intake manifold hose to the petcock?

I have an 83 XJ 750 R and the ? i have is what and for what reason is there a hose running off 1 of the intake manifolds going back to the gas tank petcock, what is the purpose of this hose and what does it do and do you need it running back to the gas tank.
Thanks

 

You have a vac operated petcock, the hose provides vac to the petcock so fuel will flow when set to on or res. Prime flows regardless of vac.

 

Thxs, that explains it..

 

ok where does it connect to though i have the same thing but cant figure out where to hook it up among other issues in having with my fuel system.

 

There should be two hoses coming off the petcock (fuel valve.)

The larger one is the fuel line and goes to the carbs.
The smaller one is the vacuum line and it should go to your #3 intake manifold's vacuum spigot.

If you replace the vacuum line be sure to use real vacuum line; other types may cave in on themselves under vacuum when they get hot.

 

What's the difference between having it on #3 and having it on any of the others?

 

Good question. I'd like to know the answer too.

My best guess is that it has something to do with ignition timing.

 

As far as I know, it was simple "expedience." (Nothing to do with ign timing, for sure.)

On the 550s, it goes to #2.
On the 650/750, it goes to #3.

I know that on the later 600s, the #2 carb alone had a couple of different settings, possibly to compensate for that. But I cannot say for sure.

 

Interesting. I just moved mine from #3 to #2 because I suspected a vacuum leak around the #3 manifold, which I believe I've fixed now. I'd like to keep the hose on #2, though.

 

Should present no issues whatsoever.