TOPIC:

Patton wrote:

Medina wrote: ...seems to happen more with nearly an empty tank...and that would lend credence to the inline filter, more gas, more weight pressing down..hmmm, Im thinking its the filter..

Imo, points more suspicion to tank vent issue -- not a matter of gasoline volume inside the tank.

Gasoline flows from the tank at the same rate regardless of whether the tank is full or almost empty.

Good Fortune!

Snuck this tibit of info out of an Article on the net.
"A critical variable in this system is the head pressure of the liquid in the product fill tank.
The delivery pressure, or head pressure of the liquid product is greatest when the tank is full.
As the tank is emptied, the head pressure drops."
it does make a difference on fuel Level, as to the flow rate.

Would test run with fuel tank cap ajar (partially open and not snapped down) to determine whether fuel starvation is resulting from insufficient fuel tank venting.

A full tank is less sensitive to vent issues.

Fuel starvation resulting from an obstructed vent worsens as the tank fuel level declines.

Imo, fuel volume flowing through the in-line filter, whether the filter is partially clogged or not, isn't related to the volume of fuel inside the tank.

Good Fortune!

Just think about it for a second, fuel is heavy, each gallon adds to the weight. A full tank weighs more than a low tank. Gravity is applying a force X Weight. More weight more force.

When emptying the bike's full fuel tank through a fuel line from the petcock, it seems to me that the flow volume remains the same from start to finish.

Same with pouring gasoline through the spout from a full gasoline can until the can empties, provided the air vent is open.
Seems the flow volume remains the same from start to finish, regardless of the can's size or shape.

A fuel pump fuel system delivers the same pressure regardless of fuel volume inside the tank, and carbs depend on this steady pressure.

Gravity-fed carbs also rely on the same steady gravity-fed fuel delivery when supplying fuel mixture to the cylinders.

I might be mistaken, but am thinking the float valve doesn't care whether the fuel tank is full or almost empty, and that a float valve won't be less prone to leakage when the fuel tank is almost empty, or more prone to leakage when the fuel tank is full.

Good Fortune!

Recall the kid using his finger to plug a hole in the dike.

He applies the same finger pressure regardless of water level behind the dike.

Good Fortune!

Edit -- pushes harder where the water level is significantly higher, say 50 feet.

It will be more prone to leak when full. Do to the Gravity pulling on the weighted mass in the tank. A tank with 8oz in it would more than likely not leak from float needle and seat. A tank with 4 Gallons in it would be more likely to leak from said float valves. Its pretty basic physics.
In another set up, you could do nothing else but increase the height of the unit of liquid in question, as it goes up the Pressure will increase.

mste.illinois.edu/murphy/WaterTower/default.html

It just seems to me that the mere six inch difference in fuel surface height between full and near empty has virtually insignificant effect on pressure exiting the petcock.

Imo, with reference to fuel starvation, the venting is far more critical than the fuel level.

Where fuel starvation is less pronounced with a full tank, the reason is most likely due to poor venting, and not because more gasoline weighs more than less gasoline or because the level of fuel is slightly higher in a full tank.

Just my 2¢

Good Fortune!

I am by no means an expert on this subject, but I think you are talking about static fluid pressure. Which if I am not mistaken, and I certainly could be, is as follows;

The pressure exerted by a static fluid depends only upon the depth of the fluid, the density of the fluid, and the acceleration of gravity. Static pressure does not depend upon the shape, total mass, or surface area of the fluid.

and is given by the expression;
P(static fluid) = ρgh [where ρ = m/V = fluid density, g = acceleration of gravity, h = depth of fluid]

The most remarkable thing about this expression is what it does not include. The fluid pressure at a given depth does not depend upon the total mass or total volume of the liquid.

This is from another web article and is more Motorcycle specific:

"The tank to carb fuel flow is gravity fed. So, the only pressure in the fuel line, comes from the fuel volume in the tank and the height difference between the carb's float valve and the highest level of fuel in the tank. The difference in fluid height creates "Head pressure". This head pressure gets lower as the fuel level decreases in the tank.

The carbs are vented and the float valves are at the top of the float chamber. The fuel level in the carb's float bowl chamber is always lower than the float bowl valve tip and seat. When the float valve opens, air can go back into the fuel line from the carb's bowl chamber, as fuel trickles into the carb float bowl chamber.

The stock fuel line routing allows these small bubbles to rise to the fuel tap and bubble up into tank, keeping the fuel line relatively clear of large bubbles. Honda routed the lines so there was a constant "fall" of liquid along the entire length of the fuel line. This allows the fuel to accumulate at the bottom of that small tube vessel and air to rise and occupy the top of it, where the fuel tap is. Bubbles can be trapped by long lines that loop down and back up to the carbs. If the lines are long enough and the amount of air excessive, the force of the bubbles trying to rise back to the top of the fuel level surface in the tank rivals or exceeds the head pressure force attempting to let gravity feed fuel to the carbs.

The result is a stoppage or reduction of fuel flow to the carbs. When the carb bowl level drops below that of the pilot jet tube, the pilot systems starves for fuel and the mixture leans out, causing loss of power.

Complicating the matter, is the saddle hump in the fuel tank, and the stand pipe, which is used before "reserve" is selected. Fuel on the far side of the hump stays on that side, unless there is fore and aft motion of the fuel in the tank, which sloshes it over to the other side, where it can feed the fuel tap inlet. Also, the head pressure into the stand pipe becomes lower as the fuel level in the tank approaches the height of the stand pipe.

The short of it is that your long loop of fuel line may well be causing your loss of power."

This seems to be pretty accurate for the problems experienced with feeding the Carbs on these bikes. About 30" of height will create 1psi of pressure. So as Patton says, the decrease would not be huge in pressure, since it would be under 1psi. But as you can see, with a Gravity system, using the wrong filter, and/ or incorrect fuel line routing. the Carbs may be starved for fuel.

The web article says: "When the float valve opens, air can go back into the fuel line from the carb's bowl chamber, as fuel trickles into the carb float bowl chamber. . . The stock fuel line routing allows these small bubbles to rise to the fuel tap and bubble up into tank, keeping the fuel line relatively clear of large bubbles,When the float valve opens, air can go back into the fuel line from the carb's bowl chamber, as fuel trickles into the carb float bowl chamber."

Here's why I think that's incorrect:

The motorcycle carbs I'm familiar with all have float bowl vents whereby air escapes from the float bowl (and without forcing its way through the float valve against incoming fuel and ultimately into the fuel line).

With a properly functioning bowl vent, there are no "bubbles" created by air backing up into the fuel line. And no such bubbles in the fuel line blocking fuel flow into the carb.

So long as the fuel line isn't kinked, long loops of fuel line are perfectly all right, provided the loops remain below the tank.

To provide sufficient room to accommodate in-line filters, longer looping lines are often necessary and commonly fitted without causing any problems.



Ample fuel flow here:



Good Fortune!

I would recommend to Test the flow rate from your petcock, before and after a added in-line filter to check the restriction. you don't want to make your carbs lean out from a restricted fuel filter.

Motor Head wrote: I would recommend to Test the flow rate from your petcock, before and after a added in-line filter to check the restriction. you don't want to make your carbs lean out from a restricted fuel filter.

+1