Yamaha XS400, Mikuni BS34: What's this hole in my carb?

I have an '81 Yamaha XS400 with dual Mikuni BS34 carbs. I cleaned the
carbs once before, but then could never get it to idle right. Had to
play with the choke to get it to idle normally, which I didn't want to
try to fiddle with while riding. So I'm at it again:

On 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?

Pic: http://i16.tinypic.com/432dtlw.jpg

Thanks...

 

Does it idle now?.......... They sorta look like they should be there. Maybe
they are some sort of airbleed to allow the diaphram to move and crap in the
carb is always bad. Does it idle now?

 

Does it idle now?.......... They sorta look like they should be there. Maybe

I wish I knew... my solar charger failed to keep the battery charged
while I had it stored for the winter. I have to find a way to get it
started now.

It's just one thing after another...

 

Suggest you do some searching on the Mikuni USA website. There's
a .pdf manual there for their normal carbs, may also be one for CVs.

--
Ivan Reid, School of Engineering & Design, _____________ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

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.

 

Suggest you do some searching on the Mikuni USA website. There's

I did find http://www.mikuni.com/fs-manuals.html, but I don't know
enough about carbs to tell which kinds are CV or what. I also found
http://i19.tinypic.com/3324bci.gif which was helpful in identifying
some of the other passages, but the mystery hole doesn't seem to be
called out in it. At first I thought it might be part of the starter
jet system, but when I blow compressed air in the little tube (not the
main jet, the other one with a hole in the side) that sticks down in
the flow bowl, air comes out the choke opening so I don't think that's
it.

Another thing I'm interested in is the pilot screw setting... but I'd
be happy just to have a bike that idles.

 

flow something, why do I see brass a short ways down the hole?

Ah, thank you!! Your explanation makes sense. I guess I'll have to do
more work to get the gum out. Yeesh, sure makes me understand the
value of adding cleaner and stabilizer to the fuel tank -- what a pain.

 

CV stands for "constant vacuum" or "constant velocity" at the venturi
under the vacuum operated slide.

CV carbs will have a rubber diaphragm or a vacuum operated piston that
lifts the slide to which the jet needle is attached.

In older, slide valve carbs, the slide was the throttle and the rider
had direct control over the slide's position. If he turned the
throttle grip too far, the jet needle could be pulled too far out of
the needle jet and, since the slide valve carburetor doesn't "know any
better", a huge gulp of fuel would be sucked into the cylinder, only
to be ejected out the exhaust pipe, unburned.

CV carburetors give control of the slides to the available engine
vacuum. Vacuum doesn't actually pull the slide up, it's *air pressure*
under the diaphragm or piston that actually lifts the slide. Engine
vacuum pumps air from above from above the piston or diaphragm, and
air from below lifts the slide.

Nice diagram of a SUZUKI carburetor, it shows the cross-drilled holes
in the pilot jet and the rubber plug under the idle jet that a Mikuni
BS34SS would have.

Did you find rubber plugs concealing the idle jets?

Maybe what you have are actually HITACHI carburetors. Some Yamaha
owners have expressed displeasure with the lack of tuneability of
Hitachi carbs, some of which didn't have interchangeable screw-in
jets.

That little brass tube IS the "straw" with the starter jet in it.

As previously stated, the number of turns on the pilot screw is VERY
dependant upon the area of the idle jet orifice, and anybody who tries
to tell you an exact beginning setting is thinking of the old slide
valve carbs where 1.0 to 1.25 turns out was a typical setting.

But slide valve carbs had AIR screws, not gas screws.

 

I also foundhttp://i19.tinypic.com/3324bci.gif which was helpful in identifying

Yes, I did.

They are stamped 'MIC" on the side... dunno if that means anything.

I'm pretty sure I got all the other jets clean with B12 and compressed
air... it's just these last two "pilot air jet" holes up on top.
Unfortunately there are some rubber seals on the "choke" (cold
enrichment device) rod so I can't dump the whole thing in a bucket of
cleaner overnight, so I'm relegated to spraying a puddle in there and
going at it with toothpick, I guess. But, I should be able to get
cleaner in there better once I remove the pilot/idle mixture screws
(after counting how many turns they're up from the bottom).

Thanks a million for the info! This is all information that should
have been in the book, if you ask me... so I'm grateful for the help.

 

I did find http://www.mikuni.com/fs-manuals.html, but I don't know
enough about carbs to tell which kinds are CV or what. I also found
http://i19.tinypic.com/3324bci.gif which was helpful in identifying
some of the other passages, but the mystery hole doesn't seem to be
called out in it. At first I thought it might be part of the starter
jet system, but when I blow compressed air in the little tube (not the
main jet, the other one with a hole in the side) that sticks down in
the flow bowl, air comes out the choke opening so I don't think that's
it.

Another thing I'm interested in is the pilot screw setting... but I'd
be happy just to have a bike that idles.[/QUOTE]

http://www.factorypro.com/tech/carbtun.html

How to set your carburetors, from people who know what they're doing. HTH

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

If you go to www.partsfish.com or (www.bikebandit.com, I find
partsfish faster to use) and look at the CARBURETOR diagram, you can
see the rubber plug, item #34.

The size of the pilot jets is not given.

Item #8 is misidentified, it's actually the Main Air Jet, which is
removeable. It supplies emulsion air to the to the needle jet tube.

The pilot air jet is not shown because it's cast into the carburetor
body during manufacture.

Just squirt B-12 through the passages and see where it comes out.

I don't recommend actually soaking an entire carburetor in
B-12, Gum Out, STP, or whatever. I have been squirting aerosol
cleaners through the passages for 30 years now and haven't damaged any
rubber parts.

The area around the cold enrichment device really doesn't need to be
cleaned that thoroughly, the air passages are relatively huge compared
to the tiny idle mixture passages.

That's what some of us are here for.

BTW, that Mikuni webpage you listed a link for is dedicated to
aftermarket racing carburetors, Harley carbs, watercraft carbs, and
carbs for old dirt bikes. It's not much use to riders who have
Japanese street bikes with CV carbs on them.

 

Bwahahahahaha! Where do you think the kids running speed shops learned
to tune carbs, if it wasn't from the original California hotrodders
who built the first superbikes and the California motojournalists who
divided their time between teaching motorcycle mechanics and writing
about speed tuning in motorcycle magazines?

 

I think Google ate my first post, so, sorry if this is a dupe.

It appears that there is brass in the middle of the passage:

http://i13.tinypic.com/4bjjvc6.jpg

Sorry the pic isn't great but it's a lot more obvious in person. It
doesn't look at all like a jet with a hole in it, it looks like a
brass plug is filling the passage. And I don't see any side branch or
passage where air might flow. Any thoughts?

 

Yes. That is actually a picture of the eyesocket of one of the aliens
captured after the UFO crash at Roswell. ;-)

I can imagine seeing a screwdriver slot in the brass if I study the
picture long enough. But there is no *removable* pilot air jet listed
on the CARBURETOR diagram on www.partfish.com, so the PAJ is probably
a bit of brass cast into the carburetor.

Perhaps Mikuni decided to plug a passage with brass? What would that
mean? The carburetor still needs a source of air to mix with the
gasoline coming from the idle jet.

Your mission, should you choose to accept it, is to find all the ports
and passages and jets in your idle mixture circuits and spritz them
out with carburetor cleaner.

If the hole you photographed doesn't contain the pilot air jet,
the *other* possible location of the PAJ is in the carburetor inlet
bell.

You will probably see a large brass jet with a screwdriver slot. That
should be the main air jet that mixes air with the gasoline coming
through the tube the main jet screws into.

It's on the right side of the inlet bell on my GS-1100's four Mikuni
BS34SS carburetors.

There is an oval hole at the top of the inlet for air to lift the
slide.

There should be a air hole on the left side of the inlet about 1/4
inch diameter for the cold enrichener.

There could be a small diameter hole in the inlet which is a vent for
the float bowl, if there is no vent that has a plastic tube attached
to it.

I only see three holes in the inlet mouth of my GS1100's carbs.

Somebody once did a thorough teardown and cleaning of at set of Mikuni
carbs on www.thegsresources.com. You might want to look at those
pictures.

 

Made some progress tonight. I appear to be running rich if I read the
above narrative correctly. When I first start it cold, it revs to
4,000 for a brief moment then idles at the correct RPM (1,200) though
with a bit of a lumpiness. Then as the engine warms up, it idles at
4,000. Doing anything with the "choke" kills it.

The idle mixture screws were set to 5.0 and 5.5 turns. It got too dark
to continue, but tomorrow I shall try screwing them in one turn each
and see where that gets me.

I think I found everything. The air jets are in the inlet bell of the
carb (called out in my crummy Haynes, no less), and I also saw the
oval opening for top of the slides and the opening for the "choke". I
see a hole in the float bowl on the opposite side as my mystery hole
which may be a vent, but I'm not sure where it goes. I'm left hoping
that my mystery hole is just a leftover from the manufacturing process
to create the passage for the idle mixture screw or the three little
transition ports.

 

Yes, your idle mixture is far too rich, so you are getting extra
gasoline through the transition ports that are prematurely uncovered
by the throttle butterflies which are too far open because the idle
mixture is too rich.

Who drilled out the anti-tamper plugs? The culprit probably screwed
the idle mixture screws out too far, not knowing what he was doing.

The rule of thumb is that if you have to open the idle mixture screws
more than 3.0 to 3.5 turns, your idle jets are too small.

I'm not saying you should run out and buy $10 worth of idle jets to
make your idle screw settings meet the rule of thumb.

I don't know what idle jets you have in there because the
partsfish.com diagram doesn't specify what they are.

But I would start at 2.5 full turns open and work with the screws,
tightening them equally and turning the master idle knob
counterclockwise until I got the fastest smooth idle with the least
amount of throttle opening.

That may be anywhere from 2.5 to 1/4 of a turn open, depending on what
size the idle jets are.

You could be right. Mikuni has made changes to carbs before, without
bothering to tell us customers about what they did.

The knowledge gradually gets trickled down from the factory
technicians on the race teams to the hot rodders and then to the
magazines. But nobody tells the commuter motorcycle owner what's up
with his carbs.

 

I have a 79 xs750. My bike often sits for months and I have the same
problem you describe every time I try to start it. I've found that it
is caused by the fuel pickup tube being blocked. If you remove the
fuel bowl the pickup tube slips into a passageway that is drilled into
the edge of the bowl. This drilling does a right angle turn into the
fuel bowl. The problem is that this passageway has a tiny jet pressed
into it with a miniscule orifice. It's very easy for this passage to
become blocked with sediment or varnish. Soak the fuel bowl overnight
in carb cleaner and then blow it out with the carb cleaner straw and
compressed air. The bottom line is you have to make sure this passage
is open because this is the idle fuel circuit pickup. Make sure that
the pickup tube is open also. This passage goes directly to the tiny
hole that you see in the carb body by the butterfly valve. There are
three other tiny holes just b4 the butterfly valve also. These are
fuel passages for the choke circuit. This is why the bike will only
idle with the choke pulled. When you engage the choke, fuel passes
through these holes and enriches the mixture. When the bike warms up
and you close the choke, fuel is supposed to pass through the single
hole that is the idle fuel circuit. This circuit should always be open
(not gunked up). The idle mixture screws are part of this circuit also

 

Absolutely WRONG! The brass tube is the "straw" that the starting
enrichener sucks gasoline through. It has absolutely NOTHING whatever
to do with the idle mixture. The pressed in jet that you see in the
bottom of the brass tube is the starter jet. It doesn't appear on the
carburetor parts diagram because it is not ordinarily replaced. You'd
have to buy a whole carburetor body to get a new starter jet.

Again, you are absolutely WRONG. The "choke" uses a separate passage
that goes around the butterfly. The starter outlet port is around
1/8th or 1/4 inch in diameter. There is NO WAY you could mistake that
single port for the three holes.

The three tiny holes by the butterfly are the transition mixture
ports. They give the engine a much needed shot of fuel air mixture as
vacuum drops off when you first crack the throttle to accelerate from
a stop.

They get their fuel from the idle jet that is concealed by a rubber
plug. The idle jet has to draw its fuel through a diagonal
passage that goes to the main jet.

http://i19.tinypic.com/3324bci.gif is a little confusing to look at.
There is no connection between the starting enrichener and the idle
mixture circuit, but the drawing makes it appear that there is.

Again, you are WRONG. The single hole is NOT the idle fuel circuit, it
is the idle mixture PORT.

You are throwing terms around that you do not understand.

The idle circuit is a branched passageway with three intake jets and
four outlet ports

The idle jet gets fuel through the main jet. The idle mixture circuit
gets gasoline through the idle jet.

The idle mixture circuit gets air through the pilot air jet.

The air and fuel mix in a passageway that branches out to feed the
idle mixture port and the three transition ports.

Only the idle mixture port is adjustable, the transition ports are
controlled by movement of the trailing edge of the throttle butterfly.

The idle mixture screw acts as a sort of "trimmer" on the idle mixture
circuit. Throttle butterfly position is critical to the operation of
the idle mixture circuit.

The mistake that amateur tuners make is believing that the engine RPM
will continue to increase as they turn the idle mixture screw
counterclockwise.

When they turn it too far, the idle mixture becomes too rich and the
idle RPM falls. They compound their mistake by turning the master idle
knob clockwise to raise idle RPM.

This uncovers the three transition ports and then the engine gets far
too much fuel at idle. It responds by idling far too fast, around 4000
to 5000 RPM.

The amateur tuner may turn the master idle knob back down, but then he
is stuck with the problem of low RPM idle and stalling.

If amateur tuners just understood how the idle mixture circuit worked,
they would know that they had to turn the idle mixture screws a little
clockwise to lean up the idle mixture.

 

Argh, I am losing this battle. Screwed the idle mixture screws down to
3.5, 3.0, 2.5, and now 2.0 but I still have to give it some throttle
(or a huge amount of master idle ajustment) in order to get it to
idle. I think it's backfiring through the carbs at 2.5 and 2.0 but at
3.0 it sounded like afterfiring in the pipes (not really sure, I am a
total noob at this). Is it really so sensitive that I'd have to do
something like 2.75? Is there a good way to distinguish lean
backfiring vs rich afterfiring?

I'm at my wits end here...

 

It sounds like your carbs are either still dirty inside, or the float
level is too low. If you have to use a lot of throttle, you create
more vacuum and make the engine suck harder, and perhaps it's getting
fuel from the needle jet, instead of the idle jet.

When you spritzed the passages out with carb cleaner, did you see a
nice stream of liquid come out each of the three transition ports and
a stream come out of the single idle mixture port? Did you remove the
idle jet and squirt carb cleaner through it to get a stream of liquid
come out the four idle ports?

If the carbs are that clean, the engine should idle down on a small
amount of throttle opening.

Some people talk about adjusting their idle screws down to 1/16th of a
turn one away or another to get the mixture perfect.

I generally do idle mixture adjustments 1/4 turn at a time. I'm happy
when I get good throttle response, the idle RPM doesn't hang up too
high when the engine is warm and the engine starts on the "choke".

I don't have an exhaust gas analyzer to adjust my carbs to factory CO,
NOX and HC emissions standards, but EGA's don't work right on engines
with 4-into-1 exhaust systems, they only manage to read an average of
the four carbs on a 4 cylinder machine.

The idle mixture screws on my FZR1000 had an incredible difference in
adjustment. One was 1/8th of a turn open, two of the screws were about
3/8ths of a turn open, but the last screw was 1-1/4 turns open!

And that was the factory setting! I don't know how the hell Mikuni or
Yamaha adjusts idle mixture at whichever factory do it, but they sure
missed the setting on my Yamaha...

Another expen$ive way to get the idle mixture set right is to drill a
hole into each exhaust pipe and weld on a threaded spigot so a
thermocouple can be screwed into the hole. When the exhaust gas
temperature is around 1150 degrees, the fuel air mixture is perfect,
as I recall from reading private airplane manuals.

But, you don't need a lot of fancy and expensive equipment to adjust
idle mixture and synchronize a twin cylinder engine.

BMW riders have said that they just make a simple manometer out of a
piece of aquarium air hose with some ATF in it.

They hook the two ends of the tube to vacuum ports on the carbs, and
adjust the idle RPM screws so the ATF is the same level on both sides
of the tube when the engine is running.

Don't twist the throttle while doing this. You'll suck the ATF into
the engine and maybe oil the spark plugs, or even hydraulic lock the
cylinder. That's bad. Rods bend when the piston suddenly stops.

Other riders have said that you can use a simple well manometer to
adjust idle mixture before synchronizing the carbs.

One end of the plastic aquarium air hose is hooked to the vacuum port
on the carb, and the other end is sitting in a jar of ATF.

You try to get the idle mixture screws and master idle knob to the
point where the engine idles at the specified idle RPM with the least
amount of throttle opening and the idle screws open the smallest
amount that results in a smooth idle.

It's the pitch of the sound. Lean backfiring is a SNAP! and rich
backfiring is a BOOM! An engine will often lean backfire out the
carburetors

Been there, done that, went through all kinds of frustration to learn
simple facts about how engines worked.