FUEL PETCOCK - PRINCIPLES OF OPERATION

[clar1jam]

When the engine starts vacuum is applied to the backside of the diaphragm.
When the engine is off and you lose vacuum, how is it bled off the diaphragm?  It appears there is a check valve on the vacuum side of the cover.

[sandy]

The vacuum just leaks back into the intake runner.

[lee]

Its bled off by the itybity hole just below the check valve in the pic.

Its actually a vacuum dampner used to smooth out the vacuum pulses.
Thats why its included in the cover set.  The little ball in the center takes a beating.

[Gryphon Rider]

There is no check valve. When the engine stops a spring pushes the diaphragm back and shuts off fuel flow.

** Edit **
I was wrong.  See Dusty's post below.
********

[Dusty]

  The check valve in the petcock is actually a rubber flap. As Lee said it dampens pulses so your main diaphragm is not moving open  and closed  when your bike is idling.  As you can see in the picture the rubber only covers the center hole and not the bleed hole. If you don't believe this rubber flap is a check  valve  the next time you have your petcock apart try  blowing air through the vacuum port . You can not blow air through the vacuum port but you can suck air through it.

Dusty

[Dusty]

Here's a picture of the bleed hole It's the tiny one at about the four o'clock position. If this bleed hole get's plugged with dirt or debris the main diaphragm will not close when the engine shuts off because the vacuum on the back of the main diaphragm is stronger than the spring.  I have had the hole plugged up on mine and I stuck  an aerosol can of carb cleaner with a nozzle attached into the vacuum nipple and blasted it a couple of times. It removed the debris and petcock worked again. I would imagine a lot of hydro lock problems originate with this bleed valve becoming plugged. Whenever I have my tank off I blast some carb cleaner in the vacuum port to flush out potential problems.

Dusty

[Gryphon Rider]

Dusty,

It seems I spoke too soon.  Thanks for the education.  

That tiny object near the central hole is the bleed hole?

[clar1jam]

Thanks everyone for your expertise!  It cleared up the question I had as to how it worked.  THANKS!

[Dusty]

 That's what we come here for is to help educate people and learn ourselves. The tiny dimple off to the side of the center hole is your bleed hole It probably only takes a speck of dirt to  plug that off and then the vacuum in the chamber holds the speck of dirt in place. I only figured this out because I tried to blow through the vacuum port with my mouth to make sure it was clear and nothing happened.

Dusty