Yet another collapsing primer bulb

The boat is a 1994 or 1995 Lund Fisherman (or at least that is what I am told; there are no model names/numbers on the hull) with a 1996 Yamaha 90hp 2-stroke. The problem is a collapsing primer bulb and engine konking out due to lack of fuel. Before you point me to other threads (which I have already studied), here is what has already been done: new primers bulbs, removal of anti-siphon valve, new fuel pick-up tube, and replacement of fuel hoses. The problem occurs only when the fuel tank is half full or less. I.e., everything is fine with more than a half tank of gas.

My guess was that there was a problem with venting for the gas tank. But I figure that would be the case regardless of the amount of gas in the tank. A quick visual check of the hoses that appear to be for venting didn't reveal anything: the hoses look to be firm and not pinching anywhere. I say "hoses" since the vent hose runs from the filler tube to the tank where it tees to become two hoses, each attached to the tank.

It does not seem to be a fuel volume problem. It takes about 5 minutes on the main motor under full throttle to collapse the bulb. The bulb will also collapse after about 90 minutes of trolling on the 9.9 hp kicker. And, as noted above, the problem does not occur when there is more than a half tank of fuel.

My next step is to remove and inspect internally the vent hoses. Maybe one is partially blocked. I really doubt I will find anything there, but I don't know where else to look. Has anyone had this problem? Even if you haven't, I would really like to hear some ideas or comments. This is driving me nuts

Re: Yet another collapsing primer bulb

My initial thought is that you have a piece of junk floating down in the tank somewhere that is blocking your intake hose. The second thought is that you are on the right track checking the vent hoses. I have seen the tips come off if an intake hose and it lodge against the corner of a fuel tank and impede fuel flow. Usually, there is some sort of pickup on the tip of the intake hose to keep it from "sucking" to the side or bottom of the tank. Obviously, if the bulb is collapsed, there is something upstream blocking fuel flow. So, obviously you are on the right track, it is just a matter of figuring out what it is.

Re: Yet another collapsing primer bulb

On a full tank there is no vacuum in the tank.
The engine is capable of pulling the tank down to a certain amount of vacuum in the tank before the bulb collapse and chokes off the flow.
Once it does that, open your filler cap and your likely to hear the vacuum suck in air.
Replace cap and I bet you will be able to run engine for another time period before it repeats.
Or leave filler cap off and I bet it will run fine until tank is empty.

Your vent line description sounds like it needs a venting fill cap. Perhaps the venting feature is plugged.

Re: Yet another collapsing primer bulb

Thank you for the suggestions and I am glad to hear you guys think I am on the right track. I already tried opening the filler cap and it made no difference. I was surprised by that since I expected the vacuum in the tank would have instantly sucked in air. So maybe there is no vacuum in the tank and the pickup tube is the culprit. I should have disconnected the fuel line upstream of the bulb at the time. That would have given me another clue to what's going on. My bet right now is that it's something floating in the tank and it gets close enough to the end of the pickup when the fuel level drops. I am beginning to doubt it's a venting issue since opening the filler cap did not help. I would expect that air could get into the tank via the filler hose when I opened the cap. I'll keep you guys posted on developments. I won't have time until next week to work on it, so don't hold your breath...

Re: Yet another collapsing primer bulb

I got into the fuel tank today. Removed the pickup tube assembly and found it intact; no screen and no blockage. I then siphoned all the gas out of the tank in the hope that I would find something floating around inside. Well, no shop rags or grocery bags anywhere. What I did find, though, was bits of some sort of semi-gelatinous material. There were quite a few clumps of it, all on the bottom of the tank. The stuff almost disintegrates when you pick it up. It also dries to a white, filmy-material when exposed to air for a few minutes. I have no idea what it is, but, from its consistancy, I cannot see it plugging anything but a very small orifice. I re-filled the tank and siphoned all the gas out again. I did not find any more of this material in the tank.

I then removed the water separator to find there was nothing weird in there either. I even butchered the canister to get at the element. I did not find any of the snot-like material anywhere.

So, I am not really getting anywhere. I am now starting to wonder if maybe all the crap in the tank has now been sucked out and it is only the remnants I am seeing. I will flush the tank a couple more times to convince myself there is nothing else inside. And I will remove and examine the vent hoses. Unless I find a problem with the hoses, I will by-pass the water separator and go straight from the pickup tube to the primer bulb (with an inline filter I can see). Then I'll fill the tank to half and see what happens.

If anyone has any other ideas, or even if you just want to commiserate with me, please speak up.