looses power at high end

I have a 1990 40 hp evinrude that stalls only when in the water at full throttle. Runs great in nuetral in the water or hooked to a hose. I have put in new tank , new gas lines , rebuilt both carbs, new fuel pump, new plugs and wires, tried a coil off of another boat that i know works. new water pump , new thermostate tried another heat sensor just to see if it was shorting out. Ive tried a smaller prop to see if that was bogging it down. I tried disconnecting the choke. Unhooked the throttle linkage and tried to run with just the carb . Nothing seems to make a difference.. PLEASE HELP

Re: looses power at high end

Is it cutting out at top end, slowly losing power, or just flat dieing?

Re: looses power at high end

Do a cyl drop test on it to make sure it's running on both cyl's. What's the compression on it ??

Re: looses power at high end

Compression is around 135on both cylinders it will not get up on plan,it Boggs out at a quarter throttle

Re: looses power at high end

Did you try the cyl drop test. That's pulling one plug wire off at a time when it's running to see if you have a dead cyl.

Re: looses power at high end

Fuel:
try running off a different remote tank to test everything upstream--especially the vent. Use a different hose to test the ball and fittings.

when it bogs, pump the ball, or hit the choke, to see if it's fuel related by there being a change in the motor.

Remove the carb cover and spray premix from a windex bottle into the carbs.

Fire:
look for shorts. I had a merc do that and we all swore it ws a fuel problem until we saw that a wire with frayed insulation was arcing against the block when you accelerated and advanced the stators. Run it in the dark, cover off, and look for sparks.

sometimes new parts are bad.

check the ground attachment for the coil, and insulation on all wires and connectors.

Re: looses power at high end

I tried all the above today , and thank every one of the post but none of it seemed to work either.I decided to pull the fly wheel to see if I had any shorts . I should have done that to begin with ,, the fly wheel key was sheered in half , it was a few degrees off. I put one from the hardware store in and it ran much better. Headed tomorrow to get the right one from the boat supply store . Thanks everyone for the replys. If that doesnt fix it I will be back asking the best place to buy a new motor !

Re: looses power at high end

That should take care of the problem. Glad you are getting the right one. The hardware store keys aren't strong enough. Also we forgot to say Welcome to iboats!