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From: billybuckshot on 15 Jun 2007 13:18
I'm having a difficult time getting a friends Honda Cl450 to quit
fouling plugs. I'm familiar with vintage HDs and Brit bikes but this
old Honda is making me tear my hair out. I've cleaned the carbs
thoroughly and rebuilt them with new jets, readjusted the valves,
checked the points, compression (140 psi) but the bike fouls the plugs
(black soot) within 10 minutes of start up. Other than replacing the
carbs completely, i'm out of ideas. Anyone out there familiar with
these old Hondas? Help is greatly appreciated. -Billy

From: Albrecht via MotorcycleKB.com on 15 Jun 2007 14:03
billybuckshot(a)gmail.com wrote:
>I'm having a difficult time getting a friends Honda Cl450 to quit
>fouling plugs. I'm familiar with vintage HDs and Brit bikes but this
>old Honda is making me tear my hair out. I've cleaned the carbs
>thoroughly and rebuilt them with new jets, readjusted the valves,
>checked the points, compression (140 psi) but the bike fouls the plugs
>(black soot) within 10 minutes of start up. Other than replacing the
>carbs completely, i'm out of ideas. Anyone out there familiar with
>these old Hondas? Help is greatly appreciated. -Billy

You didn't say what year your friend's Honda is.

http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=2323039

This piston type constant vacuum carburetor has a slide (#7) that lifts when
the throttle butterfly is opened and engine vacuum increases.

It has a plate type choke that increases engine vacuum for engine starting.
More vacuum helps the engine suck gasoline out of the float bowl through the
idle jet (#20)

#2 is the float valve seat and the float valve, which is sometimes called a
"needle valve" by car mechanics. If the carburetor is all gummed up from the
gasoline evaporating during storage, the float valve will stick and the fuel
level in the float bowl will be too high. The engine will soot up the spark
plugs if it doesn't just flood out right away.

#3 is the jet needle and the needle jet that controls the fuel air mixture
when the slide is more than 1/4 of the way open.The tapered needle makes a
variable orifice that controls the flow of fuel air mixture until the
throttle is almost wide open. No matter what position the throttle butterfly
is in, the vacuum slide will not lift high enough to get the engine to run on
the main jet unless there is enough engine vacuum at high RPM to lift the
slide.

The needle has no slots for adjustment, sometimes amateur tuners will put a
stack of small washers under the head of the needle to raise it up as they
try to make the mid range mixture richer.

#4 is the float. It has a bendable tab for adjusting the level of the fuel in
the float bowl. If the tab is bent down a little bit, the float will not be
able to close the float valve at the right time and the idle mixture will be
too rich because the engine vacuum can more easily draw fuel out of the float
bowl.

#6 is the idle mixture screw. Turning it clockwise leans out the idle mixture.
Amateur mechanics get into trouble when they try to adjust this screw,
because they think that the idle speed should increase as they turn it
counterclockwise and that the speed should keep increasing the more they turn
it. But, when the idle mixture gets too rich, the idle RPM actually slows
down. The exhaust sound becomes dull and thudding instead of crisp sounding.
The engine stalls, so the amateur mechanic turns the idle speed screw up to
get the engine back to specified idle RPM. Then, the engine begins to suck
gasoline out of the acceleration transition ports downstream of the throttle
butterfly. At best, the engine runs too rich at idle. At worst, it won't idle
down because the throttle butterflies are open so far the acceleration
transition ports are uncovered.

#18 is the main jet. It controls the flow of fuel air mixture when the
throttle is almost wide open.

#19 is the slow jet. It has to suck fuel through the idle jet.

#20 is the idle jet. It controls the flow of fuel from idle speed upward.
Less and less fuel flows through the idle jet as the throttle is opened
further, but when you are riding on the highway, you're getting at least half
of your fuel through the idle jet.

The idle jet controls fuel going to the idle mixture screw and to the
transition ports. The idle mixture screw only controls one port, the throttle
butterfly controls the transition ports.

--
Message posted via MotorcycleKB.com
http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200706/1

From: billybuckshot on 15 Jun 2007 14:57
On Jun 15, 1:03 pm, "Albrecht via MotorcycleKB.com" <u33665(a)uwe>
wrote:
> billybucks...(a)gmail.com wrote:
> >I'm having a difficult time getting a friends Honda Cl450 to quit
> >fouling plugs. I'm familiar with vintage HDs and Brit bikes but this
> >old Honda is making me tear my hair out. I've cleaned the carbs
> >thoroughly and rebuilt them with new jets, readjusted the valves,
> >checked the points, compression (140 psi) but the bike fouls the plugs
> >(black soot) within 10 minutes of start up. Other than replacing the
> >carbs completely, i'm out of ideas. Anyone out there familiar with
> >these old Hondas? Help is greatly appreciated. -Billy
>
> You didn't say what year your friend's Honda is.
>
> http://www.bikebandit.com/partsbandit/showschematic.asp?dept_id=2323039
>
> This piston type constant vacuum carburetor has a slide (#7) that lifts when
> the throttle butterfly is opened and engine vacuum increases.
>
> It has a plate type choke that increases engine vacuum for engine starting.
> More vacuum helps the engine suck gasoline out of the float bowl through the
> idle jet (#20)
>
> #2 is the float valve seat and the float valve, which is sometimes called a
> "needle valve" by car mechanics. If the carburetor is all gummed up from the
> gasoline evaporating during storage, the float valve will stick and the fuel
> level in the float bowl will be too high. The engine will soot up the spark
> plugs if it doesn't just flood out right away.
>
> #3 is the jet needle and the needle jet that controls the fuel air mixture
> when the slide is more than 1/4 of the way open.The tapered needle makes a
> variable orifice that controls the flow of fuel air mixture until the
> throttle is almost wide open. No matter what position the throttle butterfly
> is in, the vacuum slide will not lift high enough to get the engine to run on
> the main jet unless there is enough engine vacuum at high RPM to lift the
> slide.
>
> The needle has no slots for adjustment, sometimes amateur tuners will put a
> stack of small washers under the head of the needle to raise it up as they
> try to make the mid range mixture richer.
>
> #4 is the float. It has a bendable tab for adjusting the level of the fuel in
> the float bowl. If the tab is bent down a little bit, the float will not be
> able to close the float valve at the right time and the idle mixture will be
> too rich because the engine vacuum can more easily draw fuel out of the float
> bowl.
>
> #6 is the idle mixture screw. Turning it clockwise leans out the idle mixture.
> Amateur mechanics get into trouble when they try to adjust this screw,
> because they think that the idle speed should increase as they turn it
> counterclockwise and that the speed should keep increasing the more they turn
> it. But, when the idle mixture gets too rich, the idle RPM actually slows
> down. The exhaust sound becomes dull and thudding instead of crisp sounding.
> The engine stalls, so the amateur mechanic turns the idle speed screw up to
> get the engine back to specified idle RPM. Then, the engine begins to suck
> gasoline out of the acceleration transition ports downstream of the throttle
> butterfly. At best, the engine runs too rich at idle. At worst, it won't idle
> down because the throttle butterflies are open so far the acceleration
> transition ports are uncovered.
>
> #18 is the main jet. It controls the flow of fuel air mixture when the
> throttle is almost wide open.
>
> #19 is the slow jet. It has to suck fuel through the idle jet.
>
> #20 is the idle jet. It controls the flow of fuel from idle speed upward.
> Less and less fuel flows through the idle jet as the throttle is opened
> further, but when you are riding on the highway, you're getting at least half
> of your fuel through the idle jet.
>
> The idle jet controls fuel going to the idle mixture screw and to the
> transition ports. The idle mixture screw only controls one port, the throttle
> butterfly controls the transition ports.
>
> --
> Message posted via MotorcycleKB.comhttp://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200706/1


Thanks a lot for your response.
It is a 1971 CL 450 Scrambler.
I have checked the vacuum slide. it moves freely. replaced the jet
needle and needle jet. replaced the float needle and seat. also set
the float height at 20mm. replaced the main jet and pilot jet. soaked
the carb bodies in chem dip for a day, blown out all the passages.

Somehow the engine is getting too much fuel. I can turn the mixture
screw in all the way and it doesn't affect the idle. On other
carburetors i've worked on this tells me i am getting too much fuel
through the pilot jet, but i can't seem to find any smaller jets for
this cv carb. the bike runs good until the plugs foul and then it'll
pop and sputter. its got to be a simple problem. i've worked on
motorcycles professionally for 16 years but this has me stumped. is
there a problem with these carbs where the cv slide fails to raise and
lower properly? i know the later kehin cv carbs have a rubber diapragm
to aid the vacuum slide.

From: The Older Gentleman on 15 Jun 2007 15:14
<billybuckshot(a)gmail.com> wrote:

> Somehow the engine is getting too much fuel. I can turn the mixture
> screw in all the way and it doesn't affect the idle. On other
> carburetors i've worked on this tells me i am getting too much fuel
> through the pilot jet, but i can't seem to find any smaller jets for
> this cv carb. the bike runs good until the plugs foul and then it'll
> pop and sputter. its got to be a simple problem. i've worked on
> motorcycles professionally for 16 years but this has me stumped. is
> there a problem with these carbs where the cv slide fails to raise and
> lower properly? i know the later kehin cv carbs have a rubber diapragm
> to aid the vacuum slide.

The carbs are shot. See my response to someone asking about a CB500T
(same engine and carbs, really).

Bloody awful carbs on those lumps....


--
BMW K1100LT 750SS CB400F CD250 SL125
GAGARPHOF#30 GHPOTHUF#1 BOTAFOT#60 ANORAK#06 YTC#3
BOF#30 WUSS#5 The bells, the bells.....
From: billybuckshot on 15 Jun 2007 15:29
On Jun 15, 2:14 pm, chateau.murray.takethis...(a)dsl.pipex.com (The
Older Gentleman) wrote:
> <billybucks...(a)gmail.com> wrote:
> > Somehow the engine is getting too much fuel. I can turn the mixture
> > screw in all the way and it doesn't affect the idle. On other
> > carburetors i've worked on this tells me i am getting too much fuel
> > through the pilot jet, but i can't seem to find any smaller jets for
> > this cv carb. the bike runs good until the plugs foul and then it'll
> > pop and sputter. its got to be a simple problem. i've worked on
> > motorcycles professionally for 16 years but this has me stumped. is
> > there a problem with these carbs where the cv slide fails to raise and
> > lower properly? i know the later kehin cv carbs have a rubber diapragm
> > to aid the vacuum slide.
>
> The carbs are shot. See my response to someone asking about a CB500T
> (same engine and carbs, really).
>
> Bloody awful carbs on those lumps....
>
> --
> BMW K1100LT 750SS CB400F CD250 SL125
> GAGARPHOF#30 GHPOTHUF#1 BOTAFOT#60 ANORAK#06 YTC#3
> BOF#30 WUSS#5 The bells, the bells.....

I read your response. i agree.i think somehow the carbs are worn to
the point of the slides not opening quick enough to let enough air
through. the problem is if i buy another set off e-bay from some m/c
junkyard, chances are i'll have the same problem.

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