Question about MerCruiser 7.4L raw water and fuel pumps

[Bob St John]

Folks,

No gas is getting to the carb in my MerCruiser 7.4l engine. This engine has the configuration in which the fuel pump is connected to the raw water pump, and the water pump drives the fuel pump

The fuel pump has been disconnected from the water pump, and I can look into the "cavity" of the water pump where the fuel pump connects to the water pump.

First, there is no oil in the reservoir (as I think there should be), but I see nothing in the reservoir that causes the arm on the fuel pump to be pumped. I expected to see something like a lobe on shaft that rotates and contacts the arm on the fuel pump which causes the fuel pump to pump the fuel. I do see and feel what I think is a bearing on the shaft of the water pump, and that seems to rotate freely (even with the absence of oil in the chamber).

Would someone explain what drives the arm on the fuel pump?

Thanks,

Bob St John

 

[B BARKER]

My fuel pump drives off a round rod that goes up in the block to a cam lobe and drives the fuel pump arm...Not sure about your model but you can go to www.mercruiser.com and look yours up and it will show you photos...

 

[markrsimon]

The shaft within the cavity should have an oblong roller on it - that when the pulley is rotated the cam pushes against the arm of the fuel pump - pushing fuel up to the carb. If you put the fuel pump back on and try to rotate the pulley you should definitely feel it working against the lever of the fuel pump.

There should also be oil (I use gear oil) in there that should be changed once a year - as small amounts of fuel get in there and dilute it. If you have two large set screws, lower one drains, then fill from the top one till it starts overflowing.

As for your specific problem - not sure how that shaft/cam is assembled - but that appears to be your problem as you have found out. I have not had to tear into that section of mine yet.

 

[Nater Potater]

The cam doesn't rub directly against the pump arm, as it has a ring that the cam rides in. If you reach into the cavity where the fuel pump arm sits, you can spin the ring independently of the shaft, giving the impression that the cam's come loose of the shaft. Take a look at https://www.boats.net/catalog/mercr...n-v-v-8-0f602010-thru-0f801999/sea-water-pump items #15 and 16. Assuming you have the drive belt off, turn the pump by hand to see if that ring moves in and out before condemning the pump shaft.
However, if the pump drive was so far trashed as to be the cause for no fuel supply, it would have had some serious fuel starvation issues leading up to a complete failure. I can only imagine work would have been performed by the former owner prior to it getting that bad. No, my money would be on the fuel pump itself, as I have found crud in the internal check valves that causes them to go from a fully functional pump to a dead item in one revolution. You can disprove the pump drive by merely working the pump arm by hand to see if it delivers fuel to the carb. Don't overlook the possibility of a clogged pickup tube in the fuel tank.

 

[Nater Potater]

Oh, and welcome to Club SeaRay!

 

[Bob St John]

Thanks Nater ... several replies were good, but your's is complete, comprehensive, and right on target!

Bob St John

 

[Mark.60]

On my previous boat I had the same problem. I replaced the complete assembly with an aftermarket one. Much better than oem and lots cheaper.
Moore Performance located on west coast
623-357-3573 give them a call