REMEMBER -- AIR IS NOT GASOLINE, THIS TEST IS APPROXIMATEThe rubber suction bulb comes from an ear wash kit that you can get at WalMart or any drugstore -- ask your wife, you probably already have one stuck away somewhere from when you raised babies.
(if you don't already know you have one for your own ugly earwax issues)
Is the bulb too strong (stronger than the vac level provided by the bike's intake tract)?? Maybe -- but you get to pick how hard it sucks by how far you squeeze the bulb before you cram the pointed end down into the suction tube.
I guarantee it sucks harder than the bike does if you collapse it all the way, so mebbe you don't want to do that ....You can suck on the fat tube to judge the flow of gasoline at the various suction levels you can get out of the bulb -- it is very educational to play around with this for a bit just to get a feel for it.
You can put your fingers in the bulb dent and counter-squeeze just a little bit, reducing the vac suction level between "full" and "zero" at will. You can feel the change in the flow through the big tube like playing a little musical instrument .... a little death flute if you will.
You can actually feel and hear the diaphragm and the valve moving and engaging.
Lastly, leave the squeezed bulb engaged and go away for a half hour or so -- you are now doing a leak down test to see if your diaphragm is leaking or not. If it fails, glue the silly thing in place to reassure yourself that it isn't the junction between blue tip and black hose doing the leaking ....
What did I learn?
On my old dead vac petcock I had a slow leak in the diaphragm/drum system and my gas never really ever shut off completely at zero vacuum. I always had "gas on" all the time at some small flow rate. I also learned we have been ignoring leaks from the drum selector system getting old -- these can happen too.
Lastly, sucking and blowing on the fat tube taught me that the supply rate of gas on a vac petcock is a variable, and it never is "freely flowing" from the screened side the way you would think it would be.
Prime position (mechanical passageway) has a much greater max flow rate than the ON or RESERVE positions do even at full input vacuum levels. This may be a function of the relatively smaller diaphragm valve "full open" passage area compared to the simple "hole" area of the Prime passageway.
Now, if you have a used vac petcock laying around I invite you to test it and post below what you see from your tests -- anything we can learn is good information. I just have the one dead petcock, some other folks need to test theirs so we can refine this test method some more.
REMEMBER -- AIR IS NOT GASOLINE, THIS TEST IS APPROXIMATE