Other How Does a Return Line Make a Ski Run Poorly

[lanmine657]

Think of it this way, you can either supply your carb with fuel(pressurized) or make it work to suck fuel to it(un-pressurized)that will end up with poor performance, just about every engine needs fuel pushed to it no matter what, pressurized tank, fuel pump or gravity feed

 

[blaster800]

Heres how Group K explains it .............Pressure test fuel system - All pwc utilize a sealed fuel system that has a check valve on the gas tank vent. This check valve (which permits pressure in the gas tank but not out) causes pressure in the gas tank that helps deliver fuel to the carb(s). Any air leak in the fuel system that permits the leaking off of this pressure, will also affect fuel delivery to the carb(s) at low speeds. To test for leaks, follow this procedure. 1) remove the return line from the fitting on the carb. 2) Blow into the return line while sealing off the return fitting on the carb with your finger. This will pressurize the entire fuel system. In a quiet room you'll be able to hear any remaining leak in the fuel system. When you remove your finger from the fitting on the carb, fuel will eventually drip out indicating that the float chamber is primed full of fuel.

Return line restrictors - The round pump 44 Mikuni carbs were manufactured with an unrestricted return fitting on the carb. This causes them to return so much fuel to the tank that the fuel circuits can get starved for fuel. If you are using a round pump 44 Mikuni, be sure you have a restrictor jet in the return line whose inside diameter is no more than .030" (.75mm) All of the new generation square pump carbs have adequate "built in" return line restrictors.

 

[TheBuzzard]

years ago when i rode the red SJ, my feet kept slipping out of the holds on rolls. it was pissing me off! i tried new turf, sand, soap cleaning and nothing worked. finally i noticed my return line had a pin hole that was squirting gas under the hood. it never affected how it ran, but the bilge fitting was on the dash and dumped that gassy bilge water in the tray, causing my problems.:banghead:

 

[Matt_E]

maybe you should go back the drawing board because pop off pressure is made by the fuel pressure against a closed needle and seat. Once the fuel pressure has piped the beadle open it makes its way into the regulator portion. Diaphragm acting as a bowl to keep fuel in and float so when fuel runs low bypasses the pop off.

Oh ok, so that is how popoff pressure works. Be sure and let Mikuni know, too, because they seem to have made a mistake in their manual.
Thanks for the legit advice. Always glad to learn something.

P.S. any idea why the diaphragm has that center pin that pushes on the lever for the needle?

 

[speedysprocket]

So how exactly does the return line pressurize the tank...? Your carbs pull fuel out of your tank, the engine uses some and what's left goes back to the tank. So you take out more than you put in, which would create a vacuum, not pressure. I guess the gas vaporizing in the tank would build some pressure.
But how do you regulate that pressure? It will affect how your popoff pressure works...
Anyways, my Octane came with two check valves- one to let air in, one to let air out. I got rid of one and just drilled a small hole through the other one. Never an pressure in my tank and ski runs great.
I did, however, forget to tighten my fuel pickup on the tank one ride and it ran terrible as all that fuel leaked into the hull. I still believe it's the vapors that caused the problem.

 

[DangerBoy]

Not going to argue on the internet but there are 2 reasons the tank gets pressurized by the return line

first the easy one
fuel is drawn from the tank, which is immediately REPLACED with air via the breather, some fuel is used and the rest is returned (pumped) back to the tank, BUT the volume of that fuel being returned has already been replaced by air, so it compresses the air in the tank causing pressure in the tank

lets see if someone can figure out the second reason (not counting heat expansion)
hint think of boiling water

DB

 

[OCD Solutions]

Fumes are the second reason but they were already mentioned earlier in this discussion so we're not adding anything new.

Answer:

Briefly, the saturated vapor volume of an average gallon of liquid gasoline when fully evaporated is 160.4 gallons of vapor at 60° F and sea level.

Vapor volume of a liquid is the number of cubic feet of vapor resulting from the complete evaporation of the liquid. The vapor volume depends on parameters of density, temperature, pressure and molecular weight which is affected by the variety of formulas for gasoline that is comprised of a wide range of hydrocarbons.

Several approaches are available to solve this problem. One is shown below.

Using a common industrial formula:

one liquid gallon = [(8.31) x (SG) x (387 cu ft)] / (MW)

Where:

8.31 = pounds in gallon of water

SG = specific gravity of liquid being vaporized

387 = At standard conditions, one pound-molecular weight of a material will evaporate to fill 387 cubic feet of space.

MW= molecular weight of liquid being vaporized

Then using the approximate gasoline constants:

one liquid gallon of gasoline = [(8.31 pounds in a gallon of water) x (.70 approx. specific gravity of gasoline) x (387 cu ft)] / (105 molecular weight of average gasoline)

= 21.4 cubic feet of vapor volume

There is 7.481 U.S. gallons in one cubic foot.

So:

one liquid gallon of gasoline = (21.4 cubic feet) x (7.481)

= 160.4 gallons of saturated gasoline vapor


The vapor volume will vary based on the specific formulation of gasoline, pressure, and temperature.

 

[nbatch95]

Not going to argue on the internet but there are 2 reasons the tank gets pressurized by the return line

first the easy one
fuel is drawn from the tank, which is immediately REPLACED with air via the breather, some fuel is used and the rest is returned (pumped) back to the tank, BUT the volume of that fuel being returned has already been replaced by air, so it compresses the air in the tank causing pressure in the tank

lets see if someone can figure out the second reason (not counting heat expansion)
hint think of boiling water

DB

Now that makes sense. Thanks for a good explanation.

So how would not having pressure in the tank effect the way your ski runs?

 

[Vumad]

Pop off pressure has nothing to do with the regulator diaphragm.

Still don't have a good explanation of how non-pressurized fuel line will make a ski run like "crap"

maybe you should go back the drawing board because pop off pressure is made by the fuel pressure against a closed needle and seat. Once the fuel pressure has piped the beadle open it makes its way into the regulator portion. Diaphragm acting as a bowl to keep fuel in and float so when fuel runs low bypasses the pop off.

You assumed that because you use a pop-off gauge to put pressure against the needle, that must be how the carb works. You are wrong.

There is a nipple on the diaphram that pushes on the end of the rocker arm. The diaphragm pushes on the rocker arm, which opens the needle and seat. That is why you can't bend the rocker arm more than 0.020". If you bend too much one way, the diaphram can't open the needle, and you starve for fuel. Bend too far the other way, and the diaphragm always pushes on the arm, causing your needle to leak. Having tuning issues? Don't forget to check your rocker arm...

The diaphragm regulates the fuel flow through the seat. However, since we have no way to measure the reaction of the diaphram during regular running condition, we sample test the "pop-off" backwards (basically) by pushing the needle open (rather than using the diaphram to test the spring. Also, this is why there are 2 parts to checking the popoff. You 1st check the popoff PSI the way we all know, but then you reassemble and let it sit at 10psi for 10 min, if it drops, you possibly have your diaphragm pushing on your rocker arm (bent rocker, damaged diaphragm, dented diaphragm cover).

Your pop-off pressure (in the way we test it) is mostly irrelevant, however, since we can't measure the working pressure of the diaphragm, we use the only measure that is available to us, that which we get from the pop-off test.


Edit: god, I can't believe I wasted all that time writing that when I could have just said "go read page 12". Don't be stubborn. Your foundation is wrong...

 

[Vumad]

The carb doesn't build 50psi of pressure on a stock ski and blow the needle valve open... Working pressure are much lower...

Air goes through your carb at a high speed, this high speed goes through the venturi and creates a area of low pressure at the fuel opening creating a vacuum (venturi principle).

The fuel gets sucks through the jets.

The fuel in the jet area gets sucked out, creating a vacuum.

This vacuum pulls the diaphragm cover inward

Diaphragm nipple pushes on the rocker arm

which opens the needle valve

which lets fuel in

which charges your low speed jet

Doubt it? poke a hole in your diaphragm and see how that goes. Your fuel pump will still be sending pressure to the N&S, but with no vacuum on the diaphragm, the rocker doesn't get pushed, no fuel flows. It takes significantly less vacuum to push the rocker arm down than the pressure the fuel system can create to blow the needle valve open.

So again, pop-off tests don't mean anything. They just happen to relate directly to something that is important. The assumption that fuel is pushed through your carb by the pump is wrong. It is pulled through from the opposite side. We can't measure pull, so we measure push and make a relationship.

Edit: Okay, it's not completely irrelevant, because different seat sizes do change the reaction of the diaphragm. This is because the pressure pushing on the needle makes the workload of the diaphragm lower, never-the-less, the diaphragm open the needle, not the fuel pressure. It is a combination push/pull. Just trying to make a point about the pull.

 

[Vumad]

Now that makes sense. Thanks for a good explanation.

So how would not having pressure in the tank effect the way your ski runs?

The vacuum in the carb work with the pressure in the fuel pump to open up the pop-off. Pressure in the tank makes the fuel pumps job easier, allowing higher pressures, which makes less work for the diaphragm/vacuum. If you had pressure and lost it, you could in theory, have lower fuel pressure, pop-off later and run lean. If you had no pressure and then gained it (such as replacing a damaged one-way valve), you would have higher fuel pressure, popoff sooner, and run rich. Both conditions are tuneable with the low speed jets/screws.

Important edit: The fuel pressure is not what matters as long as there is enough pressure to supply the fuel at WOT. What matter is the vacuum in the carb. Higher fuel pressure means you need less vacuum! Changing your fuel pressure modifies how much vacuum causes how much reaction. It is still the vacuum/diaphragm/rocker arm doing the work, NOT the fuel pressure pushing the needle open!

My X2 never had tank pressure and ran just fine. It was tuned that way. However, when it was upside down, it dripped fuel. My lack of tank pressure didn't hurt how my ski ran, but it did contribute to my chances of exploding.

 

[naticen]

True story. The only thing I would add is not just a low pressure in the venturi pulling, but regular atmosphere pressure on the other side of the diaphragm pushing fuel through the jets. Like sucking on a straw. It won't do anything without the air pressure on the other side pushing the drink down. That's how most carbs work, but since a jet ski can't have a fuel bowl open to the elements, we have a rubber diaphragm to use outside air pressure to do work on the carb.

What a long winded thread. I guess the answer I was looking for is the pump isn't very efficient at low RPM's and needs the tank pressure to help it along.

 

[MADMAT]

Well FWIW I think I finally understand it! Woo hoo.

 

[tshank123]

Getting deep here. I think one simple concept is that the pressurized fuel system helps deliver fuel to the carbs. Its not that it wont run, it just runs better with pressure because thats how the fuel system was designed.

Have you ever pulled a fuel line off your carb without first releasing the pressure from the gas cap? Notice how fuel starts streaming out of the fuel line? Its kinda like having a lift pump in your fuel tank.

 

[nbatch95]

Thanks all for good detailed explanations it all makes sense now.

 

[waxhead]

It's not a sucking or vacuum. It's a pressure difference that makes the fuel flow. The fuel wants to go to a low pressue inside the carb. What causes the high pressure inside the fuel bowl. It's a small round black piece of rubber that has air pressure on the out when enough fuel is pushed through he diaphragm pushes down on the rocker arm and let's fuel through the needle and seat. If you have reduced fuel pressure in the system from a leak your not going to get the same fuel flow through the needle and seat. As I said before you can jet for it and your ski will run ok. But to change between them with out adjusting is going to make the engine jetting incorrect. If you can get an air leak and think your ski is ruining fine I would suggest your ski was not well tuned to start with

 

[Vumad]

It's not a sucking or vacuum. It's a pressure difference that makes the fuel flow. The fuel wants to go to a low pressue inside the carb. What causes the high pressure inside the fuel bowl. It's a small round black piece of rubber that has air pressure on the out when enough fuel is pushed through he diaphragm pushes down on the rocker arm and let's fuel through the needle and seat. If you have reduced fuel pressure in the system from a leak your not going to get the same fuel flow through the needle and seat. As I said before you can jet for it and your ski will run ok. But to change between them with out adjusting is going to make the engine jetting incorrect. If you can get an air leak and think your ski is ruining fine I would suggest your ski was not well tuned to start with

Atmospheric pressure is 14.7psi at sea level. A vacuum is defined as any amount of pressure below atmospheric. 14.6psi is a vacuum. That lower pressure inside the carb that allows atmospheric pressure to collapse the diaphragm, that's a vacuum. It's status as a perfect/partial vacuum doesn't negate it being a vacuum.

Hydraulics for engineers is much more complicated than hydraulics for pump operators, so I'm open to correction, but I'm pretty confident this is how all this carb stuff works.

 

[waxhead]

I'm talking in absolute not in vacuum cleaner salesperson speak. This is why on a car we have a map sensor. Manifold absolute pressure. It's about pressure differential this is what makes the fuel flow from one side to the other. This is why a smaller carb is easier to jet. It has a greater difference in pressure over the jets.
but in reality if thinking about in vacuum works for you that's fine and we are really only debating words used