Wide open throttle and boat just won’t move

[newboater32]

really racking my brain.
1987 35HP 2 stroke Mercury Outboard
I have a 16’ aluminum boat with seats and console etc
WOT and engine sounds labored. and does a maximum of 8-9 mph
things i have done since this problem
cleaned carb
new ignition coils.
new plugs
new switch box
new starter
new fuel lines
new gas tank with new bulb
changed impeller
changed lower oil. looked good

couple things wondering
when purchased controls cables were for a longer boat. does that have a factor.
cables sheath is broken but cables are fine

 

[newboater32]

forgot to say. idles good. throttle revs to high rpms.
but once in drive feels like we are dragging freezer behind us

 

[rickdb1boat]

Sounds like a propeller problem if the engine is running good otherwise. Possible spun hub or wrong pitch. Did it ever run correctly?

 

[GA_Boater]

Welcome aboard.

You threw a lot of parts at the motor. but you haven't mentioned any troubleshooting. What is the compression for each cylinder and is each cylinder making spark?

The most useless test in the world is revving up in idle. Only one working cylinder will peg the tach in idle because there is zero load on the motor. Do the compression and spark checks.

BTW - The cables are shot and need replaced. Too long is better than too short, but when you replace them get the right length.

 

[newboater32]

prop replaced. 11 from a 13.
compression 126 both cylinders.
both have spark.

 

[Flyin' Hawaiian]

Maybe too much stuff on the boat and you can't get it on plane? Or maybe its just too much boat for that 35hp motor. Is the motor tilted too much up or down? Does the bow stand up when you give it full throttle? If so, try putting more weight up front. I'm assuming this because you lowered the pitch on your prop, was it to compensate for extra weight?

 

[newboater32]

we thought weight so we took everything out seats etc . nothing
tried more weight at front or back. nothing

 

[racerone]

Exhaust plugged ?

 

[newboater32]

nope. cleaned out exhaust ports.

 

[Flyin' Hawaiian]

try taking a video of the problem while you're out on the water. That may give others a better idea on what the motor is doing, or in this case, not doing.

 

[dopey39]

I m having the same issue now. We are looking at a leak in the manifold gasket not head gasket. Might want to give it a look.

 

[Mark ofs]

I have a '85 mercury 35hp-2cyl on a 16' alum. boat with plywood decks,etc. Full throttle we are getting about 23mph.....one thing I didn't see in your list is rebuilding the fuel pump...Ours was disintegrating after having set for 10yrs then using with this wonderful new fuel.