so i am having a little confusion on how this bike is supposed to run and why...

let me explain. i have been having issues with my bike getting starved of fuel after riding for a little while and dying. gas tank had a woosh when i opened it up and i was lead towards the gas tank vent (thanks, pete). i cleared out the vent in the tank with a little blast of air and it seemed like i fixed the problem. however, i started riding it again and it still happened to me. my solution: reached under the gas tank and pulled the vent line that connects to the carbs and at least the bike ran okay and didnt die on me. i got home and tore it apart and found a few things out.

let me help explain so everyone is on the same page. this port is on the back of the #2 carb, throttle side:




this is the line that connects to it:



and this is the port that it connects to on the gas tank:





now, onto my situation. i decided to pull the lines apart since the tube that connects the carbs to the tank is two pieces, with a smaller plastic piece in the middle of the two which i have been lead to believe is a check (one way) valve. i found this to be my restriction in the system and decided to blow out the tubes to clear them of any blockage. however, when i got the compressor out i decided (accident) to blow the check valve out into the street and not be able to find it. i decided to call it quits for the night and went inside.

today i picked it back up, went to the store and bought some tubing to connect the two and hope for the best. here is what i found out today. with the tube hooked straight up to the gas tank the bike wont even run. with the tube disconnected from the gas tank, hooked up to the carbs, and left open to the atmosphere the bike runs, but fairly poorly (it idles really low and lean). when i put my finger over the tube and close it off the idle levels out and it runs how it should. the tube is pulling vacuum because it sucks my finger into the tube.

this leads me to believe that the check valve is set up to let air to the tank, but not from it. can someone help me figure out how this is supposed to be setup and work? why would the gas tank ever need pressure and how would the bike ever produce any pressure? would this be for when going WOT that the tank would be pressurized from the top down allowing it to make sure it doesn't starve of fuel? hopefully someone has the knowledge to give some solid input because so far i've got absolutely nothing.

help me!

 

Ya might want to factor this into the equation ......or at least check it

 

when i put my finger over the tube and close it off the idle levels out and it runs how it should. the tube is pulling vacuum because it sucks my finger into the tube.

I would fit a rubber cap over the pipe from the carbs and run it like that, for a while and see what results you get.
There is no pressure build up in the tank under normal conditions, the vacuum hose on the LHS carb opens the petcock valve and allows fuel to get to the carbs, and the vent pipe allows pressure to equalise inside the tank. I suspect that the pipe you have been connecting up from the carbs to the undertank pipe under the tank should not be connected that way, but the carb pipe be blocked off. The vacuum that you are creating by connecting it to the tank is overcoming the tanks ability to retain neutral pressure inside, and stopping the fuel from flowing.
My 2c worth.

PS I played with the pic to show what the undertank looks like - I think that should just vent to the air, through a pipe lead out under the bike. That is your vent pipe - the big one is for fuel overflow in the filler to drain back into the carbs through the air intakes (I think)
Hope this helps.

 

well, i rode the bike in this morning to work. it idles great and pulls phenomonally up top. its still a little rich cruising and down low so i probably have to change out the pilot jets for slightly smaller ones, but up top im running 12.9-13.3 AFR and it is MEAN. pictures and video to come tonight when i get off work.

i ended up capping off the tube on the carb side and letting the tank side open to the air and the bike seems to run fine. after talking with my brother for quite a while about it the only thing we can think of that could be there would be a tiny restrictor piece or pill in order to just pull a tiny amount of vacuum on the tank for EVAP purposes. seeing as how i dont care about the environment, i dont care about a functional EVAP either.

and sprock, the petcock looks perfect. i made sure to check it when i had everything apart.