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.
Vacuum operated fuel tank valve (Petcock)
11
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:
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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.
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Showing the inner gasket. What if this leaks?
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Detail on the back side of the selector housing:
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Pulled apart there are 2 gaskets and plastic plate between them:
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Spring on the outer gasket side:
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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:
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Selector side:
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These are the ports from the tank:
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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.
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The carburetor connections. Note the petcock vacuum is direct before intake into the engine:
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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.
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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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.
- 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.
1 - 20 of 161 Posts
