Flat Fuel line Bulb

[GA Paramedic]

I have a Merc 73 inline 6 on a ski boat. After about 1-2 minutes of riding, the engine will bog down and eventually die. Upon inspection, the fuel bulb will be flat. After a few moments, the bulb will inflate and the cycle will repeat. Brand new fuel lines throughout. Clean fuel fliter. No kinks in the line and vent open and clear.

What could be going on here?

 

[Les Robb]

Re: Flat Fuel line Bulb

It is highly unusual but you could have a defective hose with the tube collapsing and the outer portion looks normal. More likely the bulb itself is defective if there is no other reason for a restriction of the flow to the motor. Also check the direction of flow through the fuel filter.

Let us know what you find

 

[hkeiner]

Re: Flat Fuel line Bulb

Temporarily connecting the fuel line from the motor to a portable fuel tank is a way to help narrow down the problem. If the problem does not go away when connected to the portable tank, you know the problem is somwehere between where you made the connection to the tank and the motor. If the problem does go away, your restriction is nearer the boat's fuel tank.

You can also connect a vacuum gauge to the fuel line to better observe the vacuum level in the fuel line. Your shop manual will have the specs on how much vacuum should be in the fuel line at idle and WOT. See the below linked Mercury Advisory for additional info.


http://forums.iboats.com/attachment.php?attachmentid=26607&d=1240244928

http://forums.iboats.com/attachment.php?attachmentid=26608&d=1240244937

http://forums.iboats.com/attachment.php?attachmentid=26609&d=1240244945

http://forums.iboats.com/attachment.php?attachmentid=26610&d=1240245079

 

[daveswaves]

Re: Flat Fuel line Bulb

I have a Merc 73 inline 6 on a ski boat. After about 1-2 minutes of riding, the engine will bog down and eventually die. Upon inspection, the fuel bulb will be flat. After a few moments, the bulb will inflate and the cycle will repeat. Brand new fuel lines throughout. Clean fuel fliter. No kinks in the line and vent open and clear.

What could be going on here?

Likely there is a blockage or crud floating around in your tank which gets sucked up into the tank feed and then released after your engine stops. Try a separate tank to eliminate the in boat tank, I assume this did not happen when Wayne was looking at it or he would not have let you leave.

 

[GA Paramedic]

Re: Flat Fuel line Bulb

Likely there is a blockage or crud floating around in your tank which gets sucked up into the tank feed and then released after your engine stops. Try a separate tank to eliminate the in boat tank, I assume this did not happen when Wayne was looking at it or he would not have let you leave.

We did have some slight hesitation during our test run. We attributed it to the top to carbs needed to be cleaned, which could be done at a later date. (We were trying to avoid a thunderstorm coming our direction.) It did seem to run better when the bulb was being pumped at cruising speeds. I was trying this technique during my lake trip sunday when I noticed the bulb was flat. "Ya can't pump a flat bulb!" I am sure someone has said that at some point in time.

Regardless, I will be using a spare tank either today or tomorrow, depending on weather, and i'll see what happens then.

 

[Silvertip]

Re: Flat Fuel line Bulb

There is a very good chance the bulb is NOT the issue. Any time a bulb gets sucked flat the problem lies between the bulb and bottom of the fuel pickup tube in the tank. If you have a built in tank, you will need to pull the pickup tube and check the screen if it has one. If the tank has an anti-siphon valve (the fitting on the tank to which the hose connects) it may be defective. Remember, fuel is "sucked" through the line so a flat bulb means the obstruction is between the bulb and tank -- not after it.

If your fuel tank is a plastic portable, drain it and look carefully inside. I've seen many of these with bits of red plastic (lots of it) in side. Enough of this stuff has been seen that it clogs the pickup tube and in some cases (one of my own boats) it has punctured the screen and made their way into the bulb. When I first discovered this issue, I began checking brand new tanks at marine stores and department stores that sell them. To my surprise, many of them had this debris in them. It is the shavings from the drilling operations during manufacture of the tank.

 

[GA Paramedic]

Re: Flat Fuel line Bulb

There is a very good chance the bulb is NOT the issue. Any time a bulb gets sucked flat the problem lies between the bulb and bottom of the fuel pickup tube in the tank. If you have a built in tank, you will need to pull the pickup tube and check the screen if it has one. If the tank has an anti-siphon valve (the fitting on the tank to which the hose connects) it may be defective. Remember, fuel is "sucked" through the line so a flat bulb means the obstruction is between the bulb and tank -- not after it.

If your fuel tank is a plastic portable, drain it and look carefully inside. I've seen many of these with bits of red plastic (lots of it) in side. Enough of this stuff has been seen that it clogs the pickup tube and in some cases (one of my own boats) it has punctured the screen and made their way into the bulb. When I first discovered this issue, I began checking brand new tanks at marine stores and department stores that sell them. To my surprise, many of them had this debris in them. It is the shavings from the drilling operations during manufacture of the tank.

I hooked the motor up to another tank. No flat bulb. So.... I guess we have found that the bulb is NOT the problem. However, I do have more problems. The hole shot was poor. About 20-25 mph was all I was getting. I'll be cleaning some carbs and replacing a water jacket gasket around the spark plugs. After I get the trash out of the carbs and the water to quit leaking from around the spark plugs, I'll try it again! Till then, I am not going to even think about what else could be wrong.

As far as the existing tank in the boat, it is a 16 gal aluminum tank that is mounted in the boat with very poor access. It looks like 4 bolts on each side (starboard and port) and it MIGHT be able to be wiggled out for inspection/cleaning. The suction port is accessable to attach/detach the fuel line but there is a metal tube that is vertical in the tank. There isn't enough clearance above the tank to pull it out. And... I have about 14-15 gals of fuel in the tank with whatever else is clogging up the lines. Although this needs to be taken care of, I can atleast use my 6 gal plastic tank for lake trials and driveway runs.

I should be able to get the gasket on tomorrow and MAYBE a carb or two cleaned! We'll see!