CL350 Keihin CV Carb questions

I have a 69 CL350K1 that I've gotten put together and running, but I'm
having a bit of a problem, possibly with the carbs. I cleaned them out
so that all the jets and passages are spotless, put it back together,
and it started right up. After a bit of adjustment, it idels great,
though there is a bit of a cough at times.

My problem now is that if I don't raise the rpms above an idle very
slowly, it will die, and the engine bogs down at around 6000 rpms in
neutral or 5000 rpms while in gear. I'm assuming that this is still a
carb problem, but this is the first engine of any kind that I've taken
from a non-running state to running, so I still have a lot to learn.

Thanks.

 

Have you inspected the choke plate to see if it is binding or anything
like that? Sounds like it could be that or you might need to adjust the idle
screw a tad. Take a look at the spark plugs to see what color they are.
Are they white? black? brown? this will give you an indication as to what
you might need to do.

good luck...

Fwed

 

The symptoms you describe indicate that your idle mixture passages are
still plugged up. There are probably three transition ports near the
bottom edge of the each throttle plate, and, as you open the throttle,
these transition ports are uncovered and they supply fuel/air mixture
during the critical phase of constant velocity carburetor operation
when the engine has enough vacuum to pull idle mixture through the idle
mixture and transition ports but lacks enough vacuum to raise the
vacuum slide and pull the tapered jet needle out of the needle jet.

I can't tell from looking at old pictures of your machine whether the
idle mixture screw is an "air screw" or a "gas screw". Typically, the
idle mixture air screw will be located closer to the air intake side of
the carburetor and turning it clockwise will reduce air flow from the
pilot air jet, richening the idle fuel/air mixture.

If the carb has an idle mixture "gas screw" instead, it will be located
downstream of the throttle butterfly and turning it clockwise will lean
up the idle fuel/air mixture.

But, even experienced riders usually don't know that the idle mixture
"gas screw" is only a trimmer on the multiport idle mixture passage
arrangement.

Idle air will be drawn into the idle mixture passages through a pilot
air jet in the carbs intake. If you don't find the pilot air intake
there, it may be concealed underneath the rubber diaphragm that lifts
the vacuum slides.

Idle gas will be drawn into the idle mixture passages through the idle
jet in the float bowl. The idle (or pilot) jet can look squeaky clean,
but if the idle mixture passages or the transition ports are plugged
up, you won't get enough idle fuel air mixture to get a snappy throttle
response at lower RPM.

So, I recommend pulling carbs back off the engine and squirting a
powerful aerosol carburetor cleaner like Berryman's B-12 Chemtool Choke
and Carburetor cleaner through the idle jet. It should come out the
transition ports and the idle port and the pilot air jet vigorously.
Sometimes you won't have enough pressure for the carb cleaner to squirt
a small stream out of the ports. That's when it's necessary to cover
some of the ports with your fingers while you squirt cleaner into
another port until it flows freely out of whatever port has been
clogged.

There is also the possibility that you have pinhole leaks in the rubber
diaphragms. Hold them up to a strong light. Pinhole leaks can be
temporarily patched with rubber cement applied to the bottom side of
the diaphragm.

Torn diaphragms will have to be replaced. There has been discussion in
this NG about where to buy replacement diaphragms. You can google for
that message by a guy named Matt.

 

Thanks for your help. I adjusted the idle gas mixture a bit richer, and
that has solved my problem with revving above an idle. Now it's running
really nice, starting right away and idling, and revving up to 5000 to
6000 rpms, but it's still bogging down above that. If I run it at half
choke, it will rev up to the redline no problem. Plugs are new, points
are clean and set correctly, and rubber diaphrams are in perfect
condition.

 

Well, I don't think you quite understand what "bogging" is and how a CV
carburetor solves the real bogging problem. I think your problem is
lean idle mixture.

Basically, bogging occurs when you are demanding too much power out of
an engine for the amount of load on it and the gear the transmission is
using.

Power demand is indicated to the engine by the amount of throttle
opening. With the old style slide valve carburetor, the rider could
open the throttle too far and excessive amounts of fuel/air mixture
would be sucked into the engine to no avail. When the engine vacuum
increased with RPM, the carburetor venturi would suck even more fuel
out of the float bowls and a lot of excess unburned hydrocarbons would
be thrown out the exhaust pipe, polluting the atmosphere.

However, the CV carburetor solves this problem by taking direct control
of the midrange mixture away from the rider. If the engine isn't
producing enough vacuum to raise the slide with the tapered needle
attaches, mid range fuel/air mixture will never
increase.

The avergae rider thinks that his engine runs on the idle jets only
when the throttle is closed and that carburetor goes right from the
idle circuit to the midrange needle immediately. Really naive riders
think that the carburetor goes from the idle jet straight to the main
jet, because that's how car carburetors work.

Or maybe a rider has read some speed tuning book and thinks he's an
authority and starts talking about how the idle jet works from closed
throttle to about 1/4 throttle, then slide cutaway influences off-idle
accleration.

And that rider is only beginning to have a clue.

The truth is that there is fuel flow through the idle circuit from
closed throttle to fully open throttle. But the fuel flowing through
the idle jet is decreasing as the throttle is opened and the fuel flow
through the needle jet is increasing. Once the jet needle has lifted
completely out of the needle jet, the fuel flow is mostly influenced by
the main jet. Very little fuel comes out of the idle jet once the jet
needle has lifted up to what corresponds to about 3/4's throttle.

But, no matter where your wrist places the twist grip, the slide valve
position will be determined by engine vacuum and engine vacuum is based
on engine RPM and the force that drives the pistons downward.

You won't get much force driving the pistons downward if the off idle
and midrange mixture is too lean.

There's a noticeable difference in the comparative sizes of the idle
jets and the main jets in either type of carburetor, slide valve or CV
carb. You may say that is obvious, the idle jet is small and the main
jet is a lot larger.

But, if you compare the idle jet in a slide valve carb to a CV carb,
the CV carb has a substantially larger orifice. The CV carb's idle jet
will flow about four times as much fuel as the slide valve carb's idle
jet.

That's because your engine is running on the idle jet at higher
cruising speeds when you're riding a CV carb equipped motorcycle.
Larger displacement motorcycles will only require about 1/8th throttle
to cruise down the highway at ilegal speeds.

So, the larger motorcycles are running on the idle jets for most of
their lives.

Your smaller displacement Honda may be running at 6000 RPM to go 60
mph, but I would bet that you are still only using half throttle to
ride on the open highway.

And, from what I've said above, you should be aware that your Honda is
still depending on the idle jet for a large part of its fuel. The owner
of Factory Pro, a company that sells aftermarket jet kits, says that
the CV carb is running mostly on the idle jets up to 3/4 throttle.

So, with all this information in mind, what should you do to solve this
problem that you think is "bogging", but which I think is a lean
mixture coming from the idle circuit?

You can clean out the idle circuits while riding. Over time, the
alcohol additives in gasoline will clean the carburetors out, but you
can do it faster with easily-obtained carburetor cleaning mixture.
Don't buy anything containing petroleum distillate, those cleaners are
for fuel injectors.

Just pour about 3 ounces of STP or Gum Out, or my favorite, Berryman's
B-12, into a full tank of gas and go for a ride. Ride slowly so the
engine will have to suck the B-12 through the idle jets. As the B-12
dissolves gum and varnish in the passages, the engine RPM will
increase, so you will have to turn the idle speed down. (1)

If you find that the idle speed hangs up at 5000 RPM or more when the
engine is hot, you'll have to adjust the idle screws for a leaner
mixture.

You want the warm engine to idle smoothly at the smallest throttle
opening and the least amount of idle mixture possible.

(1) Berryman's B-12 Chemtool Choke and Carburetor Cleaner is handy,
because you can get it in an aerosol can or you can get it in pure
liquid form. B-12 is xylene, acetone, and methyl alcohol. You can get a
15 ounce can for less than $3.00 at Wal*Mart.

 

I never pretended to know what I was talking about. Like I said in my
original post, this is the first time I've ever worked this extensively
on an engine of any type. Your posts have helped me to understand how
these carbs work a lot better.

Two nights ago I tore the carbs apart once more to spray some B-12
through the passages. I think that everything was already fine, but I
noticed that the slot in the main jet holder was supposed to align with
the hole going to the pilot jet, and it wasn't, so I made sure that it
was aligned, put it back together, and now it redlines no problem. My
dad, who is weighs 195 got it up to 80 on a pretty steep section of
highway near my house, where many cars can't maintain 60. So it's
running well. Thanks for the tips.

 

Just keep reading this group and soon you'll be pretending
with the best of 'em.