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.