1984 Mercury 50 confused!

[bclongdrive]

Wondering if anyone can help. I’m out of ideas and no marinas near me have any idea.

84 merc 50 my late grandfathers motor on a 61 glastron. Always ran around 35-36mph.

Few years ago when my brother owned it started running poorly maxed out at 25mph.

I took ownership of the boat and here is what I have done.
Compression perfect on all 4
Rebuilt carbs and fuel pumps
New coils
New wires
New plugs
New switch pack
New stator
New cdi
Checked timing to make sure set to required specs

nothing I have done has made a single difference boat revs to the moon in neutral but under load falls flat on its face at 4600 rpm pushing the throttle all the way down seems to almost cut the engine out. Boat starts perfect idles perfect and accelerates fine but will not go past 4600rpm

At a total loss for ideas at this point and feel this must be something electrical but can’t seem to figure it out and local mechanics either won’t even look at it due to its age or have no idea what’s wrong.

Any ideas greatly appreciated!

Cheers all
Brett

 

[racerone]

Remove lower unit.-----Look up into exhaust housing.------Inspect the exhaust tuner for blockage.----Good flashlight required.

 

[bclongdrive]

Thanks I will try that !

 

[Chris1956]

Are both carbs opening fully? Sometimes linkage between them slips.

 

[dwco5051]

Was it stored outside when you brother had it? Flotation maybe waterlogged adding more weight. Did you brother have to replace the prop at any time? Could be that if it was it was the wrong pitch and you are over propped.