Air leaking into fuel system?

[stevenmc87]

Hey guys just got this amazing 78 wing and road it home 120 miles and it performed flawlessly. That is until the last 2 miles or so it started dying at stop lights and at very low speeds. I attached a video of air coming out of the fuel pump. I didn’t notice this before the issue started but I also didn’t look at it specifically. Forgive me if this is a dumb question or it’s been discussed before i tried searching the forums but “air” is not a searchable word lol. The carbs have been professionally rebuilt and synthesized. The fuel pump has been “overhauled ” says the guy I got it from. And all the lines seem to be serviceable and connected tightly. I don’t see or smell signs of gas leaks so I’m a touch stumped at the moment.

[ritalz]

First thing I would check is the fuel cap. It should be vented and the vent may be stopped up causing fuel starvation. Easy test is to open the cap when the problem starts to surface. If you hear air rushing in and the bike starts running better you have found the problem. Others may chime in with more ideas.

[stevenmc87]

First thing I would check is the fuel cap. It should be vented and the vent may be stopped up causing fuel starvation. Easy test is to open the cap when the problem starts to surface. If you hear air rushing in and the bike starts running better you have found the problem. Others may chime in with more ideas.

Good thought man! I tried it still no better unfortunately.

[stevenmc87]

If it narrows it down I can see air bubbles coming from the output side of the fuel pump even when the bike is off after a ride. Not sure if that’s significant or not.

[dontwantapickle]

interesting.

[dontwantapickle]

I can see air bubbles coming from the output side of the fuel pump

How can you see the bubbles?
Do you have clear fuel lines?
Is it out of gas?
Did you try puting the bike on reserve?
maybe there isn't enough gas in the tank for the fuel pump to pick up.

[stevenmc87]

I can see air bubbles coming from the output side of the fuel pump

How can you see the bubbles?
Do you have clear fuel lines?
Is it out of gas?

Did you try puting the bike on reserve?
maybe there isn't enough gas in the tank for the fuel pump to pick up.

I posted a video of the clear line coming out of the fuel pump not sure if that video actually posted with my original post or not (I’m new to using this forum) but yes there’s a clear line coming from the fuel pump to the carbs that I can see about 50/50 air bubbles to fuel. It’s not all the time the issue is intermittent. And yes it has a full tank and yes I played with the petcock to see if that made a difference but it didn’t. But all good suggestions that i wouldn’t have put past me to overlook

[gltriker]

Fuel filter? Install a new filter.
'Overloaded' means what?

[stevenmc87]

Fuel filter? Install a new filter.
'Overloaded' means what?

Typo it was supposed to say overhauled. And I’ll let you know about the filter I have one on order.

[ritalz]

I just reviewed your video and the bubbles don't seem right. The diaphragm in the pump may have failed. One other thing could be the flow from the tank. You could try hanging a small fuel source to feed the pump.

[gltriker]

Yes, I was a non-conformist, but, approximately 2 years ago I simply had to investigate the decades old? puzzler regarding WHAT is an adequate fuel level in a new fuel filter and the dynamic flow of gasoline into and out of the fuel pump. In this photo, the fuel delivery system was "dry"

20230523_180434 - new hoses and filter 2023 no gasoline in system.png

After this first photo was taken, Fresh Gasoline was poured into the clean fuel tank. Then the Engine was cranked for approximately 30 seconds and was moving gasoline into the empty carburetor fuel bowls.

This next (only one) video presents the second cranking session, after the Starter Motor had rested for a few minutes. You will see how a healthy fuel delivery to carburetors system operates. (behaves?)
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20230927_165938.mp4

[stevenmc87]

I just reviewed your video and the bubbles don't seem right. The diaphragm in the pump may have failed. One other thing could be the flow from the tank. You could try hanging a small fuel source to feed the pump.

Oh that’s a good idea! I’m going to try that and see what happens.

[Shadowjack]

If the tank is full you can bypass the pump to see if it has a problem. Gravity will feed it until the level gets down to the petcock.

[gltriker]

Hey guys just got this amazing 78 wing and road it home 120 miles and it performed flawlessly.

The carbs have been professionally rebuilt and synthesized >>>synchronized

The fuel pump has been “overhauled ” says the guy I got it from.

A "new to you" bike.
Former owner "overhauled" the fuel pump?
What's the odds the fuel tank is contaminated with rust inside?

Remove the fuel filter and examine it.

[gltriker]

The fuel pump has been “overhauled ” says the guy I got it from.

There is a procedure to dynamically test the GL 1000 fuel pump's operating pressure and fuel delivery volume.

It is posted in the Honda GL1000 Shop Manual.

.Yes, I have performed the test on my 1975 GL1000.. (I had owned the combination Sears Roebuck pressure/vacuum gauge since 1970)
You will require a very low pressure fuel pressure testing gauge. The factory fuel pump pressure specification is less than 3psi.
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read, understand, perform

Screenshot_20250812_205006_Samsung Notes.jpg

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AND BETTER LEGIBLE FUEL PUMP TEST SPECS ARE MADE AVAILABLE TO VIEW HERE:
viewtopic.php?p=453019#p453019