Fuel line and carb fouling mysteries solved!

[wattsr]

This spring I ran into a problem where the engine would start fine, run smoothly, but shut down a few minutes later. When I removed the drain plug from the carburetor bowl, there was no fuel in the bowl. Using a small screwdriver I made sure that the float was not stuck. Then I tried to pump gas into the carb using the hand pump on my mechanical fuel pump, but to no avail. I disconnected the line between the fuel pump and the carb, and tried to pump gas into a container. I got a few squirts, but then nothing, followed by a few squirts, etc. But mostly nothing. To check for vacuum leaks around the gas filter, I disconnected the gas filter and ran the fuel line directly from the gas tank to the fuel pump. Same result, although I did notice that the disconnected gas line was happy to siphon fuel down the line from the higher-mounted fuel tank (of course, I did not let it siphon very much). So the problem did not appear to be in the fuel line, gas filter assembly, or carb, leaving only the fuel pump as the suspect. I pulled the fuel pump off and noted significant wear in the mechanical components. I was unable to check the membrane due to a frozen screw. But with the wear in the mechanical components and the other evidence I had collected I ordered a Moyer electric fuel pump kit and installed it on the engine. Before attempting to start the engine I hotwired the oil pressure cutoff switch to test the installation, and noted that the pump ran as expected. I disconnected the fuel line from the carb, ran it into a bowl and test that the pump actually pumped fuel. But it did not, even after leaving to running for a few minutes! Not a drop. Now, almost completely stumped, I disconnected the fuel line at the tank and blew air through it back into the tank. No blockage there. I inspected the fuel line to see if it had deteriorated and was blocked internally, but could not detect any damage. Almost completely out of ideas, I it finally occurred to me that the fuel vent could be blocked. I removed the gas cap on the fuel tank, hotwired the pump again, and gas poured out into my bowl! So I disconnected the fuel vent line at the hull vent end, poked a screwdriver around in the hull fixture, blew through the vent host with the gas cap removed to check for any blockage there, reconnected everything, started the engine which then ran for 2 hours with no problems. I never saw what actually blocked the vent – I suspect that I dislodged it with my screwdriver and it fell out of the vent and into the water. I suspect that an insect such as the wasps that we have around here (South Carolina) that build nests out of mud may have been the villains.
One additional thing I discovered during this ordeal: the beige (diesel) Racor spin-on gas filters which look identical to the white (gasoline), but are much more readily available, rust over the winter and will fill your carb with rust particles! I have now switched to the white filters (complete with the white head), and I’ll see next spring whether they rust as well.

[dvd]

Wattsr

Good job in diagnosis and repair. You would think that a clogged Fuel vent would at least allow some fuel, for a period time, to flow until enough pressure developed in the tank. Keep an eye on it.

dvd

[CalebD]

Mud daubers - little wasps that like to build nests on overboard fittings.
Had that happen on a friends boat on the Chessy but the nest blocked the holding tank vent and made pumping out the holding tank nearly impossible.
Same solution: pointy metal object dislodged the dam and then a flow of stinky air from the tank let me know it was all clear.

[JOHN COOKSON]

The exterior metal fitting on my old fuel tank vent had a non-marine grade screen that coroded and blocked it. Had me guessing for a while. I use a mirror now to inspect the new one every so often.

TRUE GRIT

[toddster]

I had that same vent clogged, leading to a disturbing fuel burp/spill in the cockpit, the first time I took the boat to the fuel dock. Fortunately I had a rag stuffed in the cockpit drain.

There was also a radial black stain on the transom for about six inches under the vent that I suppose was a fungus growing on the gas fumes. But I got busy and washed it off before I remembered I was going to scrape off a sample to look at under the microscope.

[romantic comedy]

Good job.

I had an intermittent electric fuel pump. It drove my crazy. I found that i could get the pump to work by tapping it with a screw driver. So now if i use it, i just need to station a crew to keep banging the pump.