Ping Mr krusty and anyone else that can help! carb help required again...

Greetings! still having carb issues here 1982 KZ1000 overbored to 1105
with a set of 4-2 headers, GSXR Mikuni radial flatslide carbs. Cleaned for
the 4th time, kremed the tank this time, installed 2 high flow fuel filters
inline as well as clear fuel line (so I can visually inspect for adequate
flow) Starts great, idles crappy, popping through 1 or maybe 2 of the carbs,
when riding it it has lots of power with partial (about 1/4) throttle but
bogs and spits horribly when trying to rev past 5000rpm or semi to WOT.
tried running with the choke on, it smoothes right out at about 3000rpm, but
still pops and sputters above that with semi or WOT. Valve clearances are
all within specs, lots of compression, ignition seems ok....I'm getting
nowhere, any more ideas?? btw, might have goofed when I set the pilots at
3/4 turns out hoping to rectify this, what would the symptoms of too many or
not enough turns out on these be??

any advice/help appreciated!

 

My advice is this. Get it to idle smooth and work from there. It is
difficult if not impossible to get a good running engine if it won't idle
right. Did you try spraying a water mist on the exhaust pipes while the
engine is at an idle? Are all the pipes getting equally hot?

Have you seen this diagram?
http://www.carbparts.com/mikuni/mikuni_tuning_info.htm

pierce

 

Those headers could be letting that engine breathe easier than before. Its
possible now that the fuel mixture is a little leaner than it should be.
That overbore will contribute to that issue as well. You might want to
consider raising your carb needles up a notch. If that makes an improvement
with your mid-range then try going richer with your main jets. Let us know
how you make out :]

 

Those are the flatslides that supposedly fit a "GSXR" and you got them
from a guy on eBay? What you need to do is go to www.sudco.com and
click on the Online Catalog link. Scroll down to Mikuni Carburetors and
download the RS carburetor .pdf file, the one on jet needles, and
anything you can find on needle jets, idle jets, and general carburetor
tuning. Hopefully, you will be able to figure out *exactly* which RS
carbs you have. I don't remember if the needle jet has P-4 or Y-6
stamped right on it or not. You'd need a set of precision metric pin
gauges to identify what needle jets you have by sticking a pin down the
hole.

The inside diameter of the needle jet is very important as you open the
throttle from 1/8th to 1/4th throttle. Then the diameter remains
important until you open the throttle from 1/2 to 3/4th's throttle.
That's when the jet needle taper becomes more important. From 3/4th's
to full throttle, you're running on the main jets.

But amateur shade tree tuners don't realize that the main jets are not
the main tunable element in their carbs. They start throwing brass at
their carbs and brass costs $$$. Most of the brass I have was given to
me by shade tree tuners who found no further use for it after jetting
their carbs for the only race track around here...

There were four RS flatslide kits with the correct 77mm-93mm-77mm carb
spacing to fit Suzuki and Kawasaki inline fours. The RS34's would be
too small for your machine, but they would fit a KZ-750 with
aftermarket exhaust and K&N filters.

The RS36's would work on a KZ-1000 with aftermarket exhaust and K&N's.

The RS38's would be for a KZ1000 with aftermarket exhaust, K&N's,
overbore, valves and head work, increased compression, higher RPM, or a
complete race engine.

The RS40's would fit a KZ1000 full race engine.

The part numbers may be acid etched on the side of one of the outer
carburetors, or there might be a metal tag attached to a screw on the
carbs.

The main jet size may have been changed by a previous tinkerer, but all
of those carbs came with 17.5 pilot jets and the pilot screw was set
1/4 to 1/2 a turn open. That's a nominal setting, if a tinkerer has
installed larger pilot jets thinking he's going to get better
acceleration that way instead of adjusting the accelerator pump, the
pilot screw might need less than 1/4 of a turn open and the exhaust
sound might be "toot-toot, toot-toot, toot-toot" at idle, indicating
the idle mixture is far too rich. Black smoke might come out the
exhaust as the rider rolled on the throttle, then the engine would
clear out at wider throttle openings as more air blew the excessively
rich idle mixture out.

The RS34 and RS36 carbs came with P-4 needle jets. That's the brass
tube that the main jet screws into. The inside diameter of a P-4 needle
jet is 2.670 millimeters, and the jet needle attached to the slide goes
down into that needle
jet and provides a *variable orifice* with MORE restriction to fuel
flow than the main jet. In other words, the difference in area between
the needle jet hole and the jet needle is smaller. In other words,
gasoline is sucked through a smaller *hole* than the main jet as you
just open the throttle a little.

So, from fully closed throttle to 1/8th throttle, you're running on the
idle jet, the fuel air mixture that gies around the throttle slide and
comes through the idle bypass passages and that tiny bit of extra gas
that comes past the pilot mixture screws.

IOW, all the idle mixture doesn't come past the pilot screw, only about
1/4 of the needed fuel comes past the pilot screw to fine tune the
mixture. When a shade tree tuner starts screwing around with idle jets
and he doesn't understand that subtle point, he's going to have an
engine that gooes "toot-toot" at idle, and the RPM hangs up when he
blips the throttle because the idle jet is too big and the pilot screws
are open too far and he's compensated by adjusting the idle RPM higher,
or he's going to have an engine that spits back through the carbs,
pings on acceleration, runs too hot when warmed up, but takes forever
to warm up and is hard to start on a cold morning.

Now, back to jet needle and needle jets and you've just opened the
slides further than 1/4 throttle. The fuel air mixture is partly coming
from the idle jets and the pilot mixture, however right-sized or
wrong-sized the jets may be, and however screwed up the shade tree
tuner may have adjusted the pilot mixture screw. But, theoretically,
from 1/4 throttle to 1/2 throttle, the fuel air mixture is supposed to
be MORE influenced by the diameter of the needle jet. Again, that's the
brass tube that the tapered needle goes in and out of. The RS34 and
RS36 carbs had the P-4 needle jets stock, with their 2.670 millimeter
holes. They came with a 9DZH1 needle that was 2.500 millimeters in
diameter and the nominal jet size for a KZ1000 with an aftermarket
exhaust and K&N's was a #120 round jet.

But the RS38 and RS40 carbs had MUCH larger needle jets, they had Y-6
needle jets. And the hole in a Y-6 needle jet was 3.130 millimeters in
diameter, so it needed a fatter needle so the jet needle/needle jet
worked as a variable orifice properly. The fatter 9CHY3 needle that
came in those carbs was 2.98 millimeters in diameter, and you can work
out the areas of the hole in the needle jet and the area of the jet
needle and whatever mainjet you have in there and see if the variable
orifice is actually smaller than the area of the main jet or if you're
sucking excess gasoline like a toilet flushing, or what...

The nominal main jet in an RS38 in a KZ1000 engine up to full race
modification was a #135 round jet.

Clear fuel line will make you have doubts about whether you're getting
enough fuel flow when you see big air bubbles in the lines, bleed the
air out, and then see big bubbles again...

As I said above, the pilot mixture screw only adjusts a small
percentage of the idle fuel air mixture. The correct number of turns in
or out is going to vary if the previous owner has changed the idle jets
from the as-delivered 17.5 idle jet.

Too big an idle jet/too many turns out makes the exhaust go
"toot-toot", you get black smoke, the idle RPM hangs up when you blip
the throttle. When cold, the idle RPM is far too high, so you have to
fiddle with the choke to keep the engine RPM from racing. If you adjust
the idle mixture so it seems OK while the engine is just warm enough to
idle smoothly, you find that the idle RPM is much too high when the
engine is hot. It drives you crazy, you get drunk and kick the dog.

Too small an idle jet/too few turns out results in an engine that is
hard to start, warms up slowly, spits out the carburetor, pops out the
exhaust, stalls when you roll off the throttle.

 

Thanks to all that have replied! I have had to take a crash course in carbs
here, I deserved it for getting into this ;/ I have since last posting,
raised the jet needle to its highest position, now I can get it running and
accelerate half decently. Still spitting and sputtering from 1/2 to WOT
which based on my newfound knowledge (please correct me if I'm wrong!)
indicates I need to replace at the very least the jet needle and the needle
jet. I'm not sure about whether the main has to be resized as I'm assuming
I should get rid of this mid to wot issue first?? BTW does anyone have a
special method for removing the jet needle in these RS carbs? What a pita, I
undo the 2 allen screws, ratchet the throttle shaft back, and use a magnet
to lift it, then tweezers to snake it past the lever and plate assembly.

again thanks all for the info and shared knowledge!
Tom B

 

Did you download those .pdf files from Sudco to determine which carbs
you have and inspect them to see which main jets, needle jets, needles,
and idle jets you have?

Do the accelerator pumps work? The accelerator pump rods are supposed
to be disconnected before re-jetting the carbs to avoid upsetting the
test with excess fuel being squirted into the venturi.

The normal order of tuning is to start by sizing the main jet with wide
open throttle runs and (with slide valve carbs)rolling the throttle on
and off from full throttle to about 7/8th's throttle and listen for the
exhaust sound to change. If the exhaust starts thudding and you hear
individual bangs instead of the desired howling sound, the main jet is
too large. You want to size the main jet so the engine achieves the
highest RPM possible on the longest straight on your test course.

After you've completed the main jet selection, you can work on the
needle position. You normally should NOT have to raise the needles all
the way. That tends to indicate a main jet that's too small

As I said previously, there were two size needle jets that came stock
in those carbs. The Sudco catalog doesn't give all the dimensions for
the taper of the needles used in the RS carbs the way it does for
needles used in earlier carburetors. But, you can chart the needles
yourself if you measure them every 10 millimeters, starting them from
the top. Got a vernier caliper?

Earlier needles were charted in the catalog according to their diameter
measured every 10 millimeters starting at the top end of the needle.
The highest e-clip slot on the needle would be about 3 mm from the top,
the lowest slot would be about 7 mm from the top of the needle.

You can figure out which needle jets and jet needles you have if you
measure the upper diameter of the needle *before* it starts tapering to
a smaller diameter. If you calculate the area of the hole in the needle
jet and calculate the area of the jet needle where it is NOT tapered
and subtract the area of the jet needle from the needle jet in the
untapered area, you'll see that the difference in area is very small.

But, if you measure the jet needle's diameter at a point about 3/4's of
the way down from the top, the area of the needle jet minus the jet
needle's area should be about the same as the area of the main jet.
That's when the main jet takes over, it then controls the flow of fuel
through the needle jet jet needle orifice.

So, from 1/4 to 1/2 throttle, the needle jet diameter has the greatest
influence on mixture, the jet needle's taper takes over from half to
3/4's throttle, and the main jet controls fuel flow from 3/4's to full
throttle.

Once you get the main jet sized correctly, you can adjust the needle
position, then prime the accelerator pumps and hook up the pump rods
and adjust the stroke according to the Sudco .pdf file...

 

Well, that surge of info just cured me of ever wanting to mess with the
carbs on my bike. I've rebuilt auto carbs (including a Quadrajet done on a
picnic table) successfully, but fine tuning a set of 4 bike carbs seems like
just too much trouble. I guess this winter I work on that single carb
conversion project...

 

Whether it's a multicarb set up, or a single pot on a log manifold, the
carburetor circuit tuning sequence is the same, so you're not going to
escape the tuning problems everybody goes through.

The OP seems to have some very refined racing carburetors with radial
flat slides and accelerator pumps, but he bought them on eBay and
doesn't seem to know exactly what bore the carbs are, so I've been
shotgun troubleshooting with him to help him figure it out...

 

and I'm very grateful, for the help! Here's the most recent update. I've
swapped the mains for a larger set, don't know the sizes, everyone that was
around the house here while I had the carbs disassembled (young eyes old
eyes drunk eyes and sober eyes) took a stab at looking at the mains with a
magnifying glass but no one could decipher the markings as anything that
would identify the size. So I pirated the visually larger mains from an old
set of CV carbs from one of my old KZ's. I installed these, set the needle
at the #3 position as a starting point and re-installed them. Voila. 100%
better. Now I only have a very bad stutter at extreme WOT. However, still
spits back through the carbs, smokes at startup and low idle and stalls
intermittently till warmed up.

This pretty well describes where I'm at! I've set the pilots at about 1/2
turn out. Still have too crank up the idle knob to get it to idle with no
choke, then turn it down once it gets warm to prevent it from racing. It
does not however, hang-up at higher rpms. Smokes like the rings are shot
till I rev it too clear out the passages, then once its warmed up behaves
like it should except at wot.

thanks again

Tom B

 

Extreme WOT is an oxymoron. You've either opened your throttle all the
way or you haven't.

The diference between how the engine runs at 7/8th's throttle and how
it runs at WOT is a major CLUE about whether your main jets are too big
or too small.

It marks the final transition from mixture being controlled by the
needle and mixture controlled by the main jet.

If the engine sound is a high pitched howl at WOT, you're either jetted
about right, or you're lean. If lean, the engine won't pull as hard as
it should and it will get hot. If the exhaust note changes to where you
hear distinct thuds, the mixture's a little rich. If it stutters and
blows black smoke, you're too rich, and when you roll the throttle back
to 3/4th's to 7/8th's throttle, the needle jet area minus jet needle
area is once again smaller than the area of the main jet, so the nj-jn
area is controlling fuel flow to the venturi. The engine goes back to
its normal howl, but not as loud at it was at WOT.

Surging at WOT or 3/4th's to 7/8th's throttle indicates a very lean
mixture.

If you still have an airbox, and you've removed the snorkel, put it
back in, engines will surge like crazy with the airbox lid on and the
snorkel removed,
as the resonant frequency of the airbox is changed and air does NOT
like to flow smoothly through a simple hole in a flat surface, it gets
seriously turbulent.

During surging, the engine will not hold a steady RPM, the motorbike
bucks like an angry horse that doesn't want to be ridden. Surging is
particular annoying on the racetrack in high speed sweepers where
you're leaned over at a severe angle. You have enough problems with
traction and cross winds, a surging motor makes for a wild ride.

You might mistake surging for stuttering by the sound, but when you do
your plug chop there won't be much color on the insulator nose.

So, what does your plug chop look like? I've described how to do a WOT
plug chop many times, you can google for "spectraltarsier@aol +plug
chop" to read it. Is the insulator nose all covered with black soot, or
is there just a little bit of greyish-white at the very tip and the
rest is fluffy black?

As I've said many times, you're looking for a black sooty ring maybe an
1/8th of an inch wide way deep down inside the spark plug, where the
insulator and the steel shell meet. The rest of the insulator nose
should be white, but not rough looking. That narrow ring indicates a
safe mixture at WOT.

Two-valve engines take longer to burn their mixture and four valve
engines, a 2-valver takes about 90 degrees of crankshaft rotation to
burn the mixture, a modern 4-valver takes about 64 degrees of
crankshaft rotation to burn the fuel, so there's less fuel to deposit
on the interior surfaces of the combustion chamber and spark plug.

Since they burn their mixture in about 2/3rd the time a 2-valve engine
completes burning, a 4-valve engine will have a really narrow ring of
soot.