Fuel delivery issue on a 268

[CharlieN]

Fuel delivery issue on a 268

We did a week of boating in the Sea of Cortez last month and we had a fuel pressure problem on our 268
I have a 502 motor so there is no engine driven fuel pump. We use a Carter electric pump. We draw from the tank, then the filter, fuel flow sensor, then the pump, then the carburetor. We have used this setup for 15 years in all kinds of temperatures and conditions. It has always worked well.
The 1st and 2nd day out, it worked fine. Third day, I noticed the fuel flow down to 12 gal/hr when it has always been at 16 at cruising speed. Sure enough, it started slowing down. We have a fuel pressure gauge on the engine and the gauge showed 0 pressure. We continued on at a slower speed for the rest of that day. That night, I changed the fuel filter and a new electric fuel pump. An hour into the run the next day, it happened again. We tried different things over the next 4 days but it was always happened again.
The question is about the tank. Has anybody had the fuel tank pickup tube out and what is at the end of it.
Any other theories and suggestions would also be appreciated.
Charlie

 

[keokie]

What kind of fuel flow meter do you have? If it is a flowscan, I would check the wheel.

 

[CharlieN]

Its a Navman. We took it out of the circuit for awhile but did not seem to make a difference.
I replaced all the hose yesterday and cut open the old stuff looking for a blockage. Nothing. That leaves the tank.

 

[scoflaw]

Anti flow valve stuck ? Tank vent ? Blow some compressed air thru the pick up

 

[dvx216]

Do you still have the anti siphon valve at the tank plus have you inspected to pickup tube in the tank.

 

[CliffA]

possible clogged gas tank vent since it takes a while once running to notice the flow start to reduce.....try opening the gas cap next time it happens to see if things get better....if so you have a clogged tank vent......after running a while the gas used cannot be replaced by fresh air in the tank and a vacuum is created....this vacuum can/will restrict the amount of gas flowing out of the tank.....in very severe cases gas tanks have been known to collapse from the suction from the fuel pump when no fresh air can enter the tank...

cliff

 

[CharlieN]

Thanks guys for the replies. I thought of the vent and tried leaving the gas tank cap loose. Still does it. There is a couple things in the vent line that could cause issues. One to keep the fuel from coming out the vent and the other is a whistler to tell when its full. Time to remove them and make sure they are not the problem.
I have heard mention of an anti siphon valve and the picture I have of one looks like the fitting I have on the tank but sticking a wire into the end of the fitting and it goes all the way to the elbow. I have not tried removing it from the elbow yet but the elbow is aluminum and the tank is aluminum and they are not unscrewing. The fitting is aluminum so I am thinking it will have the same problem. Any ideas on how to get 2 aluminum fittings undone?
Thanks,
Charlie

 

[CliffA]

Thanks guys for the replies. I thought of the vent and tried leaving the gas tank cap loose. Still does it. There is a couple things in the vent line that could cause issues. One to keep the fuel from coming out the vent and the other is a whistler to tell when its full. Time to remove them and make sure they are not the problem.
I have heard mention of an anti siphon valve and the picture I have of one looks like the fitting I have on the tank but sticking a wire into the end of the fitting and it goes all the way to the elbow. I have not tried removing it from the elbow yet but the elbow is aluminum and the tank is aluminum and they are not unscrewing. The fitting is aluminum so I am thinking it will have the same problem. Any ideas on how to get 2 aluminum fittings undone?
Thanks,
Charlie

the anti-siphon valve is a simple one way valve with a spring loaded ball on the inside....its purpose is to prevent fuel from flowing out of the tank and into the bilge in the event the fuel line gets ruptured or comes loose from its connection.....

over time build up inside the valve can cause the ball not to retract enough from the suction of the fuel pump thus restricting the flow of fuel......at low engine speeds the fuel flow may be sufficient to run the engine but at higher engine speeds the restricted fuel flow may not be enough to properly fuel the engine....if you cannot remove the valve from the tank you still may be able to clean it....maybe use some brake clean with the squitter straw.....however gasoline is a pretty strong solvent so i'm not sure brake clean would remove any more build up inside the valve than gas itself would.....

if the fuel flow is normal when the engine is running at low rpms but becomes restricted at higher engine rpms a partially clogged anti-siphon valve may be the cause....

you mentioned that when you stuck a wire into the anti-siphon valve the end of the wire went all the way to the fitting elbow.....this tells me one of two things....

1. the check valve ball is not able to fully close off the flow of fuel from the tank due to build up inside the valve thus allowing the wire to slide past the ball and reach the fitting elbow.....if the check valve ball was operating properly the ball would seal off the end of the valve and the wire would not have been able to get past the ball and not be able to reach the fitting elbow....

......or......

2. someone has removed the spring and ball from the anti-siphon valve.....if this is the case this is potentially very dangerous....I would replace the valve ASAP......

cliff

 

[CharlieN]

I will take a look at the fitting tomorrow and see if I can remove it and inspect it.
Charlie

 

[CharlieN]

To all:
I removed the hose barb that is screwed into the elbow. Its just a hose barb and nothing else. I looked into the tank and it looks clean. Both pickups go all the way to the bottom of the tank and appear to be clean. (Interesting, I had always heard that the pickup to the generator did not go to the bottom of the tank but now appears it does). I included some pictures, the last one is the pickup at the bottom of the tank, a bit unfocused. I will take a look at the vent hose next.

 

[scoflaw]

To all:
I removed the hose barb that is screwed into the elbow. Its just a hose barb and nothing else. I looked into the tank and it looks clean. Both pickups go all the way to the bottom of the tank and appear to be clean. (Interesting, I had always heard that the pickup to the generator did not go to the bottom of the tank but now appears it does). I included some pictures, the last one is the pickup at the bottom of the tank, a bit unfocused. I will take a look at the vent hose next.

Looks some tight. Did you blow the tube out ?

 

[CliffA]

I would replace the hose barb with an anti-siphon valve......

to determine if you have a problem with the fuel tank you can run the engine from another source of fuel such as a portable gas can...if the problem goes away there is an issue with the tank and/or fuel lines.....

other things to check:

1. fuel line is not restricted or crimped
2. fuel line is not collapsing at higher engine rpms. if the fuel line is original it may have deteriorated over time due to ethanol fuel (if used) and weakened the walls of the line. back in 1989 there was no ethanol in the fuel (i don't think) so fuel lines were made from different materials than today's lines. the increased suction on the fuel line from the engine at higher rpms may be enough to collapse the line and restrict fuel flow. the fuel line can collapse at higher rpms due to a clogged tank vent as well.
3. if applicable check the small screen type secondary fuel filter that is in the fitting where the fuel line from the fuel pump enters the carb

cliff

 

[CharlieN]

Cliff:
All great ideas. 1&2: I replaced all the fuel line about 5 or 6 years ago. I have since replaced the ones from the tank to the pump and cut the old ones open. They look clean. I need to take a look at the tank vent and check the screens going into the carb too. Hopefully next week when I have some more time.
Charlie

 

[CharlieN]

Just to recap the finding and solutions on the fuel supply issue. I removed the vent hose and checked for any restrictions in the hose. Nothing. I removed the hard line going into the carburetor to check the filter at the carb. There aren’t any. When I replaced the hose going from the pump to the hard line on the carburetor, I noticed the 2nd hose clamp was not completely on the barbed fitting. The hose was pinched down to about a ¼ or 5/16 hose. Nothing really critical but still a restriction. I ran a wire down the pickup tube till it reached the bottom of the tank, so I know it is clear
My current thinking on the problem is the way the pickup tube goes all the way to the bottom of the tank with only a small cutout on one side for the fuel to enter. I suspect there was a small piece of debris inside the tank interfering with the necessary flow only sometimes as it drifted around caused my problem. I am also going to upgrade the pump for more volume-have not decided which one yet. These pumps do a great job of supplying pressure but not so great on sucking like a diaphragm pump would. I suspect a engine mounted pump would just suck up the debris and leave it in the filter. I have attached a picture of a piece but not really certain this is THE piece. Seems to be about 3/8 inch across.
Thanks for all the input guys. Any other suggestions are always appreciated.

 

[CharlieN]

see attached picture

 

[scoflaw]

You should trim that pickup tube. Too tight

 

[CliffA]

before you go through all that trouble you might want to run the engine from a remote source of fuel such as a gas can....this will tell you for sure if you have an issue with the boat's fuel tank....

cliff

 

[CharlieN]

I would like to get the fuel tank pickup tube out but it seems the fitting is aluminum and the tank is aluminum and they are bound together. I got 1 turn and was very afraid the fitting would snap off. It could be it is stainless and can stand a little more abuse but I can't tell which.
Cliff:
A separate 6 gallon tank could be plumbed in ahead of time with a valve would be the best choice as I would hate to have to stop in the middle of a run and plumb it in. I may try that. Have to collect the parts first. Thanks for the suggestions.

 

[CliffA]

I would like to get the fuel tank pickup tube out but it seems the fitting is aluminum and the tank is aluminum and they are bound together. I got 1 turn and was very afraid the fitting would snap off. It could be it is stainless and can stand a little more abuse but I can't tell which.

worst case scenario you could cap off the original fuel pick up tube and leave it in place and install a new tube with the length you feel is correct....

cliff