M40C won't accelerate past 1/2 throttle

This has got to be an easy fix. I'm pretty new to boat motors but i'm pretty handy with a wrench. The motor starts first pull, idles just fine, and snaps your neck when your take off...........only up to 1/2 throttle then it falls on its face and starts stuttering, spitting, and will only do 20 mph. Everything is new from the tank all the way to the lines that feed the carb as well as new plugs. I will take all of the tips, tricks, and ideas that anybody has. I know that 20 mph will get me where i want to go but i know that motor has a lot left in it.
Thank you so much for all of your help.
Jarrid.

timing advancing?

I should also add that this motor will rev through all RPM ranges like a mad man in neutral. Also i've had some problems with it dying while idling and when i immediately go to restart, it's either going to start right back up on the first pull, or flood out and i have to wait a while. Now for the idle, it's set too low and i need to adjust the screw to raise it. But what about the easy flooding. It doesn't take but what it seems like 4 or 5 pulls and it's pointless to keep trying. Will messing with the air/fuel screw solve any of my problems?

When was the last time the carb was removed, disassembled, cleaned, and adjusted? It should never flood. The float may be adjusted too high, or the float valve needle may be worn/corroded.

Professionally cleaned and leak tested on thursday. Ran it again on saturday. I had 4 people, a dog, guns, and decoys in my boat so needless to say it had a load on it and the motor would only push us to a whopping 12 mph. Fast out of the gate and then like always would fall on its face spitting and sputtering (sounds just like a flooded out 2 stroke). Here's how the day went. Started on first pull. Full tank (6 gal) of gas. 20 minutes tops round trip. Burned through 1.5 gallons (seems like a lot for 20 minutes at 1/2 throttle) Black plastic piece in front of the carb was holding and dripping fuel. Brand new spark plugs (put them in friday night) burnt black on the diode. Is it my fuel pump?

No, your fuel pump cannot "pump too much". You either have a carb issue (was the float valve replaced? what method was used to clean and adjust the carb? what on earth is "leak testing"?) or another problem. Most likely still a carb problem, but whip out your Factory service manual, and verify the electrical and mechanical systems as well. A bad reed (very rare) could cause extra spitting, but should not ;ead to a mix that's so rich (flooded) that the plugs foul.

Thanks Paul. I'll start with a carb kit. Electrical is all good as well as compression. After the carb was last cleaned, I turned the air screw 1 3/4 turns out to start and tuned by ear from there. No difference in performance. Still bogging down and gulping gas. Extremly ineffecient. Same old thing, screams in neutral but won't go over 20mph in gear. Going out of town for Thanksgiving and won't be able to run it until next week. I'll post again to keep you up on the progress, or lack there of. Thanks again

The idle mixture screw affects idle, and is pretty insignificant above idle. Do a complete disassembly of the carb, including jets, emulsion tube nozzle, etc. Then a 4-hour room temperature soak in real carb dip (available at auto parts houses in 1-gallon cans), followed by a liberal blow-out with generic carb spray. Then reassemble. Be sure to verify float height when reassembling.

The manual i had, i borrowed from a friend and he's out of town for the weekend. Can you explain to me exactly how to measure the float height and what it should be. The motor is a 1998 M40C. More and more i'm beginning to think its the culprit. Thanks Paul. Happy Thanksgiving.

New prop installed ?----correct pitch for the boat and the load ?----------I do not work on those motors but is there a " rev limiter " on that machine.

As for float height... with the bowl off, and the carb inverted, the float should be parallel to the casting.