Vacuum Issues

[Guest]

Working on a 77 GS550 with clean, stock VM carbs. The original petcock was not working properly so I just replaced it with a brand new OEM unit and am having an apparent vacuum issue. With the vacuum line hooked up to carb #3 and the engine running, the petcock does not let gas through either the RES or ON positions. I checked to confirm the vacuum line wasn't pinched. What should I be looking at? Where should I start looking for vacuum leaks?

[Cipher]

I just replace my petcock vacuum line with a nice OEM tube. The old one much to my amazement would let in air if you capped the end and sucked on it. It was porous.
I wet it and the darned thing passed no air. I never saw the like of it.

What happens when you apply vacuum to the line by sucking on it or by using a pump or having a minion suck on it?

[JPM]

I'm using a new tube and will try sucking later today and report back.

[Steve]

Are you counting your carbs correctly?

#1 is on the left, under your clutch hand. #4 is on the right, under your throttle hand.

It may sound silly, but some guys number them as they are looking back from the front wheel.
Numberered that way "#3" (which is really #2) also has a hose, but it's a vent hose, not a vacuum supply.

.

[phydeauxmutt]

Be careful with the sucking. If there are problems with the diaphragm, you will end up with a mouthful of stuff that really doesn't taste very good.

.

[JPM]

Ok, problem seems to be solved. I seems the vacuum line I was using didn't fit quite tight enough at the nipples so I assume the vacuum wasn't strong enough to operate the petcock. Switched line to a slightly smaller diameter so it fit nice and tight and the problem seems to be solved.