Manual fuel petcock question. - Kawasaki KLR 650 Forum
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post #1 of 33 Old 08-12-2016, 11:16 AM Thread Starter
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Manual fuel petcock question.

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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post #2 of 33 Old 08-12-2016, 01:13 PM
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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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post #3 of 33 Old 08-12-2016, 09:11 PM
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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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post #4 of 33 Old 08-13-2016, 01:24 AM
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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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post #5 of 33 Old 08-13-2016, 02:13 AM
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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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post #6 of 33 Old 08-13-2016, 05:29 AM
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Quote:
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!

Quote:
" . . . 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?

Last edited by Damocles; 08-13-2016 at 05:33 AM.
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post #7 of 33 Old 08-13-2016, 10:17 AM
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Quote:
Originally Posted by CheapBassTurd View Post
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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post #8 of 33 Old 08-13-2016, 04:51 PM
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Quote:
Originally Posted by Damocles View Post
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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post #9 of 33 Old 08-13-2016, 06:13 PM
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Quote:
Originally Posted by pdwestman View Post
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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Last edited by Damocles; 08-13-2016 at 06:16 PM.
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post #10 of 33 Old 08-13-2016, 06:58 PM
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Here's the iconic image I mentioned:



Both air and fuel passages.
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