KZ750 Spectre fuel tank capacity? Petcock operation confusion

I run my bike in "PRI" mode on the petcock as opposed to "ON" or "RES". The vacuum line is not connected to the petcock so it's the only way the bike will run. My question is, will the bike be able to drain the full tank capacity (including reserve capacity) in the "PRI" position? Supposedly fuel capacity is 3.9 gallons on the KZ750N, however I have never put in more than 2.2 gallons (even when I was sure the tank was empty because I saw the fuel light turn on and the bike seemed to run out of gas about 2 miles after that light turned on, I actually ran out right when I rolled up to the gas station). It really feels like the bike is not capable of draining full tank capacity, unless I am just going crazy and the actual tank capacity is around 2.5 gallons. It makes me feel like I am missing something here. Please help!

It needs to be on RES (reserve) to fully drain the tank, that's why you are running out just after the light comes on which is when you should switch to RES

Ok. That would make sense. Here's the thing though, the bike can't operate in "RES" without vacuum on the petcock, correct?

Correct, why is the vacum disconnected?

And if it is disconnected is it capped off. If it isn't the bike'll run like shit. 

It's currently disconnected because I could not find where it plugs into so I just capped it. Do you know where it should be connected? I'll just pull the tank and reconnect it. Thanks

Thank you for the diagram Warren3200GT. This will be very useful

Even connected to only one of the carb vaccuum nipples, any one available, would make it work.      Preferable though, get two at least. 

Is not two nippels for vent and one for vacuum?

gordone wrote:

Is not two nippels for vent and one for vacuum?

The carburetors pictured above each have a vacuum port (nipple). On U.S. bikes, two of them are connected to the Clean Air System, which reduces exhaust emissions. Most folks delete the system and cover the ports with rubber caps designed especially for that purpose. One of the remaining vacuum ports is connected to the fuel valve (petcock) on the fuel tank. The last remaining vacuum port is capped with a rubber plug. All four of those ports are necessary to synchronizing the carburetors.

Every carburetor must have a fuel bowl vent. Sometimes they are individual hoses, sometimes (as above) two carburetors share a vent, so there will be only two vent hoses, one for numbers one and two, the other vent hose is for three and four.