Mercruiser 5.7 Carb. has no power under load!

1995 Mercruiser 5.7L w/ Carburetor Engine on a Chapparal 21'-er. (upgraded about 6 years ago from 4.3L) Thunderbolt IV ignition system (has spark advance, but no knock sensor)

Here's the problem:
The boat was working nicely. About 4 weeks ago, the engine suddenly had no power under load. It will rev all the way up in neutral. It takes about 2 full minutes to get on a plane and it will reach about 40mph with one person. The lack of acceleration is my concern. There are no noticeable pings/knocks/misfires. The timing light indicates 12deg BTDC (the book says 10, but i had always had it set at 12). Spark advance appears to be working properly. Since the accel. pump only offers one quick spurt on acceleration (and it appears to be working fine), I don't think this is the problem. Also, I have checked the (2-barrel) carb (which was rebuilt about 2 years ago). Any ideas on what could be causing this?

Thank you!

Re: Mercruiser 5.7 Carb. has no power under load!

rpm`s in neutral mean nothing. start with making sure the sparkplug order is correct 1,8,4,3,6,5,7,2 clockwise.
fuel system test, run it on a outboard gas tank plugged into the fuel sep housing
compression test with throttle wide open and all sparkplugs removed.

Re: Mercruiser 5.7 Carb. has no power under load!

Ayuh,.... Welcome Aboard,... Inspect the contents of the fuel filter, then replace it....

Re: Mercruiser 5.7 Carb. has no power under load!

Check your prop might be coming off Slipping. Don't hurt to check if it was fuel you here a miss under load going that slow. same with the weak spark. You should get on plane quick.