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[BA80]

Clear as mud.

What don't you see?

[timktm]

The graph is confusing to your statement.

C'mon Brant, the IDLE MIXTURE SCREW is just that. It has an insignificant affect off idle.

About 1/8 off idle the pilot jet is what controls the majority of fuel flow, the main jet and jet needle come into play at 1/4 to WOT.

[BA80]

This is a generic chart from the Mikuni tuning theory. It shows a throttle valve cutaway that is used on cable operated slide carbs. Our carbs don't have that because they have a butterfly throttle valve used in conjunction with the constant velocity vacuum slide.

If you look in the engine side of your carbs with the butterfly valve open you'll see 3 tiny holes in the top of the venture. That is the first stage of fuel feed off idle. They are fed by the pilot jet only. These are VERY important and when cleaning your carbs they need particular attention. If one or more of them are clogged or if the pilot jet is the wrong size or type there will most likely be a stumble when the throttle is opened. At this point some very small amount of fuel is still coming through the idle mixture orifice but is insignificant through the rest of the throttle positions.

As the chart says, after about 1/8 throttle the needle jet and jet needle begin to come into play. Now with more air flow there is fuel being drawn through the pilot jet (the 3 tiny holes) and the needle jet. The needle jet being regulated by the jet needle according to intake demand by the diaphragm operated slide.

At about 1/2 throttle the main jet comes into play along with the other metering systems I have mentioned. This is where most cruising is done. At this point the engine is getting ALL the fuel that the pilot jet can supply plus what comes up through the main and needle jets and is still being metered by the jet needle.

At WOT the jet needle is wide open and so is the main jet and you are getting max fuel feed.

The pilot jet, needle jet, jet needle, and main will determine performance and economy. The idle mixture screw is just for setting fuel demands at closed throttle. It's insignificant beyond that.

More mud?

[MPittma100]

Screws

As Greg stated, the idle mix screws are for IDLE. The lean drop method was initiated by many manufacturers (cars/bikes) back in the day. It was one of their initial attempts to "clean up" emissions.

40+ MPG is easily obtainable with proper tuning.

[timktm]

Greg,
Great explanation. So since my bike is running so good I will leave it alone.
Tim

[BA80]

Greg,
Great explanation. So since my bike is running so good I will leave it alone.
Tim

As long as you have good plug color you will be good to go. Not everyone get's 40MPG. Everyone rides a little different but cruising mileage usually averages out 30 - 35 MPG. I've gotten as good as 43mpg or better and as bad as 25 or less. I run my jetting to the rich side of the scale. The richer it is the cooler the engine runs, there is a tipping point though.

[mack]

Tim

This may help with the chart:
This may be a good time to talk about CV carb theory. The simplest way to explain it is atmospheric pressure Voodoo. High pressure always travels to low pressure. Low pressure is induced first by suction of the pistons and valve train, and then by air movement velocity as it passes through the bell to the narrower barrel. This works to move both air and fuel through the various carb passages (circuits).
In simplistic terms, each carb has five circuits that perform a delicate ballet contingent on the throttle plate setting. Each carb has three straws to suck up the required fuel. Each of these straws is metered to control the amount and velocity of the fuel it can pass. Each straw is also provided with air so the fuel can be atomized (turned combustible).
Now the first straw is enrichment circuit pickup tube. Since the motor has to warm up a bit so the fuel doesn't stick to the cylinder walls instead of burning properly we have to richen up the mixture. So when the plunger is pulled back, it closes off the pilot circuit and fuel is drawn up the tube and thrown into the atomization manifold and the mixture enters the venture ahead of the butterfly through a large (off centered) hole in the top of the barrel which is angled directly into the intake boot. The ability to syphon the fuel is provided by the tube and metering occurs at the pressed in jet located in the bowl itself located immediately under the brass pickup tube. If you look at the diaphragm chamber you’ll notice a hole directly in line with the angled hole that points into the intake boot. This hole is only open when the enrichment plunger is engaged. After studying the set up and some consideration, I’ve concluded that the purpose of this hole is to break the low pressure induced in the bowls as the fuel is drawn up to the manifold. The best way to describe this requirement is by comparing it to drinking from of a juice box with a straw. The more you suck the harder it is to get fluid until you allow air to break the low pressure in the box.
Once the engine is warm enough, the enrichment circuit is closed and the pilot tower circuit is open again. Now the idle circuit comes into play. This circuit draws its fuel up the pilot tower and is injected into the venturi ahead of the butterfly through a small hole in the top center of the venturi. The metering is primarily done with the idle mixture screw. When fully seated the idle mixture will be lean, backing the screw out a number of turns will richen the mixture up for optimum combustion.
Up to this point, the throttle plate has remained virtually completely closed so the air fuel mixture has to be dumped into the carb ahead of the butterfly.
The last three circuits are all determined by the throttle plate position and overlap each other as the carbs transition from one to another depending on air velocity through the barrel as the plate is open and closed. They are the pilot, jet needle and main circuits. Since this thread was primarily about setting the correct float heights, I will not touch on correct jetting and associated modifications based on your rides individual configurations. However, this short video which TC made in 2008 will demonstrate the delicate balancing act that is performed between these three circuits.

https://www.youtube.com/watch?v=tR63vrfhwr8

Note that the jet needles remain fairly neutral until he rolls on the throttle hard. Then, you see the diaphragm draw the needles up rather quickly opening up the main jet nozzle. Speed isn’t involved here; but, the pressure drop induced by abrupt manipulation of the throttle plates is! This is an excellent demonstration of the fundamental operation of CV carbs. As the throttle plate opens slowly and rpm increases, you see the needles retract some but are fairly static because the pressure drop isn’t as robust. It doesn’t show it; but, at WOT (with the advances fully rotated) the needles would be fully retracted and the main jet is pumping fuel as fast as the jet orifice will allow. Meanwhile through all that, the pilot circuit is providing the seamless transition required for the motor to run smoothly from one circuit to another, including its own!

[motoman]

This may help with the chart:
This may be a good time to talk about CV carb theory. The simplest way to explain it is atmospheric pressure Voodoo. High pressure always travels to low pressure. Low pressure is induced first by suction of the pistons and valve train, and then by air movement velocity as it passes through the bell to the narrower barrel. This works to move both air and fuel through the various carb passages (circuits).
In simplistic terms, each carb has five circuits that perform a delicate ballet contingent on the throttle plate setting. Each carb has three straws to suck up the required fuel. Each of these straws is metered to control the amount and velocity of the fuel it can pass. Each straw is also provided with air so the fuel can be atomized (turned combustible).
Now the first straw is enrichment circuit pickup tube. Since the motor has to warm up a bit so the fuel doesn't stick to the cylinder walls instead of burning properly we have to richen up the mixture. So when the plunger is pulled back, it closes off the pilot circuit and fuel is drawn up the tube and thrown into the atomization manifold and the mixture enters the venture ahead of the butterfly through a large (off centered) hole in the top of the barrel which is angled directly into the intake boot. The ability to syphon the fuel is provided by the tube and metering occurs at the pressed in jet located in the bowl itself located immediately under the brass pickup tube. If you look at the diaphragm chamber you’ll notice a hole directly in line with the angled hole that points into the intake boot. This hole is only open when the enrichment plunger is engaged. After studying the set up and some consideration, I’ve concluded that the purpose of this hole is to break the low pressure induced in the bowls as the fuel is drawn up to the manifold. The best way to describe this requirement is by comparing it to drinking from of a juice box with a straw. The more you suck the harder it is to get fluid until you allow air to break the low pressure in the box.
Once the engine is warm enough, the enrichment circuit is closed and the pilot tower circuit is open again. Now the idle circuit comes into play. This circuit draws its fuel up the pilot tower and is injected into the venturi ahead of the butterfly through a small hole in the top center of the venturi. The metering is primarily done with the idle mixture screw. When fully seated the idle mixture will be lean, backing the screw out a number of turns will richen the mixture up for optimum combustion.
Up to this point, the throttle plate has remained virtually completely closed so the air fuel mixture has to be dumped into the carb ahead of the butterfly.
The last three circuits are all determined by the throttle plate position and overlap each other as the carbs transition from one to another depending on air velocity through the barrel as the plate is open and closed. They are the pilot, jet needle and main circuits. Since this thread was primarily about setting the correct float heights, I will not touch on correct jetting and associated modifications based on your rides individual configurations. However, this short video which TC made in 2008 will demonstrate the delicate balancing act that is performed between these three circuits.

https://www.youtube.com/watch?v=tR63vrfhwr8

Note that the jet needles remain fairly neutral until he rolls on the throttle hard. Then, you see the diaphragm draw the needles up rather quickly opening up the main jet nozzle. Speed isn’t involved here; but, the pressure drop induced by abrupt manipulation of the throttle plates is! This is an excellent demonstration of the fundamental operation of CV carbs. As the throttle plate opens slowly and rpm increases, you see the needles retract some but are fairly static because the pressure drop isn’t as robust. It doesn’t show it; but, at WOT (with the advances fully rotated) the needles would be fully retracted and the main jet is pumping fuel as fast as the jet orifice will allow. Meanwhile through all that, the pilot circuit is providing the seamless transition required for the motor to run smoothly from one circuit to another, including its own!

Mack, thanks for givin' the long explanation to my short version.
Greg, sorry you didn't follow my short explanation version.

[BA80]

Mack, thanks for givin' the long explanation to my short version.
Greg, sorry you didn't follow my short explanation version.

You crack me up........

[motoman]

Motoman, It works just fine for me and also for many others.

...........but not at 5,00ft. elev. and above.

[motoman]

...........but not at 5,00ft. elev. and above.

.....And BTW, have ridden my Venturer everything from PC1(sea-level Cali Rally, to top of Pikes Peak(a bit over 14,000ft. elev., and everthing inbetween). There IS an idle mixture tuning difference procedure.