8hp 2stroke fuel problem

[Jason2tpa]

So you can SEE IT vs it gets pumps into the block where you cannot see it..

Ah ha. Very good. I learned something new on this thread that has nothing to do with my engine. But if it does happen, I'll know better how to diagnose. Muchos tacos.

[99yam40]

Ok to me maybe he has a bad check valve in the pump or has a air leak not letting the pump suck fuel properly.

I am not familiar with that small motor's pump/system so I do not know if it even has a diaphragm.
but if it was leaking It should cause an over rich condition on the cylinder furnishing the pulse.
does a 8 have more than one cylinder?

[TownsendsFJR1300]

Ok to me maybe he has a bad check valve in the pump or has a air leak not letting the pump suck fuel properly.

I am not familiar with that small motor's pump/system so I do not know if it even has a diaphragm.
but if it was leaking It should cause an over rich condition on the cylinder furnishing the pulse.
does a 8 have more than one cylinder?

Two cylinder, one carb, click the link:

2000 8MSHY Yamaha Outboard CARBURETOR Diagram and Parts

Shows the diaphragms as well..

But the pump is attached to the outside of the carb, not the block..

Looks like just a strainer as well, no check valves:
http://www.boats.net/parts/search/Ya...UEL/parts.html


.

[99yam40]

how can a pump pump with out check valves?

All pumps I have seen either uses mechanical action or vacuum/pressure pulses , and check valves to move things.
unless it would be centrifugal like a electric motorized pump

I looked and #35 shows to be check valves

there is more than likely a port thru the carb to the block/crankcase

[panasonic]

S/V Mar-a-Lago here.....

Thanks for the suggestions...the engine year is 2000, the model # 8MSHY...the fuel pump is mounted directly to the side of the carb...the other thing a noticed just prior to the failure of the fuel flowing properly, was that when the engine was tilted up for the night, the next morning it would not start until I pumped the fuel bulb to get fuel into the carb...the bulb was initially soft when I started to pump it, and then would get harder when fuel went through. Does this help at all?

S/V Mar-a-Lago

Sir I would order the parts to rebuild the fuel pump, its cheap and easy to do.

Sounds like the diaphragm has a hole in it. You are the fuel pump when you pump the ball to keep it going...

Please report back with the results.

[Jason2tpa]

S/V Mar-a-Lago here.....

..the other thing a noticed just prior to the failure of the fuel flowing properly, was that when the engine was tilted up for the night, the next morning it would not start until I pumped the fuel bulb to get fuel into the carb...the bulb was initially soft when I started to pump it, and then would get harder when fuel went through. Does this help at all?

S/V Mar-a-Lago

The fuel likely ran out the failed diaphragm...

Because of the carbs being higher than the fuel tank, I thought if the engine sits for an extended period of time- either tilted up or down- fuel drains out of the bowls. No? Aside from a fuel pump issue like this thread, when i take the boat out and prepare to start engine, the carbs and primer bulb are empty. If the fuel doesn't drain back, where does it go?

[panasonic]

Because of the carbs being higher than the fuel tank, I thought if the engine sits for an extended period of time- either tilted up or down- fuel drains out of the bowls. No? Aside from a fuel pump issue like this thread, when i take the boat out and prepare to start engine, the carbs and primer bulb are empty. If the fuel doesn't drain back, where does it go?

Carb bowls are vented to atmosphere so the gas will evaporate over an extended period of time.

As for your primer bulb being empty, it must be allowing fuel to flow back to the tank. The check valves in the bulb are slowly letting it leak back. I don't how old your bulb is, but when they are new they hold pressure for a long time.

I just checked my primer ball on my portable tank (new ball last fall) and it is still firm and I have not used it for 4 months now.

Might be time for a new primer bulb?

[Jason2tpa]

Carb bowls are vented to atmosphere so the gas will evaporate over an extended period of time.

i think we'll need a chemistry professor here to calculate the rate at which fuel evaporates. I don't know the volume capacity of each carb bowl, but ill guess about a cup each. Hard to believe that much fuel would evaporate without exposure to direct sunlight.

[panasonic]

i think we'll need a chemistry professor here to calculate the rate at which fuel evaporates. I don't know the volume capacity of each carb bowl, but ill guess about a cup each. Hard to believe that much fuel would evaporate without exposure to direct sunlight.

don't know where you live but in hot places gas evaporation happens very quickly. I also don't your carbs and what size the vent holes are in the carbs and how much the bowls hold. And I also don't know how long the extended time period you are talking about.

Here's what I do know. Your float bowls fill to a certain level and then the float shuts off the flow. So no more fuel can get in the bowl until the level drops to certain point and the float needle opens and allows more fuel in. Carberator 101.

I will give you an example. I have an old 1970's bike with four 28 mm carbs on it. Each carb has a vent hole approximately 1/8" in diameter. Fuel tank is directly above carbs, so no fuel pump, gravity feed. There is fuel shut off valve on the tank which you turn off every time you park the bike for more then a few hours.

Now if I leave that bike to sit for more than a few days in the summer and then go to start it without turning the fuel valve back on it will not start. No fuel in the bowls.Turn the fuel valve back on and wait a minute or two, bowls fill up, and it starts normally.

Where did the fuel go? It evaporated from the bowls...no other place for it to go. It couldn't go back to the tank, it would have to go straight up to do that, plus there is a shut off valve on the tank. It is not leaking out as I would see that. I park it in my shop and it always smells like gas in there for a few days and then the smell disapates....the fuel has evaporated from the carbs, no more smell.

As for a cup of fuel in each carb bowl on your engine..that would be a lot I would think. When the bowl is off and in your hand it looks pretty big, but the floats take up a lot of space when it's together.

You like to do experiments.Drain one of your bowls when you know it's full, right after running, see how much you get. Refill the bowls, disconnect the fuel line, wait a week or so and drain them again and see how much you get then...

How bout the primer bulb thing...you agree or disagree on that?

[panasonic]

Oh one more thing, this is why modern cars have EVAP systems on them. I think I read somewhere that before they put these systems on vehicles that 10-15% of vehicle emissions was due to fuel evaporation. Also ethanol based fuels evaporate even faster than regular gas..

[99yam40]

need to add the heat of a motor that was running before it was turned off.
everything under the cowl is fairly warm at that point and the fuel pump is not going to push any more fuel once the motor stops running.

Only so much fuel in those carb bowls and it will disappear sooner than you would think.

I try to pump the bulb every so often when walking by the boat if it has been awhile since it was run.
helps keep the residue from forming in bottom of bowls by keeping liquid in them

[boscoe99]

The float bowl for an 8 HP motor would hold only maybe an ounce or so. No where near to being a cup. A cup is bigger than the entire carburetor.

[panasonic]

The float bowl for an 8 HP motor would hold only maybe an ounce or so. No where near to being a cup. A cup is bigger than the entire carburetor.

I think Jason was referring to his own boat engine, not the 8hp.

[boscoe99]

I will then say that I don't think a 115 carburetor will hold more than a few ounces.

But gasoline is pretty volatile. More so with higher temperatures. I could see even a cup of gasoline evaporating from a one cup fuel bowl in a relatively short period of time. More so with diurnal pressure changes.

[99yam40]

pour a little out on a brush pile and then throw a lit match in there, the fumes spread out very quickly.
fumes are from the gas evaporating, not from the liquid spreading.

Towens or Robert Gram talk about draining carbs all of the time.
they need to tell us how much comes out of a single float bowl