Carb Diaphragm Vacuum Sources - Kawasaki KLR 650 Forum
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post #1 of 17 Old 04-18-2015, 02:30 PM Thread Starter
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Carb Diaphragm Vacuum Sources

In a different thread Damocles posted:

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Originally Posted by Damocles View Post
Maybe so; I thought manifold vacuum above the diaphragm, and atmospheric pressure on the underside of the diaphragm, raised the slide.

Thus I thought (perhaps incorrectly) manifold vacuum was routed to the mixing chamber area above the diaphragm, and the area under the diaphragm vented to atmospheric pressure.
I promised a top view photo of the area below the diaphragm and here it is.

The big hole ~1/4" or 6mm on the upper side of the photo goes down at an angle to the carb throat on the intake manifold side of the butterfly. So that when the throttle is closed it puts maximum manifold vacuum below the diaphragm slamming the slide down to idle position.

You can see the brass fitting just outside the carb for vacuum to the petcock and (on gen2) to the Air Injection System gets its vacuum from the same passage.

I think the little brass jet on the left side of the diaphragm area feeds vacuum down to the Coasting Enricher Valve diaphragm lower down on the left side of the carb.
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Last edited by GoMotor; 04-18-2015 at 02:33 PM.
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post #2 of 17 Old 04-18-2015, 03:42 PM
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[QUOTE=GoMotor;400905)

The big hole ~1/4" or 6mm on the upper side of the photo goes down at an angle to the carb throat on the intake manifold side of the butterfly. So that when the throttle is closed it puts maximum manifold vacuum below the diaphragm slamming the slide down to idle position.
[/QUOTE]
Sorry GoMotor,
That is completely In-Correct!

That angled hole is only OPEN when the enricher is pulled ON. (Choked, for cold start-up.)

And the Throttle Slide still goes up and down.

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Last edited by pdwestman; 04-18-2015 at 04:48 PM. Reason: added text.
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post #3 of 17 Old 04-18-2015, 05:27 PM Thread Starter
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That makes sense since I had the enricher pin out when I rotated the carburetor. The intent must be to suck the slide down tight when the enricher is on.

The only other opening to the cavity below the diaphragm besides the slop around the slide it the little brass orifice which may go who knows where.

I am now eager to get the bike back together, so I won't fool with the carb any more. I put a ez-adjust knurled knob in for the idle mixture screw and am working on a shorty enricher cable.
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post #4 of 17 Old 04-18-2015, 06:37 PM
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Quote:
Originally Posted by GoMotor View Post
The intent must be to suck the slide down tight when the enricher is on.
Nope! It doesn't do that either!

The filtered air passes freely from the intake bell mouth 'eye brow vent', under the diaphragm, with the enricher on, with "Zero" affect on the slide position.

The small vacuum hole in the slide is working against the spring in the top chamber.

Try the enricher at 5000-7000 RPM. It makes it just a little richer. Sometimes, just rich enough for me to ride at lower than normal altitude.

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post #5 of 17 Old 04-18-2015, 07:11 PM
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The position of the throttle 'butterfly' plate is what controls the effectiveness of the enricher (choke).

Leave the throttle closed and the enricher (choke) jet can Do It's Job, on a cold start.
If one opens the throttle plate during cold start (choking), the vacuum thru the 1/4" hole is reduced, therefore enriching effect is reduced.

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post #6 of 17 Old 04-18-2015, 09:51 PM Thread Starter
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Quote:
Originally Posted by pdwestman View Post
The position of the throttle 'butterfly' plate is what controls the effectiveness of the enricher (choke).

Leave the throttle closed and the enricher (choke) jet can Do It's Job, on a cold start.
If one opens the throttle plate during cold start (choking), the vacuum thru the 1/4" hole is reduced, therefore enriching effect is reduced.
That is easy enough to agree with.
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post #7 of 17 Old 04-20-2015, 09:58 AM
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Black arrow is atmospheric, from the airbox, below the diaphragm.
Red arrow is vacuum, from the manifold, right though the slide to above the diaphragm.

Vacuum above the diaphragm raises the slide.
More vacuum = higher slide.
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post #8 of 17 Old 04-20-2015, 10:41 AM Thread Starter
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I have that diagram. It didn't look quite right to me for the carb in my KLR.

The diagram doesn't show the big hole from the lower diaphragm area down through the enricher to the manifold side of the butterfly.

My carb doesn't have the mixture screw shown on the top side of the discharge throat.

My carb does have a mixture screw on the bottom side of the discharge throat.

My carb has no opening in the front half of the area below the diaphragm for atmospheric pressure shown by the black arrows drawn on the diagram.

The atmospheric air passages shown on top of the entering air side in the drawing are on the bottom of the entering air side on my carb. When I blow air in them it comes out through the main jet holder on the bottom of the carb.

The diagram does a good job of showing conceptually how a CV carburetor works and is helpful, but it is not the KLR CVK40 Carburetor. Just close.
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post #9 of 17 Old 04-20-2015, 12:20 PM
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Quote:
Originally Posted by GoMotor View Post
I have that diagram. It didn't look quite right to me for the carb in my KLR.

My carb has no opening in the front half of the area below the diaphragm for atmospheric pressure shown by the black arrows drawn on the diagram.

The atmospheric air passages shown on top of the entering air side in the drawing are on the bottom of the entering air side on my carb. When I blow air in them it comes out through the main jet holder on the bottom of the carb.

The diagram does a good job of showing conceptually how a CV carburetor works and is helpful, but it is not the KLR CVK40 Carburetor. Just close.
#1, The pic may be of a Mikuni CV carb, uncertain.

#2, Your carb DOES have the air inlet to the underside of the diaphragm (Where the Black Arrows enter), I,ve referred to it several times. "Eye Brow Vent". About 6mm wide and 20mm long. Or this , but upside-down.

#3, The 2 or 3 small meter air inlets to the pilot jet, the emulsion tube and I think the Idle air cut-off diaphragm.

#4, Yes.

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post #10 of 17 Old 04-20-2015, 12:25 PM
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More vac = higher slide.

Vac drops when the butterfly is wide open at big rpms and that slider has
to be out of the way. This is what my T-mod was affecting. The slide plate
has to raise as vacuum drops. Still a head scratcher to this old fart.

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