1987 2357 Carver - low top speed

Any help I can get with this- Thanks in advance!
1989 Carver 2357 Mercruiser 260 -

Last year 4300 rpm W.O.T on the water.

Bought a low hour Alpha One lower unit and replaced over the Winter - and yes I'm using the prop original to the boat. Old lower functioned fine but was corroded. I'm still using my old upper. First 2 trips on the water I could get to 4300 rpm. Now I'm only getting to 3000 rpm and top speed of 15 mph.
I added the outdrive swap info but here's what I've done so far to remedy the 3k max:

Checked for carb linkage operation at W.O.T, new Quadrajet carb, new Sierra fuel pump and both inline and spin on filters,replaced fuel lines from carb to tank as well as a new pickup and anti-siphon. I've also ran the boat without the gas cap for ventilation. Old carb was originally thought to be the issue. I also removed the water shutters in the exhaust to eliminate a restriction possibility.
Replaced plugs with Delco MR43T's gapped at .035, new cap,rotor and wires with Mercruiser numbered parts on the Thunderbolt 4 dist. Nothing changed. Still 3k max.

Yesterday I upgraded to a Delco Voyager ignition- regapped plugs to .045. Still only getting 3k rpm. I'll add that the boat runs great at all speeds up to and at 3k rpm- but it's maxxed there. I have yet to do a compression check.

Anyone see anything I've missed???

Don Sindledecker said:

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.

Click to expand...

Ayuh,.... Courtesy of an old Friend,.... I guarantee the answer is in that list,....

Just a guess, Check your firing order to see that you don't have 2 crossed spark plug wires. Clockwise rotation
1-8-4-3-6-5-7-2, Port side 1-3-5-7, Starboard side 2-4-6-8

Where do you live and how long was it between (in days/months) using the boat when it went 4300 vs 3000?

What kind of prop? solid hub or plastic hub? Swap props as a easy check...........

What is the growth on the boat bottom look like?

Has the coupler ever been changed?

Also have you checked the outdrive oil condition since lower unit change?

Look to see if there is any grey matter in it as it would indicate a gear/bearing issue.....

All of the above have been checked out other than lower unit oil and compression check. Although oil was changed only a few days leading up to the issue. I'll check compression next.

Thanks Guys!