Air Pockets in Fuel Line - Normal?

[airshot]

Re: Air Pockets in Fuel Line - Normal?

Air pockets, not bubbles. More like an inch or two in length. They move around, but don't move thru...

If the air pockets are not flowing with the fuel and causing a fuel disruption to the engine then you are OK. Some air is trapped in the line and staying at that spot while the fuel flows past and that can be normal. While the pockets are a little bigger than normal as long as they do not follow the fuel you are fine. I would still check everything to be sure you do not have a fuel leak anywhere. Many time bubbles trap in a line due to kinks or bends or high and low spots in the line and are there just trapped in place and the fuel flows around them. I was under the impression the bubbles were flowing with the fuel causing engine problems. A boat is no place for a fuel leak!!

 

[NYBo]

Re: Air Pockets in Fuel Line - Normal?

If the air pockets are not flowing with the fuel and causing a fuel disruption to the engine then you are OK.

He is having problems with the motor dying.

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

That would be unusual. It is usually screwed into a bung that's welded to the tank. Tempo Withdrawal Assembly - iboats Can you post a picture of the top of your tank?

Here is a shot of the tank as I found it. The fitting has since been cleaned and replaced, the shut off valve eliminated for now.

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

If the air pockets are not flowing with the fuel and causing a fuel disruption to the engine then you are OK. Some air is trapped in the line and staying at that spot while the fuel flows past and that can be normal. While the pockets are a little bigger than normal as long as they do not follow the fuel you are fine.

That's the question, am I onto the cause of the problem (no acceleration, stalling) or am I chasing a non-issue?

 

[dingbat]

Re: Air Pockets in Fuel Line - Normal?

You can not have air bubbles in a vacuum system w/o an air leak or an empty tamp. Physically impossible, no matter how coiled, looped, bent or bowed the fuel line might be.

Double check all connections. Double clamp to make to make sure. Water separators are notorious for leaking.

 

[NYBo]

Re: Air Pockets in Fuel Line - Normal?

Here is a shot of the tank as I found it. The fitting has since been cleaned and replaced, the shut off valve eliminated for now.

View attachment 207627

Well, that's ugly.

That fitting should unscrew from the tank, but don't try that until you've eliminated all other possible sources of the air leak.

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

You can not have air bubbles in a vacuum system w/o an air leak or an empty tamp. Physically impossible, no matter how coiled, looped, bent or bowed the fuel line might be.

Double check all connections. Double clamp to make to make sure. Water separators are notorious for leaking.

I have eliminated everything from the tank to the fuel pump. So it must be the tank. Those air bubbles should work their way thru the system and the line should be solid fuel, correct?

Anything connected to the top of the tank (sender, fill, vent) should have no effect, correct? I mean, they can't cause the air leak, right?

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

Well, that's ugly.

Yeah... 28 years of crud!

A little better now:

 

[dingbat]

Re: Air Pockets in Fuel Line - Normal?

I have eliminated everything from the tank to the fuel pump. So it must be the tank. Those air bubbles should work their way thru the system and the line should be solid fuel, correct?

Anything connected to the top of the tank (sender, fill, vent) should have no effect, correct? I mean, they can't cause the air leak, right?

Only two ways you can draw air from the tank. You have a hole in the pickup above fuel or your out of fuel.

Try this....remove the hose from the tank and put it in a 5 gallon gas can of fuel. Start the motor and see if your air goes away. If you still have air, your problem is in the fuel lines.

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

I'll try that. Should be definitive, one way or the other. Tank is 1/2 full. Thanks.
Btw, any air bubbles, on startup, should work themselves thru the system, correct?

 

[Fed]

Re: Air Pockets in Fuel Line - Normal?

This is one of life's little mysteries but if you look at a typical little inline filter placed just before the fuel pump I don't think I've ever seen one full of fuel while running.
It makes no sense at all, the air should be pushed out with the primer bulb or sucked out with the fuel pump but it just seems to be happy at half full.
I may just buy a filter to smash apart & see what's going on inside.

 

[H20Rat]

Re: Air Pockets in Fuel Line - Normal?

one thing to keep in mind.. Leaking air into a fuel line is roughly the same as leaning out your engine by running a smaller carb jet. Its entirely possible to burn down an engine with an air leak in a fuel line, it happens all the time!

 

[UncleWillie]

Re: Air Pockets in Fuel Line - Normal?

That's the question, am I onto the cause of the problem (no acceleration, stalling) or am I chasing a non-issue?

I think you are chasing a Non-Issue.

The Bubbles/Pockets are stationary at the top the humps in the fuel lines.
The fuel wants to run downhill in both directions so the tops of the hills are low pressure points.

Gasoline is composed of dozens or hydrocarbon compounds. It is not just "Gasoline".
A significant component is butane, Like in your BIC Lighter. It is what makes starting the engine in the dead of winter possible.
The low pressure spots in the line cause the Butane to evaporate and form the standing vapor voids. (Simplified Explanation)

If the fuel pump is doing its job it will pull the fuel over the top of the hills.
If it were to move the vapor bubbles as far as the carburetor, the float valve would just stay open a few milliseconds longer as the bubbles were sucked down the carb throat.

As long as the Carb bowl can stay full, a few passing bubbles will have no effect. (<---<<< Important Concept!)
A fuel injected engine will re-compress the vapor back into liquid due to the high fuel pressure.

If your vapor voids are not moving, they are not an issue.
Air leaking into the line will continuously be seen being replaced by new bubbles coming from the leak.

A temporary clear line between the fuel pump and the carb would be more informative.
You may not be pumping or the fuel pump diaphragm may have an air leak.

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

That's interesting... The air pockets move around but don't advance. I don't see new air coming into the line. So, this is normal?

New fuel pump installed last week, which improved the pressure between the pump and carb. I was so sure this was a fuel problem...

 

[dingbat]

Re: Air Pockets in Fuel Line - Normal?

That's interesting... The air pockets move around but don't advance. I don't see new air coming into the line. So, this is normal?

New fuel pump installed last week, which improved the pressure between the pump and carb. I was so sure this was a fuel problem...

IA fuel pump is not a closed vacuum system. It vents to atmosphere between strokes. You will almost always have air trapped on top of the fuel column. Most air is removed from the system when the pump is active but as soon as the pumps stops, the fuel siphons back to the tank bringing with it air trapped in the fuel pump diaphragm. The anti-siphon valve keeps the back flow of air to a minimum.

The 1/2 full filter is not a mystery. Most are installed on their sides. A vacuum is formed when both ports are covered with fluid, trapping air above the flow. Mounted vertically, the air will move to the top of the filter and get "vacuumed" out.

 

[Jacalore]

Re: Air Pockets in Fuel Line - Normal?

Ok, thanks everyone - so air pockets in the fuel line is not an issue.
Onto compression check, then carb.