Carburetor air and fuel flow

[boscoe99]

For anyone that is interested, I found a fairly new illustration that depicts how both air and gasoline enter into and are mixed together within a certain Yamaha carburetor. This is about as clear of an illustration that I have found. It is for the idle RPM configuration only. Things change as the throttle is opened.

For those that might want to know why bleed air is introduced into a fuel passageway I got some good stuff that explains that as well.

[Jason2tpa]

For those that might want to know why bleed air is introduced into a fuel passageway I got some good stuff that explains that as well.

Let's not stand on ceremony, bring it on!

[panasonic]

Yes bring on the more information...

[boscoe99]

[panasonic]

There that's the fluid dynamics lesson for the day....Sure to be tested soon enough.

[rodbolt17]

most Yamaha carbs use the above design.
that is why when you alter the pilot air screw you are not changing the A/F ratio you only allow more or less of the pilot fuel mix.
restrict the pilot air and it goes rich.
restrict the pilot fuel and it goes lean.

[99yam40]

with the more or less air/fuel mix going into the cylinder from the pilots screw adjustment there is also the air that comes by the cracked open throttle plate.

so the more of the mix you let in with the pilot screw to the set throttle plate position (Air) will cause a richer mix into the cylinder.

I know we have discussed this before Rodbolt, with you disagreeing to this way of thinking, but unless the only air going into the cylinder is the mixture thru the pilot screw I do not see how the richness of air/fuel in the cylinder is not changed when adjusting the pilots screw

[rodbolt17]

at idle the shutter bypass air is fixed.
it cannot change.
at idle the pilot air and pilot fuel is a fixed ratio.
you can allow more or less of this fixed ratio into the intake.
your not altering the ratio with the screw.
if you don't belive it simply look at that diagram.
it is also why with most Yamaha carbs if you have everything else correct you simply set the screws per the manual and it works.

[99yam40]

I understand that the ratio of air to fuel going thru the pilot screw is fixed by the jets, but it is mixing with the air going by the throttle plate.
Throttle plate opening amount is set by adjusting the idle speed screw, it is not closed all of the way so air comes by it at idle

The ratio of air/fuel in the cylinder can be changed by allowing more or less of the metered mix to go by the pilot screw with the fixed air by the throttle plate.
Is this not correct?

[panasonic]

I understand it the same way as rodbolt. At idle you are only letting in more or less of the "premixed" fuel, when you adjust the screw.

I like this picture BosCoe put up a while ago.

Everybody has this debate, "Which way you turn the mixture screw to make it lean or rich" I really don't care which way it is turned. My goal is to get the engine to idle smoothly and take the throttle input off idle without engine stumbling.

So set screws to factory settings then tweak from there.

[boscoe99]

The motor is running at idle with X amount of air flowing through the main bore of the carburetor and then into the intake manifold and Y amount of air/fuel mixture flowing through the idle circuit and then into the intake manifold.

X is pure air and Y is a 1/1 mix of air/fuel.

All of the air and all of the fuel gets mixed together and hopefully fully vaporized by the time it is drawn into the combustion chamber. It is then burned when the spark plug ignites it. The air/fuel ratio is some value. Let's say that it is 14.7 to 1. 14.7 parts of air and one part of fuel. If an O2 sensor is installed in the exhaust this is the value that would be seen on the oxygen sensor.

Now let's say that someone dribbles a bit gasoline/air bubble fuel mix at a ratio of 1/1 into the intake manifold. That mix will join up with the air and fuel that was already flowing through the carburetor.

Will the O2 sensor indicate a leaner or richer mixture? If richer why? If leaner why? If no change why?

If a small amount of air only leaks into the intake manifold behind the carburetor it has been stated that the motor will run leaner. Is this statement correct or not?

[rodbolt17]

we will use an 05 s150 as an example.
correctly running the pilot screw is 1 1/4 from lightly seated.
turn it out 2 1/4 and you wont know it.
turn it out 3 and a trained tech familiar with that motor may notice it.

set it at 3/4 turns out and it will start lean sneezing.

varnish up the pilot fuel jet and it sneezes no matter what you set the screws to.

[99yam40]

from that statement it sounds to me turning the pilot screw in too far will lean out the fuel to air ratio to the point of lean sneezing.
And turning it out too far will cause a rich fuel to air ratio in the cylinders

[rodbolt17]

lets say that the ratio of the mix from the pilot FUEL and pilot AIR is 14.7/1.

when you move that screw in or out you decrease or increase the amount of that 14.7/1 mix.
not enough of that mix and that cyl starts burning the limited amount in the crankcase.
that is what makes it "sneeze" and can shut it off.
it is an explosion under a piston on it's down stroke and tries to reverse the crankshaft direction.
do it violently enough and it can shear flywheel keys and pop fuel pumps.
at idle it has two fixed air sources and one fixed fuel source.


unlike some carb designs that actually meter fuel or air with that screw Yamaha did not choose to do it that way.
some carbs are set up with no idle speed screw and no A/F screw and it is all fixed jet.

[panasonic]

[QUOTE=boscoe99;137325]The motor is running at idle with X amount of air flowing through the main bore of the carburetor and then into the intake manifold and Y amount of air/fuel mixture flowing through the idle circuit and then into the intake manifold.

X is pure air and Y is a 1/1 mix of air/fuel.

All of the air and all of the fuel gets mixed together and hopefully fully vaporized by the time it is drawn into the combustion chamber. It is then burned when the spark plug ignites it. The air/fuel ratio is some value. Let's say that it is 14.7 to 1. 14.7 parts of air and one part of fuel. If an O2 sensor is installed in the exhaust this is the value that would be seen on the oxygen sensor.

Now let's say that someone dribbles a bit gasoline/air bubble fuel mix at a ratio of 1/1 into the intake manifold. That mix will join up with the air and fuel that was already flowing through the carburetor.

Will the O2 sensor indicate a leaner or richer mixture? If richer why? If leaner why? If no change why?

If a small amount of air only leaks into the intake manifold behind the carburetor it has been stated that the motor will run leaner. Is this statement correct or not?[/QUOTE]

Man you ask complicated questions....

If you mean there is an air leak where the carb is attached to the manifold/block, which I think you do. The engine will run leaner as you are only introducing more air downstream of the butterfly plate. No more fuel is being put in.

So yes that statement is correct.

As for the rest of it...I will have to think on that a bit.