Fuel leaking from exhaust on cold start

[99yam40]

most lawm mowers and a lot of other small motors have the tank higher that the carb and those floats seat the needle well enough to close it off,so I would think these Yamahas would be able to also.
even if fuel leaked by the needle and seat, that would raise the float higher when the level went up in the bowl and put more pressure on the needle to seat it just like you were saying more pressure forcing fuel into the bowl would.
right? or am I overlooking something?

[panasonic]

most lawm mowers and a lot of other small motors have the tank higher that the carb and those floats seat the needle well enough to close it off,so I would think these Yamahas would be able to also.
even if fuel leaked by the needle and seat, that would raise the float higher when the level went up in the bowl and put more pressure on the needle to seat it just like you were saying more pressure forcing fuel into the bowl would.
right? or am I overlooking something?

If the needle is not sealing and the tank is above the carb the fuel will start over filling the bowl and forcing it up through the main jet into the carb throat...and into the engine.Gravity just keeps allowing the fuel to flow.

When the tank is below the carb the fuel has to go uphill...thus the primer bulb to get the engine going and the fuel pump to keep it going....as we all know.

Is there any over flow ports on these carbs? My motorcycle has a overflows on the carbs right at the top of the bowl so if the needle sticks the fuel goes out on the ground and don't fill the engine up with fuel...thus the shut off valve on tank.

I don't what is going on with this particular engine exactly... but I would bet cleaning the carbs and replacing the needles will stop it..

[boscoe99]

most lawm mowers and a lot of other small motors have the tank higher that the carb and those floats seat the needle well enough to close it off,so I would think these Yamahas would be able to also.
even if fuel leaked by the needle and seat, that would raise the float higher when the level went up in the bowl and put more pressure on the needle to seat it just like you were saying more pressure forcing fuel into the bowl would.
right? or am I overlooking something?

Normally that is the case. It is an anomaly when it happens. The inlet valve failing to stop the flow of fuel that is. Very rare, but it has been known to happen.

Based on what the OP is saying I am guessing that might be what is going on in his motor. Only a guess. He says that fuel flows when the motor is off but does not do it when the motor is running. Of course with the motor running any excess fuel may be burned along with the normal fuel.

I might suggest that he pump the hell out of the primer ball with the motor not running, when fuel is flowing from the exhaust, just to see what happens. The extra fuel pressure, might, just maybe, be enough to seat the inlet valve.

[boscoe99]

If the needle is not sealing and the tank is above the carb the fuel will start over filling the bowl and forcing it up through the main jet into the carb throat...and into the engine.Gravity just keeps allowing the fuel to flow.

When the tank is below the carb the fuel has to go uphill...thus the primer bulb to get the engine going and the fuel pump to keep it going....as we all know.

Is there any over flow ports on these carbs? My motorcycle has a overflows on the carbs right at the top of the bowl so if the needle sticks the fuel goes out on the ground and don't fill the engine up with fuel...thus the shut off valve on tank.

I don't what is going on with this particular engine exactly... but I would bet cleaning the carbs and replacing the needles will stop it..

There are vents in the top of most of the carburetors. Might be a good idea for the OP to monitor those vents when the engine is not running, with gasoline flowing from the exhaust, to see if any over flow from the carburetors is present. I can see gasoline flowing from the carburetor at the same time it over flows into the intake manifold and then on to the motor and the exhaust.

[99yam40]

Normally that is the case. It is an anomaly when it happens. The inlet valve failing to stop the flow of fuel that is. Very rare, but it has been known to happen.

Based on what the OP is saying I am guessing that might be what is going on in his motor. Only a guess. He says that fuel flows when the motor is off but does not do it when the motor is running. Of course with the motor running any excess fuel may be burned along with the normal fuel.

I might suggest that he pump the hell out of the primer ball with the motor not running, when fuel is flowing from the exhaust, just to see what happens. The extra fuel pressure, might, just maybe, be enough to seat the inlet valve.

to me the extra fuel pressure would be pushing the needle open , not closing it.
the float is the only thing pushing the needle into the seat, and if fuel is leaking by over filling the bowl then the float will be up as far as it can go even if the motor is off or running

[boscoe99]

The float is the device that is actually pushing upward on the needle valve. But the deeper the float is in the gasoline then the greater the push will be. Some times you can force enough gasoline into the float bowl so that the fuel level rises quickly, making the float go deeper into the gasoline. The deeper the float the greater the push. The greater the push the greater the seal. Within reason of course.

I suspect that the distance between the top of the fuel level in the float bowl and the fuel nozzle outlet can also affect this. I have heard of cases where if the float is improperly set then it will allow fuel to flow on out without actually seating the inlet valve.

[99yam40]

I think you need to think a little harder on what forces are being presented on the needle.
the higher the level in the float bowl the more pressure is exerted on the needle to close it off.
It does not matter what pressure is exerted by the pump onto the needle except how hard it is trying to open it up.

the only thing that matters is the level in the bowl.

on a motor that has the fuel source higher than the carb, if the needle is not seating properly when motor is not running then the bowl fills higher and pushes more on the needle to keep the incoming fuel from coming in,
on a running motor with a pump the needle has to counter act the pump pressure to close the needle.
the level of the fuel in the bowl puts the same pressure on the needle no matter what the fuel pressure is coming in.
now can the fuel pump overcome the needle when running, maybe if the float has hit it's max highth and cannot exert more presure to block it off

[donnie remesco]

thanks for the input, I already changed the pump, i'll check the carb next.