carb to tank vacuum line

[dtkid]

I have set-up an auxillary gas tank to use when I am adjusting and tweeking carbs but I have a question about the fundamental nature of the vacuum line that runs from the manifold side of the carbs (CV's) to the gas tank.

QUESTION: Exactly what does that hose do for the tank or carbs? Is it creating a vacuum (drop in pressure) at the petcock specifically or is it decreasing overall internal tank pressure.

The reason I ask is that if it decreases overall internal tank pressure than it would seem that this would decrease fluid delivery to the carbs.

Also, when I use my auxillary set-up, should I plug the vacuum line or leave it open and suckin'?

[tpeeples]

It operates the petcock...opens the valve for fuel delivery to the carbs. I plug it (hose with a bolt to plug the end) when working on the carbs without the tank on.

I hope that helps.

[JayH]

Plug the vacuum line. A golf tee will plug it off just fine.

[rphillips]

Has nothing to do with tank pressure. Put petcock to pri position, & as said, plug the vacuum line from the carb, or golf tee or nail. Bolt or screw may possibly leak vacuum thru the threads

[outofcontrol]

I was just explaining this to a buddy.
Do all the things these guys say to do.
Plug the vacuum line while you test with your gas IV bag. BTW, I use a 1L plastic pop bottle with 2 holes in the cap. One for my fuel line. I fill the bottle with gas, plug one hole with my finger and squeeze the bottle to get the line primed. If you zip tie the bottle to the handlebars (higher than the carbs) it will syphon itself, and the 2nd hole now acts as a breather hole.

That vacuum line actuates a diaphram that's connected to a valve in your petcock. Have you ever noticed that gas doesn't flow out of your tank even when the petcock is in the "on" position? If you suck on the vacuum line with the other end connected to the petcock, you can hear the diaphram operate, and fuel will flow. This is designed to keep your tank full when the bike isn't running. If you notice that your oil level ever gets higher even though you know your bike burns oil, this could be the culprit.

If you get gas in your mouth, you can assume that the diaphram in the petcock is bad. This will also really screw up your #2 cyl mixture because fuel is going directly in to the downwind side of the carb through the vacuum line.

Ok that was really too much info, but I'm bored at work right now.

-Kevin

[dtkid]

exactly the feedback I was looking for.

Thanks for the knowledge.........