XS400 carb swap from later model

Hey, maybe someone here can help me. I have been building a track bike
out of an old junked xs400 1980 model. Harness seemed in good shape,
engine great, carbs rusted and full of crud from mud wasps. Found a
set from an 82 XS400 Maxim in perfect shape for $20. Modified the
mounting brackets to fit wider mounting (they are closer together on
the 82 head). Figured they would be very similar, found that main jets
were .010 smaller, and pilot jets were bigger. Also, the idle jets do
not have holes in the side of the jet like the older ones. I swapped
all of the old jets to the new carbs, but I still can't seem to get
the thing to idle correctly. With the newer jet setup, it will crank
and run but once it is warmed up you get bad throttle hang when you
let go of the gas (the cable is not binding), and bogging at full
throttle. With the newer jet setup it runs great at full choke but
won't idle. High flow cone filters and straight pipes. Anyone know a
way to get this thing to idle and run without bogging?

 

Pardon, on that last part the older (1980) jet setup causes no idle,
save at full choke. That is using the 1980 jets in the new 1982 carb
bodies. Using the 82 jets in the 82 bodies allows for idle, but bad
bogging at WOT and bad high idle hang at closed throttle.
I am assuming that these carbs are essentially the same bodies save
different jets, floats, bolt ons.
Correct me if that is not the case.

 

Idle RPM hangup when you close the throttle is due to having the idle
mixture screws turned out too far, and *somebody* compensated for
excessively rich idle mixture by turning the idle speed up too far.

This uncovers a pattern of acceleration transition ports that are
controlled by the edge of the butterflies.

This doesn't work worth a damn, the engine bogs on too much gasoline
at very small throttle openings and doesn't clear out until you get
the throttle open more than 1/4 of an inch.

The acceleration transition ports are not supposed to be uncovered
until you barely crack the throttle open as you try to accelerate from
a stop.

The single idle mixture port is rather small on a CV carb, so engine
vacuum can't pull enough mixture through the one hole, so it has to
suck through the other three holes.

Of course you shouldn't be trying to use cross-drilled emulsion type
idle jets in a carburetor that didn't come with them. Emulsion type
jets premix air and gasoline before the idle mixture gets to the idle
circuit.

If you want to look at pictures of all the different jet types, go to
the Factory Pro website and also look at their CV carb tuning guide.

 

Thank you so much for the info. I didn't intentionally richen the mix,
the screws were set at 1.5 where they were. I did try to increase the
idle speed, going to play around with it. I am going to put the stock
jets back in, and leave the larger main jets to help compensate for
the pod filters. Another point. The newer carbs did not have the
rubber plugs in the little jet tube, is that normal or simply the
owner forgot to replace them? All the carbs I have dealt with have
them, but I am sure some don't. It also seems to want to idle on one
cylinder, until the mains kick in and then you get both firing. I
think that might be a dying coil/CDI module, possibly unrelated to the
carbs. Again thank you, I really want this to work, the bike turned
out very nice, got my new tires on and covered my seat pan, I want to
ride her!

 

You can get into trouble swapping idle jets if you don't understand
how the sizing works. You need to understand how the numbering system
for Mikuni and Keihin works.

A #100 main jet has an orifice hole that is 1.00 millimeters in
diameter, so a #125 mainjet has an orifice hole that is 1.25 mm in
diameter.

A #30 idle jet has a hole that is 0.300 mm in diameter and a #42 idle
jet has a hole that is 0.420 mm in diameter. Work out the area = pi X
radius squared formula, and you will see that the area of the #42
pilot jet which you probably have is much larger than the #30 jet's
area.

Carburetors with #30 pilot jets will usually require the idle mixture
screws to be turned out 3.0 to 3.5 turns from lightly seated, but a
carb with a #42 idle jet may need the idle screw to be turned out less
than 1/4 of a turn.

The 1982 XS400J came with #125 mains and #42.5 idle jets. I can't
imagine you needing anything larger for an engine that small.

I don't see any rubber plugs on the parts fiche. Maybe your Yamaha
came with Hitachi carbs, not Mikunis?

It sounds to me like you may be talking about Mikuni BS34SS carbs
which had a
rubber plug in the float bowl concealing the idle jet, and the idle
mixture screws came down from above.

The BS34SS idle jet is supposed to get its fuel through a diagonally
drilled passage
that gets fuel after it passes through the main jet.

A BS30/96 idle jet has ONE cross-drilled hole near the end. You can
look at the Factory Pro website I mentioned previously, or you can
look at Sudco International's website.

I suspect that is actually caused by LEAN idle mixture. The single
idle port underneath the idle mixture screw may be plugged up. It's
easy enough to squirt some Berryman B12 or GumOut or STP down that
hole.

CV carbs don't go onto the main jet until the slide lifts all the way
at 7/8ths to full throttle. Until that time, the tapered needle
restricts the fuel flow.

That's doubtful. As much as a parts geek at a $tealer$hip might want
to sell you a CDI module or a coil, a CDI usually either works or it
doesn't, and the worst problems with ignition systems nowadays are
just bad connections in the wiring harness.

 

Thanks a ton. You really know your stuff. I sort of figured the same
thing about the CDI, either I might have a weak battery or the mix was
wrong. But the battery is new... The 80 XS400 had 135 mains, and the
same idle jet save the crossdrilled holes. The 82 carbs have 125 mains
as you said. The carb bodies APPEAR identical Mikunis #s and all, save
the drain screw is different in the 82 model & it had no rubber plugs,
and has plastic floats.BS34SS 34mm's.
I am going to tear back into them, I assumed I had everything clear
but I will take another look and clean again.
With the pod filters and straight pipes, and assuming that the 80
model originally had 135s, should I use the 125 mains on the carbs, go
with the 135s, or maybe 140s? Thank you again, I am confident I am
close.

 

Your Yamaha wouldnot have a real CDI, which means "capacitor discharge
ignition", but ignorant mechanics and parts counter dorks have gotten
into the habit of calling transistor ignition modules "CDI" and we're
stuck with it.

Ignition modules work, or they don't work, there is no in-between
operation where they sort of work occasionally.

The 80 XS400 had 135 mains, and the

Getting all the branches of the idle mixture circuit requires spraying
Berryman B12 or GumOut or STP carb cleaner through the idle jet and
the pilot air jet and having a good stream of carb cleaner squirt out
through the single idle mixture port that is controlled by the idle
mixture screw and the three acceleration transistion ports down stream
of the throttle butterfly.

The pilot air jet is sometimes in the carburetor intake, and is
sometimes underneath the rubber vacuum diaphragm.

I can't imagine how an engine that small can use #135 main jets. I
can't use #135's on a GS1100 with pistons that are larger than your
XS400 pistons. I use #125's in my large bore bikes.

Now, if you were running wide-open throttle around Daytona, you'd need
to waste a lot of fuel to keep that air-cooled motor cool but I doubt
that you will ever ride full throttle for more than a mile.

There is a way to tell whether your mixture is marginally lean. When
an engine pings, it deposits little black specks on the nose of the
spark plug that look like pepper. That tells you that you are as lean
as you can safely go on the total jetting.

The next step after pinging is aluminum throw-off. When the total
mixture is seriously lean, the top of the piston starts melting and
you see little silvery aluminum balls on the spark plug.

Tuners used to tell you to look for a light tan color on your spark
plug nose to indicate proper mixture, but that was back in the days of
leaded gasoline.

Nowadays, unleaded gasoline leaves a sooty black ring about 1/16th to
1/8th inch wide deep inside the spark plug where the hot insulator
meets the cooler steel body
and that's why you see race tuners peering down inside the plug with a
magnifying flashlight.

With the advent of 10% ethanol gasoline, an engine that is jetted lean
from the factory will run even leaner, because ethanol carries some of
the oxygen needed to burn it.

If you read the "carburetor tuning guides" that tell you to start
jetting by selecting the main jet that allows the engine to run best
at full throttle, you may get the impression that the main jet and the
idle jet work only at different throttle settings, but this is not
true at all.

The engine sucks more and more fuel through the main jet as you open
the throttle more, but it sucks fuel through the idle jets until the
butterflies are wide open.

It's just that the engine sucks less and less fuel through the idle
jets as you open the throttle more. But that's why I say that you need
to work on your total jetting and get
your idle mixture set for optimum throttle response as you crack the
butterflies open.

 

Will do. I will let you know what happens. I am not sure why it has
135s, I thought that was kindof large as well. Possible that someone
put them in there, but it had stock everything and only had 4000 miles
on her when she went down (for 20 years in my cousins barn).
Hopefully, with a little luck, I will be able to run her this spring.
I appreciate all of your information and experience!!! I have a
GS850G, BTW, I love that bike more than any other. Any good way to get
the rear end to stop sliding? Seems like everytime I hit a hard corner
or brake it hard, it wants to come around. Just being shafty I guess.

 

Well, those motorcycles came with hard tires and narrow rims and the
feeling I got in corners was "weak arse", the rear end felt like it
was sinking, no matter what the tire manufacturer was.

A shaft drive might tend to resist the sinking sensation, but the CV
carbs drop their slides suddenly when you roll off the throttle so
that makes the rear end squat a little bit.

A wider rear wheel with a softer tire that fits the wider rim would
help, but the only manufacturer that ever made wider wheels was
probably Lester, back in the early 1980's. If you could find a 3.00-17
or 3.00-18 Lester rear rim and a 2.50-18 Lester front rim you could
make the 850 handle better.

A set of Works Performance shocks would really help rear traction too.

But, why don't you just buy a cheap old sportbike? They came with
wider rims so you can use wider radial tires...

 

The XS400 was points ignition.

 

And ignorant dorks don't realise that the XS400 was points ignition.....

 

For Christ's sake. The GS850G is a shaft drive bike, so rear wheel swaps
are not really an option.

 

Depends how determined you are, at least one guy on the Concours Owners
Group forum has had a 17 inch rim welded to the hub of the ZG1000's OEM
16 inch rear wheel. About $450, IIRC.

 

Yes, true, and I've seen a few shafty sidecar outfits with interesting
(and expensive) rear wheel mods.

But in this case, my money is on Krusty not realising that we were
talking about a shafty, and weighing in with his usual ignorance.

 

Maybe those ignorant dorks are British dorks.

Item #18 IGNITOR UNIT ASS'Y 1 $482.21 Call Us

 

What's with this "we" shit? You weren't even in the thread until you
stuck your pointy nose in with disinformation about the XS400 having
ignition points. It didn't.

And Lester made wheels for BMW's, how do you know they didn't make
them for GS850G's?

 

Wrong.

 

http://shop.wemoto.com/pictures.dyn?u=2147869qqq5827940

http://shop.wemoto.com/pictures.dyn?u=2147869qqq5827939

They made two types :-/

 

http://shop.wemoto.com/pictures.dyn?u=2147869qqq5827940

http://shop.wemoto.com/pictures.dyn?u=2147869qqq5827939

They made two types

BMW rear wheels were interchangeable between many models, so it was a
case of one size fits all. And a lot of BMWs were sold. Also, a lot of
BMWs of the era had spoked wheels, so people were in the market for
alloy wheels. The GS850G had alloy wheels as standard, and Lester didn't
make wheels for them.

End of.

 

http://shop.wemoto.com/pictures.dyn?u=2147869qqq5827940

http://shop.wemoto.com/pictures.dyn?u=2147869qqq5827939

Heh. They made two types :-/