Keihin carburetor tuning problems

Hey folks, I've a 1972 Honda CL350 with some apparent carburetor
issues. The jets are the factory ones, so this doesn't have to do
with jet size. I have gone through the standard procedure of tuning
them ( Warm the engine, adjust the idle to ~1100rpm, turn the mixture
screws on each until engine idles at it's maximum, equalize exhaust
pressures via the idle speed screw again, until the engine falls back
to 1100 rpm...) Even after running through this my plugs end up carbon
fouled after riding for a little while, and no matter which way I turn
the idle mixture screw, it still happens. I noticed that the primary
main jets in each carburetor do not have an O-ring on them, and it
looks as if they should by the groove(s) around their circumference.
Possibly, the lacking O-ring is allowing too much fuel to be sucked up
around the jet holder and into the bore??

Another very silly question: which direction do you actually turn the
mixture screws to lean it out? From what I've gathered, turning them
out (counterclockwise) should be lean, but even when I turn them out
like 4 or 5 turns, the plugs still foul. Any advice for an amateur
like myself will be greatly appreciated.

 

There are two or three serious carb' experts here who know much more
than I do and they usually pipe up so I won't try to give detailed
advice. I looked at
http://www.bikebandit.com/houseofmotorcycles/honda-motorcycle-cl350k4-1972/o/m9205

which is for a cl350k4, but I can't quite figure out which screw is the
mixture screw and which side of the needle it's on. If the screw you're
adjusting is on the engine side of the throttle piston/needle, I think
that would be a fuel screw and 4 or 5 turns out is probably way too
rich, maybe the screw is about to fall out of its threads! If I've got
that right, turning it clockwise would make the mix leaner. Maybe
somebody will suggest a starting point, the range for many Hondas seems
to be between 1 1/2 and 3 turns. It's easy to bungle synchronization if
you pick a starting point that's way out of range.

 

Hey, thanks. Yes, in that blow up, the mixture screw is labeled part
number '8'. I should have mentioned above that even when the mixture
screws are turned in nearly all the way (when fully seated the engine
just putters down and dies) and I ride it for a bit, I still get
carbon fouling! It's quite boggling, really. Also, my choke is not
on. That's the first thing to check, of course.

A factory spec-sheet that I found said 3/4 of a turn, which I've
tried. I've also tried, a full turn, 1 1/2 turns, etc. It shouldn't
have to be quite so accurate, anyhow. I don't even have different
grades of carbon fouling, no matter how I adjust the carbs, if I ride
it for about two miles and pull the plugs, they are coated in carbon.

 

Idle mixture screws stop having any effect on the idle mixture
delivered to the idle mixture port after about three turns
counterclockwise.

A site called oldmanhonda.com used to have a list of jet sizes and
starting points for setting the idle mixture screws and air screws of
all the first and second generation Hondas, but I can't find it.

I recommend that you screw your idle mixture screws all the way in
until they just gently bottom out, then turn the idle mixture screws
counterclockwise one full turn and see how that works.

If you turn each idle mixture screw clockwise 1/4 of a turn and give
the engine a chance to burn off any rich mixture and then the idle RPM
increases a little bit, turn the idle SPEED screw a little bit
counterclockwise so the engine slows down to the specified RPM in the
manual.

Then try turning the idle mixture screws clockwise another quarter
turn and let the engine use up any exces mixture and see where the
idle RPM goes.

Some manuals recommend that you use a precision tachometer to
determine when the idle RPM drops off 50 RPM as you reach the critical
adjustment on the idle mixture screw.

When it drops off 50 RPM, you tweak the idle mixture screw
counterclockwise
again to get that "perfect" idle mixture (for the particular weather
on that particular day).

 

Turning in goes lean. Carbon on your plugs really has little
to do with your Idle mixture screws, unless you ride around
99% at idle.

Clay

 

I see some of the big guns haven't answered yet, so I'm curious whether
the "fouling" is the only apparent "problem"? Does it start easily in a
second or two when cold? Does it respond well at different amounts of
throttle? Does it smoke? When starting from cold, when you put your
hand on the headers for the first minute or so, do the cylinders seem to
warm up equally? Are the plugs new and was the bike not ridden very
fast in recent history (if the bike has a history of mostly being ridden
slowly, it may take a long time for the plugs to burn off some of that
carbon). Sounds like the idle jets are clear, could the float levels
be way out of whack or the piston needle be set very high up? Not
asking for answers, maybe one of those questions might trigger an idea.

 

When you ride a motorcycle with CV carburetors, you *are* riding
around on the idle mixture most of the time. That's why it's so
important to get those idle mixture screws set right.

 

I didn't see that message yesterday while I was posting my reply to an
earlier message.

If the carburetor is unresponsive to the idle screws, the first thing
to do is turn the idle speed screws counterclockwise to close off the
acceleration transition ports.

Most amateur tuners don't realize that there is a pattern of three
acceleration transition ports that are controlled by the lower edge of
the throttle butterflies.

If you have a set of CV carbs on the bench and look into the
downstream side, what you should see is just half of *one* of the
three transition ports, the other two will be covered up by the closed
butterfly.

The transition ports are not affected by the adjustment of the idle
mixture screw at all. They are controlled entirely by the position of
the butterfly.

You cannot set the idle mixture if the throttle butterflies are too
far open because the engine sucks unregulated mixture from the
transition ports.

However, if the excessive richness is *not* due to the butterflies
being too far open, the other possible culprits are stuck float valves
or floats that are adjusted too high, causing too much fuel to
accumulate in the float bowls.

When the fuel level is too high, the engine can easily suck too much
fuel through the idle jet because it takes less engine vacuum to suck
the fuel up.

 

Does the bike start easily when cold? Idle well when warmed up? How's the
acceleration? From idle to 2K? From 2K-5k? From 5k-redline? Does it
stutter or hesitate at any throttle position? You'll have to set up the
correct main jet first. If the jets in it are originals and there are a
zillion miles on the bike (or if some previous owner changed the size) they
may be oversize from factory stock. Is the needle height adjustable?

 

Needle height really isn't all that important, but amateur tuners will
often stack a few 0.5mm washers under the head of a non-adjustable
needle.

They think they are really smart when they do this, but what they are
doing is just causing the needle to allow gasoline past the needle jet
slightly sooner, they aren't affecting the peak power of the engine,
they are affecting the midrange, and it's possible that the engine
isn't producing enough vacuum at lower RPM to lift the slide enough to
take advantage of extra needle height anyway.

www.factorypro.com has some *really* good tips about CV carburetor
tuning and troubleshooting that the manufacturers of competing jet
kits don't want you to know.

For instance, there are pictures of needles that are badly worn and
needle jets that
are *subtly* worn in an egg shape so that gasoline can get past them
at small throttle settings, like less than 1/4 of the slide lift

 

FactoryPro is a great resource.

Just replaced the emulsion tubes that the needles slide into on my
Y2K Pegaso. It would idle and it would run at speed.
But, off the line it would stumble and up to 3k rpm it was rough.
That is all gone now with the new, non-oval tubes.
And, fuel economy is up about 80%.

JohnA in LA, CA USA