V8 Mercruiser with no top end power

[13crazyhorse]

I purchased a boating project last year. It is a 24 ft four winns vista. It had a MCM260 V8 engine that turned out to be no good. I purchased a brand new crate motor from Michigan Motorz which was an exact replacement. I added a new ignition system. Set the timing with a digital gauge. Changed over the intake and carb after rebuilding the carb. Also changed over the manifolds and risers. The motor starts and runs perfectly sitting at the dock. After warming it up I can easily rev the motor up to over 4K rpms very smoothly. But when I'm on the water it runs perfect up to about 2500 rpms. When I give it more throttle it goes BLAHHH and won't go over about 2800 rpms. It seems and sounds like this is when the 4 bbl kicks in. Speed at this point is about 10 mph. I did change the y pipe that was leaking and the replacement necks down to 3 inches from 4 inch risers but I don't think this would make any difference since they all neck down to a small hole before exiting the boat? I have also tried a different carb with basically the same result. I also put new seals in the outdrive last year and while I had it apart checked the gear ratio which is correct. Any input or experience with this would be greatly appreciated. Thanks.

 

[Bondo]

I purchased a boating project last year. It is a 24 ft four winns vista. It had a MCM260 V8 engine that turned out to be no good. I purchased a brand new crate motor from Michigan Motorz which was an exact replacement. I added a new ignition system. Set the timing with a digital gauge. Changed over the intake and carb after rebuilding the carb. Also changed over the manifolds and risers. The motor starts and runs perfectly sitting at the dock. After warming it up I can easily rev the motor up to over 4K rpms very smoothly. But when I'm on the water it runs perfect up to about 2500 rpms. When I give it more throttle it goes BLAHHH and won't go over about 2800 rpms. It seems and sounds like this is when the 4 bbl kicks in. Speed at this point is about 10 mph. I did change the y pipe that was leaking and the replacement necks down to 3 inches from 4 inch risers but I don't think this would make any difference since they all neck down to a small hole before exiting the boat? I have also tried a different carb with basically the same result. I also put new seals in the outdrive last year and while I had it apart checked the gear ratio which is correct. Any input or experience with this would be greatly appreciated. Thanks.

Ayuh,...... I Guaranty ya, the Problem can be found in #15 in Don'S Adults Only section, at the top of this forum,....

 

[FreeBeeTony]

I'd check the valve adjustment..........did you do it or leave it as it came fro Michigan Miotorz?

 

[13crazyhorse]

I didn't do anything to the valves engine is brand new crate motor from GM I would assume the valves are fine?
Thanks Bondo for the info I've already eliminated some of the items on the list I'll double check some of the other ones.

 

[MikDee]

First check your firing order, a small block Chevy engine can run perfectly smooth with 2 certain plug wires crossed, I had it happen to me.