trying to make this bike right in all ways .... i decided to take of the tank and replace it with a better one that i have.

so when i look at them, i can see that the tap on my tank is not the original?? the new tap has a vacuum tap type.

so after taking off the tap i notice that there is a small brass inlet/outlet on the back of the tap.

my question to you is ...What should it be connected to?

should there be a hose from it to the carbs?

Thank for any help you lot can give me

XS 400s in the late '70s to early '80s came with a vacuum petcock....which failed frequently.

The inlet connects to a vacuum source which is usually a nipple on the rubber cab holder/carb boot. Mine ('79) was connected to the right carb holder, but it does not matter which one. It just matters that whatever one is not connected is blocked off.

Because of the failure rate of the vacuum petcocks...mine refused to stem the fuel flow and overflowed the left carb, many people replace the vacuum petcocks with a manual one. I did. there are rebuild kits available for the vacuum petcock and used petcocks (not sure if new ones are out there0, but frankly, opening the fuel flow at the start and turning it off at the end is not that big a deal.

turning my fuel tank tap off is not a problem .... i do that as a matter of course with all of my bikes.

i can run the bike using that tank and fuel tap by setting the tap to "prime" and have done so in the past.

it just seems odd to have an outlet/inlet that appears to go nowhere! ..... i have looked on my carbs and there appears to be no connection for it there?

i`ve also got the Haynes Manual, and it does`nt give it a mention?

i will take the tap to a Yamaha dealer tomorrow and see what they have to say

Do your carb holders look like this?

http://www.z1enterprises.com/detail.aspx?ID=2635

That brass port is what provides the vacuum for the petcock.

If the vacuum petcock goes bad, it does not matter what position you turn it to, the gas will keep flowing.

turning my fuel tank tap off is not a problem .... i do that as a matter of course with all of my bikes.

i can run the bike using that tank and fuel tap by setting the tap to "prime" and have done so in the past.

it just seems odd to have an outlet/inlet that appears to go nowhere! ..... i have looked on my carbs and there appears to be no connection for it there?

i`ve also got the Haynes Manual, and it does`nt give it a mention?

i will take the tap to a Yamaha dealer tomorrow and see what they have to say

HoughMade answered your question there.

Connect the vacuum nipple on the petcock to the nipple on either carb boot (manifold). They're attached to the cylinders and the nipples are on the bottom. One will need to have a cap.

Doh! now i know thanks lads it was there in front of me all the time!

i knew the answer was there, but i could`nt see the woods for the trees.... another issue resolved and now on to the next bit