Post by t***@gmail.comOn the top of the carb where the diaphragm is, there is one hole for
the choke, and another hole for... ? What? Mine were plugged up with
crap, and I got some of it out to the point where I can see shiny
brass a little ways down. Are these holes supposed to flow air or
fuel, meaning I should continue my cleaning attempts, or is this just
a hole leftover from the casting/assembly process? If it's supposed to
flow something, why do I see brass a short ways down the hole?
Because it's the non-removable PILOT AIR JET. When you have a CV
carburetor that has the idle mixture screws on top, you're usually
going to find the pilot air jet under the diaphragm.
It regulates the flow of air through a fixed orifice. The carb sucks
gasoline up through the pilot or idle jet in the float bowl and the
fuel and air mix high up in the carburetor. The idle mixture screws
just control one of four idle mixture ports, the other three ports are
fixed transition ports that allow the engine to get extra gasoline
while vacuum drops off slightly as you
just barely open the throttle.
Transition ports are the CV carburetor's equivalent of the accelerator
pump that a car carburetor has.
You need to have all the idle passages open and all four ports clean
or you'll have to run on the "choke".
Why can't you get the engine to idle right without using the "choke"?
Your carbs don't have a "choke", they have a starting enrichener
device, which is like a tiny carburetor built into the side of each
main carburetor. When you pull on the "choke" knob, you're opening an
air valve and the vacuum downstream of the throttle butterfly sucks
gasoline up a small diameter passage like a kid sucking milk through a
straw.
It's a very rich mixture, but that's what your engine needs to start
when it's cold.
So, your idle jets, idle ports, and idle passages are all plugged up.
Tighten the idle mixture screws all the way until they seat lightly.
Count the number of full turns and fractions of turns and write the
information down. Then remove the screws and keep each screw, spring,
washer and o-ring in a separate container so you can get each screw
back into the carb it came out of.
If you don't find a tiny rubber o-ring, look into the hole the idle
screw came out of.
Should you remove the idle jets for cleaning? Only if you have a small
slot screwdriver that fits the slot in the jet perfectly.
And, what will you see when you get the jet out? Yamaha idles jets
aren't cross-drilled, according to the owner of Factory Pro, who makes
aftermarket jet kits.
Spraying carb cleaner though the idle jets as installed will probably
get them clean enough.
Get youself an aerosol can of Berryman B-12, Gum Out, or STP
carburetor cleaner or whatever is cheap and available.
Use the red plastic tube to precisely direct the flow of carb cleaner.
Spray the carb cleaner down each hole. It will come out the single
hole directly aligned with the idle mixture screw. Put your finger
over that hole and keep spritzing down the idle mixture hole.
You should see carb cleaner squirting out of the idle jet, the three
transition ports next to the throttle butterfly, and carb cleaner
should squirt up out through the pilot air jet.
By using your fingers to block whichever holes are clear, you can
force the carb cleaner to go through the other passages.
When you can see you're getting an unobstructed flow of cleaner out
all the holes, reassemble each idle mixture screw with its spring,
washer, and o-ring and screw it back into the hole it came out of.
Then turn each idle mixture screw back out the same number of turns
you wrote down. When you get the engine started and warmed up, you can
tweak with the idle mixture screws to get the throttle response you
want.
The mistake that amateur tuners make is that they expect the
idle speed to continue to increase as they screw the idle mixture
screws counterclockwise.
At some point, the idle mixture becomes far too rich, the exhaust note
becomes dull and thudding, and the idle RPM slows down and the engine
tends to stall.
Then the amateur tuners compound their misunderstanding by
turning the master idle knob until the engine idles at the specified
RPM.
When the engine gets hot, it idles far too fast, hanging up at 4000
RPM or more. So they turn the master idle knob down and they are back
to the stalling from excessively rich idle mixture.
The whole idea of adjusting the idle mixture is to get the engine to
run smoothly with the smallest amount of butterfly opening and the
least number of turns of the idle mixture screws.
So the amateur mechanic whose engine is idiling far too fast when the
engine is hot needs to turn each idle mixture clockwise the same
amount, and turn the master idle knob counterclockwise a bit and turn
the idle mixture screws clockwise a bit and turn the master idle knob
counterclockwise a little until smooth idle at the specified RPM is
achieved.
How many turns out should the idle mixture screws be?
It's impossible to say, without knowing what size the idle jets are.
The area of the fixed orifice is dependent upon the radius of the
hole, squared, times pi. If you have a #30 idle jet, the diameter of
the hole is 0.30 millimeters. If you have a #40 idle jet, the diameter
of the hole is 0.40 millimeters. You would think by just looking at
the numbers, the #40 idle jet would flow
25% more fuel than the #30 jet.
But, when you work out the area of the orifice, you can see that the
area of the larger jet is about 75% greater than the smaller jet. So
nobody can accurately guess how many turns you should set your idle
mixture screws at, unless they know the idle jet sizes.
With #30 idle jets, you might turn your idle mixture screws out 2.5 to
3.0 full turns to start tuning. If you have #45 idle jets, your
initial idle screw setting might be as little as 1/4 of a turn from
fully tight.
