Merc 120 I/O 4 cylinder problems

[royclaypool]

Ok. I'll start by saying I am a boating newbie who just bought his first boat. That being said. I bought a 1984 Renkel with Merc 120 I/O. Took it to the lake and the engine started. Ran fine. Drove for about 15 minutes just fine. Had 2 adults and 4 small kids loaded up. After about 15 minutes of just trying to learn controls and get acquainted, I smell a burning smell. Not a good kind either. I panic and immediately steer for the docks. Once the burning smell was present, I noticed the boat starting to lose horsepower. At the first, RPM's did not seem to be affected, then after about 15 seconds of the HP loss, RPM's gradually fell and the engine died. Luckily, had just enough momentum to make it to the dock. Looked in the carb and there was boiling gas in both barrels. At this point we loaded up and went home. Upon inspecting the engine/carb further at home, I noticed on the side of the carb, there was a clear plastic line that was cut and melted shut. I'm thinking this is a fuel return line or where the fuel pressure regulator connects to. Any idea's? Any help is greatly appreciated.

ON a side note, the engine flooded the first 2 times I tried to start it. I chalked it up to user error, however, maybe not?

 

[royclaypool]

Here is a photo.

 

[Scott Danforth]

first, welcome aboard

second, with any new to you boat, you need to do all the maintenance items

on your 1984 mercruiser that would be the following at a minimum:

• raw water pump change
• thermostat change
• gear lube change
• bellows change
• gimble bearing check
• u-joint check
• alignment check (adjust as required)
• engine oil and filter change
• fuel filter change
• cap, plugs, rotor, points, wires, plug change

back to your problem on the burning smell - most likely the issue is your raw water pump is shot and you over-heated the motor. Hence the reason that little item is the first on the list above.

did you ever start the boat motor with the boat out of water without muffs? if so, you murdered the raw water impeller (only takes seconds to melt it without water)



on to your flooding issue, it could be a few things:

• ruptured fuel pump diaphragm
• bad needle and seat
• improper float level

 

[Alumarine]

The clear plastic line that is cut/melted should go to your fuel pump.
If the fuel pump diaphragm breaks then instead of fuel dumping into the bilge it will go into the carb.
If you see fuel in that line then you know to repair your fuel pump asap.

Someone either cut the line for some reason or replaced the original fuel pump with a car pump that does not have the nipple for that line.
If that's the case you need to replace the pump.

In any case that did not cause your overheating problem.
Scott has you covered on that.

 

[royclaypool]

Thank you for the info guys. Additional info I found so far. 1. It appears the gentleman I bought the boat from tried replacing the carb. Although, he did so with a carb that has the vent/fuel return connector. So, neither the rocker cover nor the fuel pump has a connection for a fuel return or a vent, whatever it is called. Upon further research, there is an older model engine that used the fuel return/vent and a newer model that doesn't. I'm guessing he bought the carb and it was for the older model. The carb was completely rebuilt within the last few weeks.

Is there a way to make this carb work on this version engine?
How do I confirm what version engine I have?

 

[Bt Doctur]

Start with the fuel pump . does it have a port for a hose.if not it is a auto fuel pump and must be changed From what it looks like the vent was added to the side of the carb .This is not an issue IF the carb was marine to start with . The purpose is to route fuel from a ruptured diaphram directly into the carb and not into the bilge

 

[royclaypool]

Also, are the coolant tubes supposed to kinda balloon up when it first starts?

 

[Grub54891]

Also, are the coolant tubes supposed to kinda balloon up when it first starts?

No, they should stay the same as when sitting. At least mine do. You may need new hoses.

 

[royclaypool]

So latest update. I changed the fuel pump to the correct one. New thermostat. New coil. Have a new impeller but haven't installed it yet, Evidently, the engine did not overheat. The heat gauge stayed at about 160. Ran great for 5 minutes. Lost power. Engine bogged and shut off. Carb was flooded and gas boiling/steaming out near one of the rod connectors on the carb. Carb was just rebuilt. Any ideaS?