I have a 2003 FXD which has the vacuum-controlled petcock. I tried draining the tank by taking a vacuum on the port on the back of the petcock, and found that I had to continuously draw the vacuum to keep the valve open and flowing. Once you stop pumping the pump, the vacuum quickly goes away. I later took it apart, and there are no torn diaphragms or anything like that. I did discover a tiny vent hole in the body of the petcock, and if you cover that hole, you get a good solid vacuum. My question is, if you have to continuously provide vacuum to open the petcock (as opposed to drawing the vacuum and then pinching it off to maintain the vacuum), isn't that the equivalent of a vacuum leak? I guess I just don't understand the reason for the little vent hole that continuously lets air get drawn in all the way back to the carb. Can anyone enlighten me?
Its intended that once the vacuum ceases to hold it open it will close on its own. Think about it, if it didnt close once the vacuum source is removed, what would be the point in having a vacuum operated valve? Its so you dont have to remember to manually close the petcock.
Yup what they said its just a vent so the diaphragm closes off the fuel supply when the vacuum is gone . Basically if the hole was pluged you would have a vacuum on the back side and it wouldn't close to shut the fuel off .
The vent hole is on the non vacuum side of the diaphragm. If there's vacuum there, then there's a leak either in the diaphragm, or around the edge where it seals.
The OP should be able to apply vacuum to the petcock and pinch the hose off, gas should continue to flow. That's how I drain carbed tanks, suck some vacuum thru the hose with my mouth then pinch off with a hemostat.
There's a test in the service manuals, I think it says the petcock should hold 3 - 4 inches of water. Pump the gauge up and the inches shouldn't drop.
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