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Do I Need the Petcock Vaccum?

  • Thread starter Thread starter Gearz
  • Start date Start date
G

Gearz

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I acquired a set of VM29SS carbs for my 79 gs750. When I put them on, I realized that I no longer have the port to hook up the petcock vaccum line. With these new carbs, do I need that to function or can I just have fuel going to the carbs at all times? I does not appear that my petcock has the prime setting. Is there a way for me to get fuel through that petcock without vaccum?
 
the vacuum Petcock on a GS has three settings: On, Prime and reserve. On, is the vacuum setting. Reserve allows you to use the last .8 gallons of gas in the tank, so if you have the tank set to ON you run out of gas you can switch to reserve and have enough gas to make it to the next gas station. Prime is set as a gravity feed. Let's say you take the carbs off to clean them, when you put them back in they will have an empty bowl so you set the petcock to prime to start the vaccum. So if you don't have a vaccum line in the carbs they could still get gas if they petcock is set to Prime. That of course doesn't guarantee the bike will run or run well.
 
yes, you can open up the petcock and disable it in several ways....the original opened by using vacuum against the force of a spring. You can maybe move the spring to the OTHER side of the diaphragm, or, just remove it AND the plunger o-ring , or, simpler yet remove the entire assembly ofdiaphragm , plunger and spring and regasket the join with a paper gasket. I think youwill still have the "reserve" function depending on the petcock but as mrhedges is saying. to paraphrase, this will not shut off the gas so you are relying on float needles alone...carbs may flood and overflow when you leave the bike for awhile. You might therefore want another tap to shut off the gas.
 
I do have another shutoff valve downstream of the petcock. Thanks for the help.
 
I see they make a bunch of adapter plates. Do you know which plate would work on a 79 gs750. If not, I plan to measure the bolt pattern and see what matches.
 
Gearz said:
I see they make a bunch of adapter plates. Do you know which plate would work on a 79 gs750. If not, I plan to measure the bolt pattern and see what matches.
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I dont remember off hand and I just recently threw out the package but if you contact them I am sure they will help you out. It is such a smooth operating and good looking piece. I get on the bike I turn it on, I get off and I turn it off. Simple valve with no muss.

The hole in the tank is oval and the pingel stem is round. I needed to ever so slightly enlarge the area the stem would go up in the tank. I did it witha simple hand file. You only need to enlarge where the stem goes by about 1.5 mm.

pingel.JPG
 
Gotta tell ya, I'm waayyyyy too forgetful to use a Pingel on my bike. I'd forget to shut it off and be constantly out of gas and over-filled on oil. lol

I have used Pingel's before on a couple ATVs and they work great.

I'm still not sure I why I can leave my ATV parked for 3 months with the valve on and not leak any gas, but if I leave my GS valve in the "PRIme" position for 2 weeks, I can lose a gallon (or more) of gas into my crankcase.
 
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as motorcyclist and especially motorcyclist riding old bikes there is already a checklist to get going and one to stop so I just incorporate it into my ritual. I turn off the key I reach down and shut the gas then hit the kickstand, remove my helmet and so on. I know I dont have to turn it off if I am just running into the post office or other brief stops but I do anyway as part of the ritual. I have been running it about a year now and have never left it on but I had two other bikes with them and have some ingrained memory now.
 
TxGSrider said:
Gotta tell ya, I'm waayyyyy too forgetful to use a Pingel on my bike. I'd forget to shut it off and be constantly out of gas and over-filled on oil. lol

I have used Pingel's before on a couple ATVs and they work great.

I'm still not sure I why I can leave my ATV parked for 3 months with the valve on and not leak any gas, but if I leave my GS valve in the "PRIme" position for 2 weeks, I can lose a gallon (or more) of gas into my crankcase.
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It's that way because that's the way it is...

The float needles on GS carbs are simply not designed to and are not capable of holding back a tank of gas indefinitely. It's not a bug or a fault in the design, and pretty much most motorcycles are this way. Your ATV's carb may use more closing force and/or rubber-tipped needles.

One wee drop at a time, if you leave the gas on and wander away, gas will slowly seep through and make its way into your engine.
 
bwringer said:
It's that way because that's the way it is...

The float needles on GS carbs are simply not designed to and are not capable of holding back a tank of gas indefinitely. It's not a bug or a fault in the design, and pretty much most motorcycles are this way. Your ATV's carb may use more closing force and/or rubber-tipped needles.

One wee drop at a time, if you leave the gas on and wander away, gas will slowly seep through and make its way into your engine.
Click to expand...

About the only theory I came up with was maybe since we had 4 carbs there was 4x the chance for a leak. Or maybe my atv carb is leaking a little but its not noticeable, but if there was 4 of them leaking it would an issue (like on my GS). Also, I notice that when our GS's leak past the float valve most of the gas seems to run into the engine, but on my atvs they leak out of an overflow tube and to the outside of the motor.
 
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this will not shut off the gas so you are relying on float needles alone
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actually I might modify this....if you turn the tap halfway between ON+RES it may shut gas off....I've tested this years ago but I can't remember results! as I like the vacuum type... but it's easy to test if you are tinkering.
 
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