1985 50 hp Merc gonna drive me to drinkin

[Rsully]

Motor struggles to get over about 3200 rpm. Compression on all 4 cylinders = 90.

Previous owner replaced:
switch board.

I replaced:
Starter selenoid
Plugs
Impeller
stator
Fuel tank
Fuel hose and bulb
Fuel filter
Fuel/water seperator
Carb complete disassemble, dipped, rebuilt, including new floats.
Trigger
Rectifier

I performed link and synch.

Tested plug wires.
Tested coils.

What am I missing?

 

[Chris1956]

OK, has it ever gotten over that RPM with that prop on that boat?

What do the spark plugs look like after running a while? Brown, black or new?

 

[racerone]

Remove lower unit.----Use strong flashlight.-----Look into the exhaust housing / exhaust tuner.-----Possibly plugged with carbon.------" I can't breathe "----- might be the problem.

 

[Rsully]

I think the highest rpm we ever hit was 3800 and it was only for a couple of seconds. We removed the lower unit and looked up into the exhaust and it wasn't obstructed at all. We just removed all plugs and took some pics. 3 and 4 are a little bit darker than 1and 2. I should probably also mention that it runs very erratic. As in when driving full throttle it cuts in and out constantly. Like 1/2 second cuts where rpms drop 300 or so. I will attempt to upload pics of the plugs.

 

[Rsully]

The first pic is before I wiped the oil off.

 

[Chris1956]

OK, what boat and pitch of prop are you running?

Can you pump the primer bulb while at high speed? Does that fix it?

 

[Rsully]

Pumping the primer bulb while underway has no effect. The boat is a 17 ' aluminum v bottom tracker. The prop is a 3 blade but that's all I know about it.
Is the pitch of the prop stamped on it somewhere?

 

[Chris1956]

Yes, the pitch is usually in the part number. Mercury part number will likely start with 48-several numbers-15, where 15 is the pitch. Aluminum props usually have odd pitches.

I may need to start drinkin with you.

 

[racerone]

Perhaps remove bypass covers on starboard side to look at pistons and rings.------90 psi on my tester would dictate REBUILD time for that motor !!-----Run with a timing light one cylinder at a time. ------look for spark issues.-----Are the boost venturi pieces in place on the carburetors ??----Could also be bad lower crankshaft seals are leaking !

 

[huntxtrm]

Remove lower unit.----Use strong flashlight.-----Look into the exhaust housing / exhaust tuner.-----Possibly plugged with carbon.------" I can't breathe "----- might be the problem.

Seemed to be the majority of the problem on mine!

 

[Rsully]

Seemed to be the majority of the problem on mine!

I was following your thread for the last few weeks because our problems were so similar. When that fixed yours and not mine, I knew I needed to create an account so I could post this thread.