1977 merc 150 outboard bogs at WOT

[rustyjcole]

Hello all and thanks for looking.

I have a 1977 merc 150 outboard engine that bogs down at WOT, and i cant seem to figure out my issue, it will flat out "scream" while on muffs outside of the water but soon as i get the engine under load it bogs down. Here is the list i have already been through

Complete rebuild on all three carbs
All new fuel lines
Spent forever adjusting carbs
Tried separate spare fuel tank
Rebuilt fuel diaphragm
New spark plugs
New impeller

Any insight would be appreciated

 

[merc850]

DON'T scream it in the driveway with muffs! You have to set the low speed needles to 1 1/4 turns out in F in the water underway @ 800 rpm.

 

[Chris1956]

Pump the primer bulb while at full throttle. If it stays at high speed, rebuild the fuel pump. The fuel pumps on those are really taxed at WOT, so make sure the valves, diaphragm and pulse hoses are good, and the screen is clean. The pulse hoses have springs inside them to keep them from collapsing.

Take a compression test. #1 and #3 can be damaged by lean mixture or overheat.

There are screens under the brass elbows on the carb covers. They almost never clog, but check 'em.

Carb adjustment is only for idle quality. It has no effect after 2000 RPM.

What boat and prop are you running?

 

[jimmbo]

First thing is a Compression Test
Next, does the Ignition System, provide a Spark strong enough to Jump a 3/8" Gap at all Plug Wires?
Did you set the Floats correctly?
After you replaced the Carbs, did you verify the Link and Sync between the Spark Advance and Carbs, and adjust to spec if needed?
Do you have an actual Mercury Service Manual?

 

[rustyjcole]

Pump the primer bulb while at full throttle. If it stays at high speed, rebuild the fuel pump. The fuel pumps on those are really taxed at WOT, so make sure the valves, diaphragm and pulse hoses are good, and the screen is clean. The pulse hoses have springs inside them to keep them from collapsing.

Take a compression test. #1 and #3 can be damaged by lean mixture or overheat.

There are screens under the brass elbows on the carb covers. They almost never clog, but check 'em.

Carb adjustment is only for idle quality. It has no effect after 2000 RPM.

What boat and prop are you running?

I did try pumping the bulb while running WOT and i still bog down, boat is a 1977 17' fiberglass larsen. As for the prop im unsure and currently at work. Compression was next on my list

 

[Chris1956]

I would think a 17 or 18P prop wpuld work, if the boat weighs what a 17 footer should.

I ran that motor on my 16' speedboat with a 21P prop and could get 60+MPH. The hull is pretty flat bottomed.

 

[jimmbo]

To get the revs up to 5800, a 17" is likely the Pitch that is going to be needed. The 150 runs great from 4000 to 5800, but below 4000, it can be an unhappy camper.