down to a 14X 15 prop. I had the same problem with my 4.3 After a lot of troubleshooting I found the boat was too heavy for engine. Weigh your boat and compare to original boat specs. I ended up putting a V8 in mine.

rl65028 said:

You could go down to a 14X 15 prop. I had the same problem with my 4.3 After a lot of troubleshooting I found the boat was too heavy for engine. Weigh your boat and compare to original boat specs. I ended up putting a V8 in mine.

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I agree with the boat bieng to large for a small engine.

My opinion is anything over 19 ft long should have a V8 motor.

If you max out at 4000 rpms with the out drive trimmed out at full throttle then yes a pitch change is needed.

Is your carb a 2 bbl or 4 bbl? If it is a 2 bbl then dont expect amazing speed. Maybe 45 +/- mph.

Every pitch change typically will give you a ~ 200 - 300 rpm change.

So as suggested, a drop in pitch from a 19 to a 15 (which is a standard proppeler pitch change of two pitches if you will) should increase your max rpms to ~ 4600 rpms that would be ~ 600 rpms.... it will also give you a much better whole shot (plane off and come out of the water faster).

Dont worry about size or diameter it is factored in with the prop design.

There are other odd props, 4 bladed come in many pitches also and sometime some find as an example a 18 pitch 4 bladed gives best speed and performance.

There are many factors as to why some boats need such smaller pitch vs other boats. Normal design parameters typically will have a 19 pitch as the right prop pitch. So it is what it is.........

Do you have a 4bbl, or 2bbl? Also, do you have the Thunderbolt IV, or T-bolt V ignition? Your prop diameter is fine, we need to find out if 19" is too much pitch for your engine? More info would help.

Usually there is a small canister filter in the body of the carb behind the fuel inlet line nut. It needs to be checked, or changed each boating season. The filter is made of some type of waterproof white plastic (for marine use), sometimes people put stock automotive "paper" filters in there, they don't hold up! Have you checked this?

its a 2bbl, not sure which ignition system, i entered the part number but it says IV/V which doesnt help me. it may end up with a v8 this winter, we will see after i run some health checks on it. its hemorrhaging oil to the tune of a quart an hour almost, looks like its coming from the rear main, so im yanking it out this winter
also the carb filter is clean
with the pitch change, if i understand correctly, changing the pitch to raise my rpm will give me a lower top speed, which i dont want as im maxing at 37 right now