To be continued

[boscoe99]

For those that have Yam carburetors and are interested in how they might work. Now keep in mind that not all carburetors are created equally. This example is for an F30.

[cpostis]

Morse Code?

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[99yam40]

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had a great thought maybe and then needed to take a quick break?

[boscoe99]

Completed.

[rodbolt17]

note with this design, as with most Yamaha carbs in the past 20+yrs, you CANNOT adjust the A/F ratio at idle.
you can allow more OR less of the fixed ratio but you cannot alter the ratio with the screw.
also note that the idle circuits supply fuel at all RPM ranges. clog it and not only will it not idles but will be lean at high speed.

Yamaha used this basic design on a lot of both two and 4 stroke engines.

[greasyshaft]

You cannot alter the air fuel ratio moving through the idle cicuit but you can alter the ratio going into the combustion chamber using the mixture screw (at low speed)

[boscoe99]

You cannot alter the air fuel ratio moving through the idle cicuit but you can alter the ratio going into the combustion chamber using the mixture screw (at low speed)

I agree with you 100%. I think I know what rodnut is saying but not sure why he is saying it. Is he trying to riddle us? Is he trying to make us think? Whatever he is doing, I believe that it causes some folks to have a wrong understanding of the inner workings of the carburetor. It infers that changing the idle mixture screw will not affect the air fuel ratio. It does. We just need to be clear about what ratio that we are talking about. The ratio through the idle circuit is irrelevant. Not sure why he brings it up, unless it is a puzzle that he is presenting.

Let's presume that the air fuel ratio going through just the idle circuit is at some high value. I will use 1/1 but this is only an example. I don't have any idea what the actual ratio is. The air going through the carburetor inlet is of course 100/0 (nothing but air).

We know that once the 100/0 pure air gets mixed with what ever amount of 1/1 air fuel, which passes through the idle circuit, that the resultant will be in the neighborhood of 13/1 or so in the combustion chamber. A bit on the rich side of things if you will.

At RPM the amount of 100/1 air fuel is not going to change. If, however, the idle mixture screw is turned outward, allowing more of the 1/1 air fuel to be provided to the 100/0 air fuel mix then the result is going to be less than 13/1. The air fuel ratio in the combustion chamber is now richer than it was before the idle mixture screw was turned outward. Let's say 12.5/1.

So, are we in agreement that the idle mixture screw being turned inward or outward can and will have an affect on the final air fuel ratio that the engine is running on?

Where the hell is Xeno?

[boscoe99]

You cannot alter the air fuel ratio moving through the idle cicuit but you can alter the ratio going into the combustion chamber using the mixture screw (at low speed)

From reading the Yamaha information provided above, turning the idle mixture screw outward (allowing more of the idle air fuel mix) will affect the air fuel mixture at all engine speeds. Not just low speed. Although the majority of the fuel entering the carburetor at high speed is through the main jet some fuel continues to be delivered through the idle jet as well. Add more fuel through the idle jet at low RPM's results in more fuel being added through the idle jet at high RPM.

Damn I wish I had a carbureted Yamaha to which I could add an air/fuel meter. We could then see the effects of changes to the idle mixture screw on the gauge.

[rodbolt17]

ok
for the sake of argument lets set up some paremeters.

the combustion wants 14.7/1 air/fuel.

the idle circuit is designed for let us say 13.7/1.
the other 1 part air is past a fixed throttle plate at idle.

no matter what we do with the mixture screw we CANNOT alter the FIXED mix ratio through the two orifices(jets).

all that screw can do is allow MORE or LESS of the FIXED A/F mix. now if you move the throttle plate it will alter the A/F.

this is why until the EPA carbs you set the screw by the book and it worked or something was wrong.
the engineers did all the work all the tech had to do was set it up correctly when reassembling things.

so typically with this design,assuming you set the screw at the correct turns out, IF it lean sneezes you either have a restriction in the pilot fuel circuit,an airleak in a seal or the shutter is to far open trying to compensate for a different issue.

so ya gotta explain how we can change the idle A/F mix by simply turning a screw.
the jetting,for the sake of argument is going to be fixed at about 13.7/1.

if ya don't belive me try taking an engine with this carb design, set it up so the idle speed is correct.

now slowly start increasing the idle speed using the idle speed screw.
about 250-300 RPM above its idle speed you will start getting a lean sneeze almost everytime.

those tiny idle and off idle passages can only flow so much A/F but we are adding air via the shutter making the A/F lean in the intake.

[boscoe99]

ok
for the sake of argument lets set up some paremeters.

the combustion wants 14.7/1 air/fuel. 14.7/1 is the stochiometric value but in real world combustion this value is seldom desired or used.

the idle circuit is designed for let us say 13.7/1.
the other 1 part air is past a fixed throttle plate at idle. Not sure what is being said here.

no matter what we do with the mixture screw we CANNOT alter the FIXED mix ratio through the two orifices(jets). Through the jets I agree with.

all that screw can do is allow MORE or LESS of the FIXED A/F mix. agreed now if you move the throttle plate it will alter the A/F. For the sake of discussion the throttle plate is not moved.

this is why until the EPA carbs you set the screw by the book and it worked or something was wrong. Prior to the EPA there was a range for the idle mixture screw. So that the idle mixture setting could be changed a tad to make the engine run a tad better if needed. The EPA is only concerned with emissions. Could not give a damn about the motor not running as best as it can.
the engineers did all the work all the tech had to do was set it up correctly when reassembling things. Yamaha US will still provide the ranges to the mekaniks if they call and ask of them.

so typically with this design,assuming you set the screw at the correct turns out, IF it lean sneezes you either have a restriction in the pilot fuel circuit,an airleak in a seal or the shutter is to far open trying to compensate for a different issue.

so ya gotta explain how we can change the idle A/F mix by simply turning a screw. Adding more fuel mixed with a smidgeon of air to a fixed amount of air will result in the air fuel ratio changing.
the jetting,for the sake of argument is going to be fixed at about 13.7/1.

if ya don't belive me try taking an engine with this carb design, set it up so the idle speed is correct.

now slowly start increasing the idle speed using the idle speed screw.
about 250-300 RPM above its idle speed you will start getting a lean sneeze almost everytime.

those tiny idle and off idle passages can only flow so much (yes, but the flow can and will change a tad as the idle mixture screw is turned inward or outward) (which is why the EPA does not want mekaniks to get access to the idle mixture screw) A/F but we are adding air via the shutter making the A/F lean in the intake. Shutter remains in the closed position. Air flow through the shutter is fixed. As more air/fuel (1/1) are allowed into the motor via the idle mixture screw the final air fuel mixture seen in the combustion chamber will change.

That's my story and I'm stickin to it.

[boscoe99]

Let's look at another way.

Let's assume that the designer used no air bleed for the pilot jet. Only fuel flowed through the jet and through the needle valve. All of the air gets into the motor via the throttle valve.

At some setting of the needle valve the amount of fuel flowing through the idle jet and the needle valve, when mixed with all of the air that flows through the throttle valve, results in an air/fuel ratio of 14.7/1.

If by turning the needle valve outward more fuel flows through the carburetor, mixing with the same amount of air, aren't we now going to have a richer mixture? More fuel, same air, should enrichen the mixture. Correct or not?

[Jason2tpa]

Nice boscoe, great post.

[rodbolt17]

nope, no one ever did it that way.
however mercs and jonnyrudes in the days gone by fixed fuel and the low speed adjustment actually changed the amount of air .\
throttle plate stayed the same.
later jonnyrudes and mercs simply eliminated any adjustments.
fixed fuel and air.
this worked for 20+ yrs.

lets go about this yammie carb again.
50 cc of air is allowed past the shutter at the correct idle speed.\

we can alter the amount of pilot fuel/pilot air that is FIXED by maybe 20-30cc
you really really aint changed the A/F ratio as it is freaking fixed.
if on the above carb the spec says 1 1/4 turn out.
you could turn it 2 1/4 and never know the difference,, assuming the rest of the engine is running correctly.

if anyone ever cared to actually look, the orifice size for the IDLE in the actual carb body is about the size of the needle tip.
you simply cannot change the idle A/F (rich) to really make a difference.
you can add to much fuel/air but your not going to change the fixed ratio that much.

unlike the old mercs suzukis and jonnyrudes the later style mikuni carbs simply wont alter the ratio much at all with the screw.

try it for yourself.

[boscoe99]

Will increasing the size of the pilot jet alter the idle air/fuel ratio?