Fuel petcock

[TomRodgers]

This may be crazy..but im fixing this 81 1100 standard for a neighbor, but have never had a bike myself that had petcocks on the fuel tank. There are 3 positions PRI RES & ON... ON seems to shut fuel off....fuel dumps out on PRI and RES. There is no "octopus" fuel assembly, just fuel lines directly to the carbs.
What gives??? Thanks

[egsols]

I'm not that familiar with the Standards but I don't think they had the octi. I believe a vaccuum line runs right to the petcock to shut the fuel off when the bikes isn't running. That's why no fuel flows while its in the ON position. If you apply vaccuum to vaccuum line, hooked to the petcock, you should get fuel to flow in the ON position.

PRI (Prime) is used when you have run the float bowls dry and need fuel to get the bike started.

[TomRodgers]

then what shuts the gas off to prevent drowning the carbs/cylinders???

[egsols]

then what shuts the gas off to prevent drowning the carbs/cylinders

The lack of vaccuum. No vaccuum no fuel. Yours appears to be working if no fuel is coming out in the ON position. It should only flow in the PRI position when the motor is off. I'm not sure about the staus of the reserve position if it can flow with the motor on or off.

[TomRodgers]

I may have to check the vaccuum lines.....true..it doesnt leak when bike is off...but when running, it seems to starve for fuel....

only wants to run when on PRI or RES.....

[John]

Fuel should not flow in either run or reserve when there is no vacuum applied. The fuel should flow in the prime position when there is no vacuum. Are you sure you have turned the petcock handles in the right direction? The handle should cover the function that is on the chrome ring around the handle. The vacuum sorce for your carbs is at the nipple on the #2 & 3 intake boot. The brass vacuum nipple you see on #2 carb body is for the hose that leads to the vacuum mechanism that is under the left side engine cover.

[dbeardslee]

If it runs ok in the reserve position your vacuum should be ok. I'm not sure if Prime draws from the upper (on position) or lower (reserve position) part of the tower that is inside the fuel tank - I'm thinkin' lower, but could be wrong. What kind of condition is the inside of the tank in? Rusty? There is a filter screen that surrounds the tower, but it's pretty delicate stuff and often tears which can allow foreign material into the petcock. You might have something plugging the upper part of the tower. I'd take the petcocks off and take a good look at them. If you take them apart, make sure you keep the parts for each petcock together, as not all the parts are interchangeable. The petcocks on these things are notorious. My 2 cents worth.