Whats this vac petcock stuff?

I've never seen this before until my 84 gpz550. I have an 83 gpz305 petcock that's a single outlet manual. Should I change this out? My 550 vac petcock is gummed so bad it looks like its full of mud and it would not turn. I was going to get a rebuild kit but I have a good spare 305 petcock that looks like the same mounting. I don't understand the vac petcock theory. Looks like a few guys on other posts want to get rid of it.

The vacuum operated petcock was invented so you do not have to turn the petcock off every time you park the bike. It only allows fuel to flow when the engine is running.

What issues can these cause? I'm money ahead if I put on the 305s petcock cause it works and don't leak. Do I just plug the connections on the carbs? Do I need to plug the valve that's above the cylinder head?

Just put a rubber cap on the vacuum nipple. It is usually between #1 & #2 carb. And remember to ;turn the petcock off whenever you stop the engine. it is smaller in diameter than the fuel line.

koolaid_kid wrote: The vacuum operated petcock was invented so you do not have to turn the petcock off every time you park the bike. It only allows fuel to flow when the engine is running.

I suspect it was because of legal liability. Leaving the petcock on can cause a gas puddle if the floats/valves are not in good position. I'll wager somebody somewhere had a house burn down when that puddle made enough vapor to get to the flame on the hot water heater.