Not achieving RPMs

[SeaEsta-12]

Ok, so their are zero mods to my engine. All stock engine and outdrive. (GM 5.7L GXi-A with a VP duo prop F4 blades)
I took the boat out today and could only reach 4000 RPMs. At that range I was getting 32 mph.
My specs say I should get 41 - 43 at 5000 RPMs. I can't even get my motor to go that high.
I just had the trottle cable replaced and I had them check the throttle range and he said it was a to spec. No play at all especially with the new cable.
What else could it be that I am not reaching my proper RPMs?
Thank you for all your help!
Side note:
Trim and trim tabs were all adjusted during my run.
I had a full tank of fuel and one passenger on board. Maybe at most I was at 7000 pounds.

 

[dypcdiver]

Did the boat reach 5000 RPM before the cable replacement? If so then your problem is cable related.

 

[ericga]

Were you able to reach higher RPM before? Several things affect WOT RPM. The most common being the propeller size and pitch.

Also a common mistake with the 5.7Gxi-A is the use of the wrong spark plugs and gap. While most newer Gxi call for the expensive Iridium/Platinum plugs GAP @ 0.06, some early Gxi use the standard copper plug Champion RS12YC GAP @ 0.045. The wrong plugs/gap would affect your WOT RPM.

Volvo spark plugs for early Gxi:

5.7GXi-A, 5.7GSiI-A, 5.7GXiI-A = 0.045

5.0GXi-A, 5.7Gi-A, 5.7GXi-B, 5.7GiI-A, 5.7GXiI-B = 0.060

 

[SeaEsta-12]

It never reached 5000 rpm before the throttle cable change and a lot of that was because of the wrong props that were on it. I ended up calling Chris Craft and asked what the props were that came with the boat from factory and they said an F4. So that is what I changed it to. After that, it went from a max of 2800 RPMs to 4000 and that is where I'm stuck at.
I'll try checking the spark plugs as I am not sure the last time they were changed. Good idea.

 

[SeaEsta-12]

Do you guys think I should stick with the champion plugs, or try something different?
Is there another brand that you all recommend?

 

[ericga]

Champion RS12YC
AC Delco MR43LTS
NGK BPR6EFS

Some prefer NGK for the quality of their products, some prefer Delco as they come factory with the engine, some prefer Champion because that's what Volvo use. Your pick.

 

[SeaEsta-12]

Ok, so not sure why when getting cross reference numbers, they all conflict.
When I go on the NGK website, I punch in all my motor info, and it gives me this number for a spark plug.
NGK 3951 TR55 V-Power Plug
but it also says this is the number: NGK BPR6EFS

What the heck! ha

 

[Bondo]

AC Delco MR43LTS

Ayuh,..... AC Delco is all I use,.....

 

[ericga]

Rely on the volvo penta store website. Part number is: 3851857 which is a pack of 4 copper Champion RS12YC gap @ 0.045. Lot of places including NGK cross reference assume all gxi use the TR55 gap @ 0.060. There is an exception for these early gxi models: 5.7GXi-A, 5.7GSiI-A, 5.7GXiI-A.

Both the TR55 and BPR6EFS fit the GM 5.7 Vortec but with different specifications.

 

[SeaEsta-12]

Rely on the volvo penta store website. Part number is: 3851857 which is a pack of 4 copper Champion RS12YC gap @ 0.045. Lot of places including NGK cross reference assume all gxi use the TR55 gap @ 0.060. There is an exception for these early gxi models: 5.7GXi-A, 5.7GSiI-A, 5.7GXiI-A.

Both the TR55 and BPR6EFS fit the GM 5.7 Vortec but with different specifications.

Eric,
I was thinking the same thing as well, since so many manufactures list the correct engine, but also give me a diff part cross reference number.
I guess the best thing to do, is just stick to the OEM and specs that my engine calls for.
Looks like I wlll be buying the Champion plugs.

 

[ericga]

While you should always check the gap before installing new plugs it is particularly true with the Champions.

If older than 2-3 years consider inspecting or replacing the cap & rotor at the same time.