Question about Mikuni Carbs on a late 80s 600 Katana.

 

Hi all,  I got a few question about Mikuni Carbs on a late 80s 600 Katana.
And I figured this is one of the best places to post my questions.  My hope
is there are a few people out there who could share with me real working
knowledge for this carbs.

Here is my story,  I picked up a semi running Katana that had been sitting
on some guys porch.  The  bike barely ran, he offered to sell it to me.  His
story was that the bike started to run rough, so he took apart the carbs to
rebuild them, and then it ran worse.  He then picked up another set of carbs
off another bike, which ran better but still had problems.  So he rebuilt
them and then once again, the 2nd set of carbs made the bike worse.  I went
ahead and took a chance and bought the bike and the extra set of carbs.

I took the 2 sets of carbs apart and mixed and matched the best looking
parts to build the best single set that I can.

This is what I have observed, after several tear downs, reassemblies, and
tests.

1)  The bike idles good.  Pretty clean response from idle to about 2000 rpm.
2)   If the CV slides lift 100% open at wide open throttle the bike has
excellent acceleration.  No missing or stumble for the engine.
3)   The slides have a real hard time climbing past the middle.  It's almost
impossible to get them to climb.  This happens at about 5000 to 7000 rpm.
4)   Not sure what an acceptable level of slide up/down jitter is, but my
slides occolate up and down about a centimeter when I try to apply 50% or
more throttle until it climbs up.  This jitter is probably about 20/30 times
a second.
5)   All 4 slides act the same when the throttle is blipped, they all
stutter the same way.
6)   I think the carbs might be a set of BS34SS Mikuni's.
7)   The slide/diaphram on the carbs appear to be still within the service
limit.  The rubber isn't super soft and virgin, but there isn't any sort of
holes in them either.  However the slide cap (located on top of the side)
that sandwiches the the rubber diaphram to the slide had about 1 to 2
thousands play in them.
8)  When I dissassembled the carbs the first time I found the  main needle
and it's associated parts inside the slide installed in the following order.
Small nylon dounut washer, needle with e-clip installed, spring seat washer,
slide return spring.
9)  The bottom surface of the slide has 3 holes drilled in it.  The middle
hole is for the needle, the other 2 are vaccum signal ports for the chamber
above the slide.  All 3 of these holes are exactly the same diameter.
10)  The engine rpm can almost never climb past 7000,  At this speed the
engine stutters and seems to go super lean.
11)  The slides don't seem to be stuck or hanging from mechanical friction.
And the bearing surface where the slide moves seems good.

These are my questions.

1) As a practical rule, how much high speed slide jitter is acceptable?
None?  1% of the total slide travel? 10% of the total travel?

2) When installing the needle it appears that parts go in the slide in the
following order,  Needle first, nylon donut, spring seat return, then
spring.  Not the way I found them, which was nylon donut, needle, spring
seat, return spring.  Which is correct?

3)  What size should the 2 signal ports on the bottom of the slide be (Mine
are about 2-2.5 millimeter each)?  Mine are the exact same size as the
needle diameter.  This seem much, much, too large for me.  Is it possible
the prior owner was clueless and overboard these out to make them "more
responsive"?  Anyone know the factory spec to the diameter of these holes?

4)   Do the slide/diaphram assemblies on Katana carbs have a high failure
rate?  Maybe where the plastic slide cap is rivited? Or are they about as
good as any other CV?

5)   On other CV carbs how big are the vaccum signal ports on the bottom of
the slides?  10% of the needle diameter?  Do they Jitter?

6)   With mid range fuel problems what are most likely things to look for?

7)   Does anyone know, or could you look up for me what the stock needle &
jet size where from the factory?  I don't own a service manual for the bike
and have no idea what the right needle and jet sizes should be.  The needles
that came with the carb are non stepped for the E-clip.  I assume this is
the was factory needles were, so that people couldn't easly adjust them.

8)  Anyone have any other tips or suggestions?


Thanks in advance.

TheWesman

 



So your pilot jets are working well enough.


So your diaphragms aren't "holed" or torn. If they were, the slides wouldn't
get up to the top.


What do you mean by having a hard time? Have the slides worn an indentation
in the slide "rails" or have the needles developed a "step" at the half way
point?


That's natural and if you think hard about a CV carb, you'll see why. The CV
means "Constant Velocity" and that relates to the speed of the air going
through the carb. In order to achieve a constant velocity, the carb has to
"suck" the slide up and that's done by vacuum. The amount of vacuum is
governed by the speed and position of the piston when the inlet valve is
open. When the piston moves down, there' a massive vacuum and the carb slide
is dragged up. As the inlet valve closes, the vacuum drops and the carb
slide drops back with it. Up and down goes the carb slide and the inlet air
speed remains constant. Great for throttle repsonse AND fuel economy AND
emissions, but crap for wearing out carb slides:-)



Indeed they will.


It's not likely to be on an 80's Katana:-)


but there isn't any sort of

Don't worry about it.



As much as it needs to maintain a constant velocity. It depends on throttle
position and engine load/speed.


I wish I could tell you, but I can't. If you've got a pic of the CORRECT
way, do it that way, not as they were on the bike if the bike was running
rough.


You need a manual for this bike you do. Clymers or Haynes would help.


Wear rate is usually dependent on use, but everything is replaceable.


They all "jitter"


Needle height and float level.

Depends on the country they were destined for, as does jet sizes. American
jets are bigger than European jets because your fuel is different to ours.


Clymers or Haynes and a credit card:-)



--
Beav

Reply to "beavis dot original at ntlworld dot com" (with the obvious
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