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When I joined this forum, I kept reading about how the fuel tank petcocks cause so many problem. At the time I didn't understand the issue because my 93KQ had a simple manual shut off valve with positions ON OFF and RES. Then I acquired 3 more 93/94 KQ's and the 94 had the valve with positions ON OFF and PRI and a vacuum port to carb. This is the type that causes so many problems. However, on my 94, the vacuum ports were plugged on the valve and carb and was run in PRI with no problems. What's this thing supposed to do? Fortunately I had one available for autopsy:



This is the back side cracked open. Turns out this thing is an automatic fuel shut off valve. A spring loaded poppet valve seats into the selector housing to plug the chamber that leads to the fuel pump feed port. Suck from the carb pulls it open. The reason there is no OFF position on the selector is because when its not running the poppet seats closed and shuts off the fuel, hence automatic OFF...when its working right.



Showing the inner gasket. What if this leaks?



Detail on the back side of the selector housing:



Pulled apart there are 2 gaskets and plastic plate between them:



Spring on the outer gasket side:



The spring is attached to this plate. The suck is put on this plate to open the poppet to let fuel into the fuel pump feed chamber:



Selector side:



These are the ports from the tank:



Back side of the selector note the chamber cut into it. Rotating it directs fuel from the various tank ports. Green is normal ON feed and is not switchable. Yellow is RES feed. It connects the reserve port to the normal port. With RES selected fuel is drawn from both the normal and reserve pickup tube. ON and RES go to the poppet valve. Red is the PRI position which is actually a bypass. It connects from the reserve port direct to the fuel pump feed chamber bypassing the poppet shut off valve. The selector pictured is oriented to the PRI position. You can visualize folding it back on top and see how the selector chamber directs the fuel.



The carburetor connections. Note the petcock vacuum is direct before intake into the engine:



I believe the problems these things cause is when the poppet gaskets leak, the vacuum to operate the valve sucks in fuel. The rouge fuel enters the engine intake after metering and carburetion causing an uncontrollable rich condition. Possibly in some scenarios a siphon or gravity feed with the engine not running causes fuel to flow into the piston area and subsequent crankcase fuel contamination.

BS system. I can't imagine a good reason to design this type of tank valve. The on/off type is fine and why drag more vacuum off the engine? Its a solution looking for a problem. Probably a regulatory edict. Does sound like something a government bureaucrat would think is necessary.

If you suspect your vacuum operated tank valve causing problems check the vacuum line for fuel contamination. You can run it in PRI with the vacuum ports plugged, disconnecting the rouge feed. Risk here is if your tank is dirty, the lower level position of the reserve tube will pick up the contamination.

I intend to use the on/off valves on all my KQ's.
 

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Nice breakdown of the vacumm operated fuel petcock Tim. This is actually very insightful and will probably be very helpful to some. Think this thread belongs as a sticky.
 

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Discussion Starter #3
Good to hear and hope it is helpful. I had a spare valve and did not care if I broke it taking it apart. I really wanted to know what this pesky thing was about.
 

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Discussion Starter #5
No, should not be fuel in it. The PRI position bypasses the poppet valve and feeds fuel direct to the pump which in turn feeds fuel to the float bowl chamber. Only place fuel should be going into the carburetor. PRI is the same as ON in the simple valve. And ON -get this :) - is the same as OFF until the carb supplies suck to open the valve, assuming its working right.

Though what you say is intriguing. As if it is a fuel boost before the intake. I don't think that would be an intentional design but I'll look at the porting just to be sure.
 

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ok on a 1985 lt250ef how is the petcock setup i have a promlam of starting it i think i dont have the vacume going to thew right place
any help would be greatly apreciated
 

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Since you have an older model quad hawaii you may have the older style petcock on yours. To check simply look at it and see if it says On/Pri/Res on it, if it does then you have a vacumm operated fuel petcock. If you have the older style petcock which says On/Off/Res on it then it's not vacumm operated and shouldn't have a vacumm line at all.
 

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i was looking at the vacuum diagram on the carb. the line going to the fuel pump port where does that line come from? on mine it is ran up the handle bar as a vent.
 

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On the fuel petcocks that are vacuum operated you'll note two lines, the bigger line with what looks like a spring around it is the fuel pump feed where as the other smaller line is the vacuum line which leads to the carb.
 

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My wife's 03 Vinson 500 sat for 18 months. I finally got around to putting it up for sale and OF COURSE it won't run. Crankcase full of fuel, fuel is entering through the vent hose that runs into the air cleaner housing. Fuel is getting there through the carb, somehow. I got it dried out, and cleaned up the carb, checked float, it floats, needle and seat LOOK good. I think the petcock might be the culprit. Got it started, ran great at idle, and at mid throttle, heavy throttle and the fuel started pouring into air cleaner housing from the carb, also it flooded the motor.

This leads me to think it is the petcock and the issues detailed here. Is it rebuildable? is there a kit, or should I just order a new one? Thank you for the help, Bob
 

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My wife's 03 Vinson 500 sat for 18 months. I finally got around to putting it up for sale and OF COURSE it won't run. Crankcase full of fuel, fuel is entering through the vent hose that runs into the air cleaner housing. Fuel is getting there through the carb, somehow. I got it dried out, and cleaned up the carb, checked float, it floats, needle and seat LOOK good. I think the petcock might be the culprit. Got it started, ran great at idle, and at mid throttle, heavy throttle and the fuel started pouring into air cleaner housing from the carb, also it flooded the motor.

This leads me to think it is the petcock and the issues detailed here. Is it rebuildable? is there a kit, or should I just order a new one? Thank you for the help, Bob

IMHO...the best thing to do is replace with a new OEM petcock. If cost is an issue...you might be able to find a gentle used one on eBay.

As an example...I purchased a complete fuel tank with working petcock for like $35. Must less...than going with a new one. Of course...I had already purchased a new one...so the used is a spare. :laugh:

Good luck...


biz
:wink:
 

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The fuel petcock is not rebuildable. You either need to get yourself a new one, or get a used one off Ebay. To test your fuel petcock pull the vacuum line off at the fuel petcock (Smaller Line), and if any fuel comes out the vacuum line or the the port where your removed that line then the fuel petcock is bad.
 

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My 97 King Quad wont start...rolls and rolls but no fire. I have spark, a clean carb and a new spark plug. After reading these posts I am wondering if that petcock is my next target. In Tims excellent breakdown there are pictures of gaskets within the petcock...are those available? Anyone think I'm on the wrong track and have overlooked something? Thanks for help
 

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When I first got my 05 Eiger I was wondering myself where in the heck is the fuel shut-off position, so since there is none I'm thinking very seriously about installing a in-line fuel shut-off valve just in case mine goes bad. Any one done this?

Tony
 

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Tried to get mine going again tonite...nothing. I am jumping it so it's not a battery issue. I think it is a fuel issue...it acts and smells flooded right from the get go. I think the petcock is screwing up. Any ideas?
 

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I bought a brand new one after running on the broken one all summer. 106 and change. Sucks I got to pay for this stupid little piece of metal.
 
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