95 cbr 600 f3. Sorry, newbish question, and i tried looking through the manual but it doesnt say anything about this.

while flipping through a how-to carb cleaning thread, i saw a guy's post (of his F2) how he had a vacuum line going from his number 1 carb to a nipple on the bottom of the gas tank. On my F3, there is just a screw going into that hole. (the number 1 is the one i'm referring to).

 

There is a vacuum line that goes from the tank to the carbon canister, to a valve then to the #1 carb or from the tank to valve to carb #1 depending on your bike. See the photos below. If you have the canister look at the first photo tubes #1, 4 and 11. If you don't then see the second photo "tubes #5 and 11"

Number 11 on both diagrams goes to carb #1, but not to the side port.

The F2 had a vacuum controlled petcock- which may have been connected to that port- but the f3 the side port should have a screw or bolt and it only used for syncing the carbs. (or if all is correct)

 

correct. the side hole (screwed in) on the f3's is used for carb sync.

 

umm i dont have that canister...but you pointed out tubes 5 and 11.. i dont have none of that stuff on my bike. sorry for being difficult, i just dont understand.

let me rephrase my question... should i have a vacuum line going to my tank? right now all i have is a fuel feed coming off the petcock going to the fuel pump. there is 2 white nipples further back that have nothing attatched to them...

 

those for me run down do the base of the bike and connect to nothing, they simply let fuel and pressure out id guess.

kinda like the float bowl drain tubes, they go down to the base of the bike, so fuel doesnt drain or get onto any bike parts.

i can only imagine one is for the excess fuel spilt around the top of the tank (when you refuel) under the gas cap, and the other is to let pressure out. i could be wrong on those, but occasionally i never connected those dead end hoses and its never affected me

 

it just seems to me that as the volume of gasoline drains from the tank, something has to replace that lost gasoline... ie: air. so im supposing the smaller of the 2 nipples is a line for air to come into the tank as the gasoline drops down. i was having vapor lock in the tank so i suspected that nipple being blocked and i poked it with a wire and some dirt came out. just dont know where that nipple is directed to... like to the top of the tank or something. how far i should stick the wire in etc.

 

One of those tubes is for rain water or excess fuel that spills under the fuel cap to drain out. Open your fuel cap and on the lip u will see a hole. that is one of the drains. The other one is a vent to releive pressure and u might be right about getting air in. It goes all the way up past your fuel filler if you are sitting on the bike it runs just to the right of the hole. If u get a flashlight you can see it. You are not going to be able to get a wire past about 6 inches cuz it takes a 90 degree turn after that. But blow compressed air up that hole and it should clear out any crap in there. Unless u or someone sealed the inside of the gas tank like someone did on mine and sealed over the hole. I spent a week trying to get mine clear and brought it to 3 different dealers to have them try but no luck. I found a vented gas cap on ebay that ill have to switch to soon. Good luck
Mike