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With an empty fuel tank/line that's filled/reconnected it seems to take an seemingly endless amount of cranking to get an unmodded KLR to light off.

Is this alleviated with a manual petcock swap? Does the fuel line/carb fill quicker and without a near battery draining process?
 

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The vacuum operated petcock opens immediately for full flow on the first stroke of the engine, so there is no difference in flow rates between the two petcocks. Leave the throttle closed so that full vacuum gets to the petcock diaphragm.

Mine takes less than 7 seconds to fill and start with the throttle closed. If it is cold, use some choke/enricher.
 

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subvetssn,
With an OEM Vacuum fuel valve and an empty carb float bowl, say from winter storage or carb cleaning, this is my procedure.

Fuel valve in the 'ON or Reserve' position.
NO CHOKE and NO Throttle input! It reduces the Vacuum!

CRANK IT! For a count of twenty! To fill the float bowl.
Still cranking 'Pull the choke ON'! They usually start instantly if Everything Else is Correct!
 
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If you are really concerned about how much cranking it takes to fill an empty fuel line and an empty float bowl, you can pull the vacuum line off the petcock and hook a tube to the vacuum nipple and suck on it to fully fill the system. Then re-hook the vacuum line and start the engine. It's not something you would be doing very often.
 

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I took apart the petcock, punched a hole in the vacuum diaphragm, put it back together, and
capped off the vac nipple in back and the one on the carb. Free full flow unit. Starts immediately
and a possible failure point is eliminated.
Cost: Nothing.
Labor: 10-12 minutes.
Ya have to turn it on and off like the old days when we had to pay attention. Before clutch switches, neutral
switches, kickstand safety switches, etc. The days where ya looked down first then took off.

Those retaining the stock setup carry some electrical tape in their
kits for vac-leaks out in the bush country or traveling.
 

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NO CHOKE and NO Throttle input! It reduces the Vacuum!
While I don't think the "choke" (starting enricher) is involved, opening the throttle (butterfly air valve) reduces the air flow velocity, and consequently the venturi vacuum, seems to me.

Or . . . Bernoulli was just funnin' us! :)

" . . . an increase in the velocity of a stream of fluid results in a decrease in pressure."
CBT: Merely punching a hole in the stock petcock diaphragm converts it to a manually-actuated valve? That's all it takes?
 

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I took apart the petcock, punched a hole in the vacuum diaphragm, put it back together, and
capped off the vac nipple in back and the one on the carb. Free full flow unit. .........................
You don't have to damage the diaphragm to disable it. Just remove the little spring that closes the valve plug. Without the spring the valve stays open. Then the first time you smell gas in your oil you can replace the spring and have peace of mind.

The hard part is remembering where you stored that little spring. Maybe zip tie it to the back of the petcock.
 

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While I don't think the "choke" (starting enricher) is involved,

Or . . . Bernoulli was just funnin' us! :)
Damocles,
The enricher plunger Opens both an Extra fuel passage and an Extra Air Passage.

Ya' better have another look at yer' Carb. ;)
 

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Damocles,
The enricher plunger Opens both an Extra fuel passage and an Extra Air Passage.

Ya' better have another look at yer' Carb. ;)
Well aware of that fact, PDW! Have posted diagram of the air-and-fuel enricher passages (from "Care and Feeding") on this forum several times. However . . .

Compared to the 40 mm diameter venturi, the starting enricher air passage area is miniscule, with minor effect on the intake air velocity and corresponding Bernoulli effect (vacuum), I would think.

Not to say, "zero" effect; I perhaps should have said negligible effect on venturi vacuum, compared to the butterfly valve.

Or not. The slide hasn't begun to rise at the instant of start-up.

With a vacuum gauge on the carburetor vacuum port to the petcock, the spark plug wire pulled off, and the engine cranked with the throttle closed and open oughta tell the tale.

After all, the CVK40 is called a "constant vacuum" carburetor! :)

The vacuum may be other than constant at start-up.
 

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My reasoning could be wrong, but here 'tis:

Assumption 1: At steady cranking rpm, the volume of intake air pumped by the engine is constant, say, "X" cubic feet per minute.

Assumption 2: The butterfly valve, when closed, acts as a restriction on the incoming airstream when in closed position; thus . . . the air flow velocity would be greater, at the same volume, with a restriction (throttle/butterfly valve closed), than the airflow velocity would be with the restriction reduced by open throttle/butterfly valve.

If these assumptions are correct, I'd expect higher vacuum (lower pressure) from the higher flow velocity with the throttle closed.

DISCLAIMER: This analysis treats start-up cranking rpm conditions only, not higher-rpm sustained engine operation.

Could be, totally in error; has not been validated by testing; corrections and clarifications sincerely welcomed. A, "Yes, but," lurking out there may devastate my rationale! :)

===================

As the eminent futurist Tofler wrote, when his ideas and perceptions didn't work out:

"When the theory and reality conflicted, I had to either abandon the theory, or abandon reality."
 

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I took apart the petcock, punched a hole in the vacuum diaphragm, put it back together, and
capped off the vac nipple in back and the one on the carb. Free full flow unit.
CBT,
I can't understand how that could possibly work.
If the diaphragm closest to the fuel ruptures, usually raw fuel leaks out of the atmospheric Air Vent which is molded into the edge of the gray plastic diaphragm divider.
If that air vent is plugged and Both diaphragms have a hole/rupture in them, raw fuel will be sucked thru the vacuum hose and flood the engine.


Here's the iconic image I mentioned:



Both air and fuel passages.
Damocles,
I will suggest that a 5-6 mm diameter opening is a pretty Serious Vacuum Leak. Especially considering that the (fuel valve) system is only working with a pulsed signal and only Every Other Down-Stroke of the piston at that! So any additional opening reduces vacuum to the fuel valve diaphragm, imo.

Other point, The vacuum operated throttle slide is permanently limited to a minimum of 'about 1/4 open throttle'.
The butter-fly plate controls the air flow from idle to about 1/4 throttle. Then the air flow past the air lift hole in the slide become sufficient enough to start working in harmony with the butter-fly plate, correct?
 

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Maybe!

Let's put the vacuum gauge on the carburetor vacuum line (to the petcock) port, pull the spark plug wire, and crank the sucker--see whether vacuum is greater or lesser with the throttle open or closed! :)
 

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Maybe!

Let's put the vacuum gauge on the carburetor vacuum line (to the petcock) port, pull the spark plug wire, and crank the sucker--see whether vacuum is greater or lesser with the throttle open or closed! :)
I see that you are speaking of the vacuum applied to petcock vacuum port in the carburetor throat not the vacuum at the jets.

The way I see it, with the throttle plate closed the intake valves open on the intake stroke and produce a vacuum from the intake manifold through to the closed throttle plate including the petcock port. This vacuum remains except for some amount of leakage after the intake valves close and for the next three strokes when it is refreshed by the next intake stroke. If the throttle plate is opened, the vacuum is immediately lost at the end of the intake stroke due to air rushing in through the open throttle plate.
 

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Very plausible, GoMotor!

Let's connect the vacuum gauge to the vacuum port and see what happens!

I'd expect less vacuum, measured at the petcock vacuum port, with the throttle open as you suggest. Conversely, greater vacuum with the throttle closed (all at steady cranking rpm).
 

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Very plausible, GoMotor!

Let's connect the vacuum gauge to the vacuum port and see what happens!

I'd expect less vacuum, measured at the petcock vacuum port, with the throttle open as you suggest. Conversely, greater vacuum with the throttle closed (all at steady cranking rpm).
I think we are all in agreement that the pressure at the petcock vacuum port on the carburetor is lower with the throttle closed and an empty bowl will fill faster with the throttle closed during cranking.
 

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Maybe not relevant to the vacuum level from the particular KLR carburetor port we're discussing, but . . . I've noticed, driving vehicles equipped with manifold vacuum gauges, the vacuum level goes down when the accelerator pedal does . . . opening the throttle reduces manifold vacuum.

Men say manifold vacuum modulates PCV valves on positive crankcase ventilation engines; greater vacuum at idle (closed throttle) closes PCV valve more fully, limiting airflow from this source into the engine.

Men also say the check-valve in the PCV valve prevents intake backfires from igniting flammable aerosols in the crankcase, but . . . that's another story! :)

DISCLAIMER: A distinction between intake manifold vacuum and venturi vacuum is acknowledged; the above phenomena may not apply to the instant discussion.
 

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PD Paul,
I may have pulled the whole insides out, trimmed the gasket, and
capped off the vac nipple. If I'm remembering right it didn't work right
on the first try and went back in for more radical surgery. Was getting
a new one and nothin' to lose. I believe it's hollow with the valve at
the base last 7 years. (One of the initial mods) Pulled the diaphram and
spring n' put it back together? lol Yeah, and that gray thingie that leaked, too.
 

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Maybe!

Let's put the vacuum gauge on the carburetor vacuum line (to the petcock) port, pull the spark plug wire, and crank the sucker--see whether vacuum is greater or lesser with the throttle open or closed! :)
Damocles,
Yes, please do and show us the spread sheet or pics or video of your testing please.


Very plausible, GoMotor!

Let's connect the vacuum gauge to the vacuum port and see what happens!

I'd expect less vacuum, measured at the petcock vacuum port, with the throttle open as you suggest. Conversely, greater vacuum with the throttle closed (all at steady cranking rpm).
Damocles,
That sounds correct.

But you also need to show us the difference that the start-up enricher makes (or doesn't make), on your video.
 

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There's a spring inside the petcock that preloads a valve to the "off" position ( internally, not the handle) that the vacuum has to overpower for the petcock to work. You can take that spring and move it to the other side of the diaphragm to preload the internal valve to the "on" position. That's all that's needed to positively defeat the vac petcock.

IF the diaphragm were to rupture you could get a steady stream of fuel coming out of your vac nipple ( it can do this in stock configuration,too) , so it's a good idea to cap off that port like Tom mentioned. No need to buy any block off kits.


Now, if your diaphragm has failed ( like mine did) and you want to permanently block off the vac port, you can toss the diaphragm and spring altogether and replace the diaphragm with a solid gasket. Just make sure it's fuel-grade gasket material. I used an old Rochester carburetor base gasket to make mine. Still not a bad idea to cap the vac port with this method, even though it's not needed. It's simple to cap and double safety on fuel related issues can't hurt. But I wouldn't let it prevent me from riding until I sourced a vac cap. ;)

Just another option...
 
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