relentless carbon fouling problem

And can we say that that is not necessarily because of too much fuel,
but may be because of fuel not being burned, maybe because of an
ignition problem.

I will pursue the idle mixture question, but please let me ask one
question about ignition.

There is a wire that supplies +12V nominal to the primaries of both
coils. When I measure the voltage, it is about 1.2V less at the coil
primaries than at the positive battery terminal. I claim that those
voltages should be the same. It would seem to be evidence of faulty
connections. Somebody please confirm or deny that.

To test, I would bypass the ignition switch and the kill switch and all
the wiring in between by running a wire directly from the +12V battery
terminal to the coil primaries.

 

I took the filters out of the airboxes and put in new plugs again. It
seemed to have very good pep at first, but soon again the performance
got bad, and the left plug was almost completely black after three
miles. The right plug looks fine, though.

I checked the timing after that test ride, and it is still okay.

 

*Ding*

 

It's true that motorbike engines have critical problems with fuel/air
ratio requirements because of the low voltage output of their
relatively tiny coils. The fuel/air mixture has to be richer than
needed for stoichmetric combustion for two reasons. The first is to get
a mixture reich enough so that molecules of burnable hydrocarbons will
randomly be near the spark when it occurs. The other reason is that an
IC engine needs to waste fuel to cool itslef internally...

With the relatively weak coils, I would definitely want all the primary
voltage I could get at the + terminal of the coil. But, with a 1000:1
turns ratio, if you have 90% of the primary voltage, logic says you
should be getting 90% of the secondary voltage at the plug gap...

Well, you certainly don't need my permission to try that experiment. It
shouldn't hurt anything to try...

 

Maybe it's better to have a wrong idea than a strong belief that's
based on a wrong idea. Ideas can be changed, beliefs are obdurate. Who
was it that said ideas were harmless and beliefs caused wars?

My answer to your question is based upon my belief in what the
motorbike magazines and the technical manuals tell me. It's not my own
idea...

A CV carb draws a lot of the gasoline it needs to run at cruising
speeds through the idle circuit. The bigger the engine, the more it's
running on the idle circuit. Really small engines will be running at
full throttle, passing little mixture through the idle circuit. If you
ride a big bore motorbike on the highway, you're only using about 1/8th
throttle, and running on relatively huge idle jets...

Performance carb tuning guides disinform shade tree mechanics who
really don't understand how the CV carb works. They get the impression
that the idle circuit works from closed throttle up to about 1/4
throttle and that fuel just quits flowing through the idle circuits
when the throttle is open more than 1/4...

But there are published graphs that show the fuel flow through the idle
circuit continuing up to full throttle. More idle mixture does flow
through the idle circuit at low throttle openings when vacuum is
highest at the outlet port. The amount of idle mixture flowing out of
the idle ports drops as the throttle is opened until the flow
approaches zero at full throttle when vacuum downstream of the venturi
approaches nil...

Fuel flow through the main jet is opposite. Almost nothing flows
through the main jet at closed throttle, and you don't want fuel flow
through the main jet. It makes the idle mixture to rich and it's hard
to tune the idle mixture if fuel is dribbling pass the needle jet/jet
needle combined orifice. More and more fuel does flow past the NJ/JN
until the needle is pulled so far out of the hole that the area of the
main jet orifice and available engine vacuum controls the flow of
fuel....

Main jet fuel flow is maximized at full throttle, and idle jet fuel
flow in minimized at full throttle. If you look at a fuel flow graph,
you will see that the two fuel flows are equal at some point between
1/4 and 3/4ths throttle. The point where more fuel flows through the
main jet than flows through the pilot jet is called the "cross over
point" and knowledgeable tuners would talk about "cross over" in
discussions many years ago...

But, back to the problem of fuel flow dribbling out the critical needle
jet/jet needle combination orifice. Even if the idle mixture screws are
set correctly, the float height is correct, the float valves are
working right and the needle clip is in the 2nd slot from the top, you
could still run into excessively rich mixtures at small throttle
openings if your aluminum jet needles have been vibrating against the
brass needle jet tube...

Sometimes the needle will have notches worn into it and sometimes the
brass needle jet will be worn oval and this may not be noticed by the
newbie shade tree mechanic. Nothing he can do will solve the
excessively rich off-idle mixture until he notices that the hole in the
needle jet is oval...

 

I'm thinking that the voltage difference indicates a resistance or a
leak that might cause a bigger voltage difference at load. I will try it.

 

I did not mention mixture needles in the post that you responded to...

Theoretically, the jet needle and needle jet are so close to the same
size that little or no mixture should drool past them to upset the idle
mixture. In the real world, the jet needle rattles against the needle
jet and makes the needle jet orifice oval shaped so even more fuel
drools out of it at closed throttle to 1/8th open...

The Sports Car Club of America once had an open-wheeled Spec racer
class called Formula 440. Everything was sealed at the beginning of the
season so drivers wouldn't "tweak" any part of the engine *including*
the carburetors. The Kawasaki 440cc snowmobile engines used in Formula
44 vibrated so badly they would ruin the carbs by making the slides
loose in their bores and ovalling out the needle jets...

What were you doing on Christmas Eve of 1968, Brady? I was laying on my
side in a corner of the lower equipment bay of Apollo 11, safety-wiring
the
spacecraft side connectors coming from the umbilical. I was a
Spacecraft Mechanic. When I wasn't fixing spaceships, I was building
and racing motorcycles. I didn't exactly fall off the turnip truck and
roll to a stop in rec.motorcycles.tech. I've been at this a LONG
time...

When I was skeptical about drilling out idle air jets, it's because I
learned a long time ago that one just doesn't go experimentally
drilling on expen$ive original equipment parts. There are probably 10
million sets of junk carbs out there in salvage yards to replace the
set the Matt might roach follwing your recommendation, but if he
doesn't even begin buggering
holes, he won't have to go fruitlessly searching for pristine carbs...

 

I tried hotwiring it, but it didn't seem to help.

 

I adjusted the idle mixture on both sides. The tach is not real
precise, so I went mainly by ear.

For the left: Starting from a closed needle valve, the rpm was very
responsive to adjustment. But after I'd opened the left one 1 3/4
turns, rpm didn't seem to respond when I opening the valve more. Maybe
I should have kept opening it until there was a definite slowdown, then
set it at the average of the range? I just left it at 1 3/4 turns open,
thinking that less is better and because I've seemingly been having
overrich problems. That is open about 1/2 turn _more_ than it was
before I disassembled and cleaned the carbs. Maybe since I've gotten
the idle mixture closer to correct, I've been able to turn the idle stop
screw to close the throttle a little?

For the right: used the same method, leaving the screw open about 1 1/4
turns. That is about unchanged from where I found it when I cleaned it.

I was surprised to notice a significant improvement in performance. It
idles better (more stably). It returns to idle faster when I let go of
the throttle grip. There is less exhaust noise. Previously when the
engine was hot, I was sometimes hearing noises like a loose nut rattling
around in the engine. Maybe that was pinging. Now that noise is mostly
or completely gone. By the way, I have been using premium gas. The
left plug now shows somewhat less fouling. The right plug looks fine,
as before.

Sorry I am a little weak on terminology. By 'needle' I guess you mean
the sliding needle controlled by the diphragm. I don't think there is a
way to adjust that. It seems you wrote elsewhere that vibration can
wear out that needle and its orifice. It could be that the needles were
a little flat on the sides when I disassembled the carbs. I see that
new needles are included in the Keyster rebuild kits.

 

Thanks for your ideas.

I have this possibly dumb question:

If the bowls don't overflow and the gas is deep enough to cover the
bottoms of the jets, isn't that good enough? You seem to be indicating
that heavy floats can cause an overrich condition. I don't know that
they can't, but I don't see why they would.

The bowls are not overflowing. You know that each bowl has a little
standpipe connected to a drain hose---I guess gas would run out of the
hose if the float valve were stuck or if the floats didn't float at all.

 

Part number KH-0142 at http://www.siriusconinc.com/carb/parts.asp
Click to see photo.

 

That's what mechanics do, they listen for sounds that tell them what
the engine is doing. It takes years to experience all the "funny
noises" and associate them with a particular problem...

When you turn the idle mixture screw counterclockwise and the idle
speed stops rising you've turned it far enough, you're just dumping
extra gasoline into that cylinder to foul the plug. Twin cylinder bikes
with two mufflers are fairly easy to tune. Rich mixture on one carb
makes the exhaust sound on that pipe sound dull and thudding. You can
smell the rich mixture coming from the muffler on that side...

No, if you turn the idle mixture screw *clockwise* until it slows down
you're going the right way. Old 2-barrel automobile carbs had two idle
mixture screws right in fron of the carb. They were easy to set. We
would set the idle speed about right, and then turn the idle mixture
screws in until the engine started running rough. Then we would back
the idle screw out 1/4th of a turn and rev the engine twice and smile
and slam the hood and go get a beer. You could adjust your motorbike's
idle mixture the same way, but don't go for the beer. It's hard enough
to balance a motorbike without a beer in you...

I guess you're probably still too rich on that cylinder

That's how the manuals tell you to tune a motorbike engine. Open the
gas screws to raise the idle speed by richening the idle mixture on one
cylinder, then turn the master idle knob down a bit to return the idle
speed to spec. Go to the other cylinder and raise the idle speed again
by opening the *other* idle mixture screw and then lower the engine RPM
back to spec by turning the master idle knob down again...

That's not surprising. If you were the first person to remove the idle
mixture screws and you count the number of turns out from lightly
seated, record that information, and re-assemble the same idle mixture
screws in the carbs the came out of, you should be very close to the
right idle mixture...

Soon you will be a carb tuning expert...

Glowing bits of carbon or sharp metal edges in the combustion chamber
will cause pingign, which sounds like a very loose valve. Premium
gasoline burns cooler, so the engine pings less...

The tapered needle is called the "jet needle". It moves up and down in
a brass tube called the "needle jet". The main jet screws into the
bottom of the needle jet. There are two types of needle jets. One type
is just a brass tube with a hole down the middle. That's called a
"primary type" needle jet. The other type of needle jet has
cross-drilled air bleed holes for better mixing of air with gasoline
before being drawn into the carburetor venturi. That type of needle jet
is called a "bleed type"...

To add to the confusion over terminology, old timers call sometimes
call the float-operated fuel shutoff valves "needle valves" and they
call the fuel shutoff valve seats "needle valve seats"...

If you go to the website at www.factorypro.com you can see pictures of
needle jets that were worn egg-shaped by vibration in only 5000 miles.
This is particularly a problem in semi-downdraft carbs used on
sportbikes that have their cylinders laying down at a 45 degree angle,
like my Yamaha
FZR-1000...

 

I see that the kit contains about all the brass parts which might wear
out over a long period of service.

The kit appears to contain the following parts that are shown on the
carburetor fiche at www.partsfish.com

2. Valve Set Float
3. Needle Set Jet
14. Holder Needle Jet
15. Nozzle Main
26. Jet #68
27. Jet,Main #110
28. Jet, Slow #35

The jet needle appears to be a non-adjustable type with a fixed head on
it like a nail. The spring probably pushes on the flat top or on a
plastic spring seat, I can't tell from the drawing. In order to raise
the jet needle and make the mid range mixture richer with less lifting
of the vacuum slide, tuners will put washers underneath the head of the
jet needle. Sometimes amateur mechanics accidentally install jet
needles with various associated plastic washers in the wrong order,
raising or lowering the jet needle radically...

I don't know what the Keihin designations for your needle jets and jet
needles are, but I can look up the parts in the Mikuni 33mm smoothbore
carbs I have on one of my Suzukis. The needle jet has a hole that is
2.64mm in diameter, and the non-tapered part of the jet needle is
2.51mm in diameter. So the difference between the two parts is 0.13mm
or 0.005 inch...

That's a big gap between the needle jet and the jet needle, but those
carbs are racing carbs and they are known for always "drooling" fuel
out of the needle jet. I compensated for that by fitting some of the
smallest pilot jets available. Before I did that, the engine ran far
too rich and was always puffing out big clouds of soot at low engine
speeds...

If your original jet needles have vibrated against the needle jets and
made the hole egg-shaped you'd get the same "drooling" of gasoline that
I experienced. So a visual inspection may be in order...

 

Heres a question, When you say a number of turns. 1 turn is equal to
turning the screw 180degrees or 360degrees????

Jimmmr in Lubbock

 

One turn is 360 degrees...

 

By a 'turn', I mean 360 degrees.

BTW I knew somebody who apparently meant to say that he had changed his
opinion on a certain topic. He told me he had done "a complete three
sixty". Very confused guy, I guess. Also referred to his children as
his ancestors.

 

Here's what I think I see in the photo:

idle mixture screw, spring, and O-ring
bowl-to-body O-ring gasket
jet needle and needle jet
small package of float-valve parts (needle, seat, O-ring, flat gasket)
unidentified jet (main jet?)

 

I believe I will order two kits and do the rebuilds. I plan to swap
between the carbs any removable parts that aren't included in the
rebuild kits (eg the floats). Then if the problem moves from the left
cylinder to the right, I will be pretty sure it is a problem with one of
the swapped carb parts.

 

I didn't spot any paper gaskets in the kit, but it looks like enough
parts to completely *overhaul* a carb, if that's what you want to do.

Did you look at your needle jet and jet needle to see if they are
actually worn out? Do you believe that you need all those parts? It's
your money, it's up to you whether you buy the kit or not...

But I remember an incident while working in an electrical accessories
repair shop in the Air Force. A motor driven alternator called an
"inverter" was used before solid state inverters could be obtained...

The inverter needed brushes and the brushes came in a depot-level
overhaul kit. So I opened the kit and found a whole mess of parts
besides the brushes and I figured that I really should use as many
parts as I had and make that inverter just like new...

The sergeant in charge of the shop saw me spending a lot of time on the
job and came over to see what I was doing. I told him that I was
installing
all the parts from the overhaul kit...

He told me to just finish installing what I was working on and throw
the rest of the kit away. I felt bad about wasting all those brand new
parts, but I trashed the rest of the kit as he ordered...

That's usually the way it is in carburetor kits. They are cheaper to
buy than buying worn out parts one piece at a time. And the old rule
of, "If it ain't broke, don't fix it" applies too. You might bugger the
body of the carb up while trying to replace a part that you were
suspicious of, but had no specific reason for replacing...

 

It's amusing to play "devil's advocate" with a confused person when I'm
bored. I find something that he firmly believes in, subtly get him to
adopt the opposite opinion (180 degrees from where he started), then
debate him back to his original position (360, right back where he
started). Then I try debate him back to 180. If he has an ounce of wit,
he'll figure out that I'm yanking his crank, or he'll just adopt the
position that the true answer to a problem is unknowable...

Such debate is a way of passing the time of day at a boring job...