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post #1 of 10 (permalink) Old 01-27-2014, 01:58 PM Thread Starter
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petcock

quick question.

will the stock vacuum operated petcock hold a vacuum on the vacuum port, or is it supposed to bleed down?
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post #2 of 10 (permalink) Old 01-27-2014, 02:19 PM
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If you were to add vaccum to the petcock, it should hold that vacuum since it is a sealed unit internally. fortunately it doesn't take much to open the valve inside.

The vacuum coming from the carbs is a pulsed vaccum and may bleed down farily quickly on it's own.
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post #3 of 10 (permalink) Old 01-28-2014, 08:26 AM
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When I flowed my petcock, it opened before 2"of vacuum. My mityvac starts at 2".
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post #4 of 10 (permalink) Old 02-12-2014, 04:13 PM Thread Starter
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ok, just to clarify my question, before I go out and rebuild the petcock..

if I apply a vacuum to the petcock, will the petcock HOLD the vacuum, or does it have an orifice or something built into it to prevent it from staying open due to vacuum. In other words, does it have a small leak in it by desigN?


reason I am asking, is occasionally my bike acts like its starving for fuel.. most notably when its cold, although it has happened when warm. In fact I was suffering from it this morning, then all at once, like flipping a switch, there was gobs of power avail. I understand it may be carb related, but when I cleaned the carbs just a few weeks ago, I noticed if I pulled a vac on the petcock, it did NOT hold it at all.

as far as mods and such, just ear shave, stock jets, no shims, stock exhaust. filters are UNI pods. I believe I have the mixture screws about 2.5 or 3 turns out.

oh, a plug check showed them about as clean as when new... very little signs of any deposits at all... I have not changed plugs since I owned the bike, so that would make them at least 1 yr old
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post #5 of 10 (permalink) Old 02-12-2014, 07:48 PM
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Are you sure it's not POOGS ?

Not sure if the petcock "holds a vacuum" seems like it would, but it doesn't need to. As long as the engines running its continually applying a vacuum at the petcock.
However , if there's a tear or rip in the petcocks diaphragm it could effect the flow of fuel.

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post #6 of 10 (permalink) Old 02-12-2014, 07:57 PM Thread Starter
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not a 100% sure poogs is out of the question.. wont have a chance to really dig into it until this weekend.

another strange symptom that occured at the same time.. when starting, with choke pulled.. most of the time, it will start, and fast idle beautifully. however, the moment I crack the throttle, it sounds like its dieing lean. and it will die if I dont close the throttle.

however, if I time it right, I can open choke and after a couple of cracks of the throttle, it will run fine, and accel ok.

a question regarding the shims under the needles.. are they to provide a slightly richer mixture just off of the idle stop?
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post #7 of 10 (permalink) Old 02-12-2014, 08:50 PM
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Quote:
Originally Posted by michiganteddybear View Post

a question regarding the shims under the needles.. are they to provide a slightly richer mixture just off of the idle stop?
The only things shims do, they cause the needle to clear the main jet faster. The needle will start to raise at the same time. Most Keihjn needles do not have much taper, so they rich en the intermediate circuit very little. If the airflow is insufficient to siphon when the needle clears, you can have a large hole in the fuel curve.

With the needle still in the jet, you can start the intermediate circuit much sooner than possible otherwise. Like trying to siphon liquid though a 1/2 " hose versus a 1" hose. Larger main jets also delay effective fuel metering. Larger mains + shims is something I would not even consider.

HTH

edit-the petcock will bleed off quickly when using a mityvac.

Last edited by wmsonta; 02-12-2014 at 08:55 PM.
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post #8 of 10 (permalink) Old 02-13-2014, 08:39 PM
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If you suck on the petcock vacum line with your mouth it will hold a vacum. The vacum being applied to the petcock vacum diphram will vary a lot (maybe 4 to 18 psi) as the engine is constantly changing the air being sucked through the carb. With only one cylinder the vacum really changes fast and a lot but since the petcock diaphragm requires so little vacum to operate your petcock should stay open. A few people have swapped their petcock (Wolfie for example) to a non vacum petcock.

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post #9 of 10 (permalink) Old 02-13-2014, 08:55 PM
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Is there a leak in the vacuum line?may be bleeding off

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post #10 of 10 (permalink) Old 02-20-2014, 06:14 PM Thread Starter
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ok. getting ready to spring for a new petcock.

the TW200 fits, right? is that a dual outlet? (all the pics I see show it as single).
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