Bike Runs rough at 1/4 tank - reasons?

DanP:

Our tanks are not pressurized. They flow due to gravity (ask anyone who has ever had a crankcase full of fuel, almost all of us). Air does need to get in to replace the gasoline that flows out through the fuel valve.

 

I pulled my petcock apart after this thread and found a lot of buildup in the portion from the vacuum into the unit. I'd pull it apart and clean it properly to be sure.

 

That's a question to which I'm still trying to find the answer. Had fantastic weather here in NoVA this past weekend and Sun aft I got to join the hordes of other two wheeled motorized conveyance's.
Had very specifically dialed the tripmeter to zero last fill up and when it read 143 mi. the engine died at a traffic light. I needed to use PRI for restarting but then went to RES underway. I took off my helmet and rocked the bike so as to hear the gas in the tank. Didn't sound like much but could hear some sloshing.
Probably no more than two miles further down the road, stopped at another light on a slight incline the engine sputters out again. Pushed to side of road and tried to restart but no luck. Fortunately a bicyclist going by told me there was a gas station down the road just a bit and I was going to push it there when I decided to try starting it again.
Dang if it didn't fire up and I dashed across the intersection even though light was still red (no traffic coming) and saved myself the push walk. Engine was dry coughing a bit but at least it got me to the pump.
When I filled up I only managed 2.5 gal to top and I thought there was a 3+ gal total cap (3.4?) which should mean I still had at least a half gal of fuel but it wasn't getting to the carbs. Also, getting 143 miles out of 2.5 gal = 57mpg which I find hard to believe.
BTW, tank has been emptied of gas and rinsed out of any crud, didn't find much, in the last two hundred or so miles. Also have new in line filter.
There's a mystery going on here. Any speculations or wild congectures will be appreciated and considered.

 

I'd say you aren't getting fuel in the reserve setting. You could confirm this by draining enough fuel from the tank so that you need to use reserve (fuel level below the pickup for the normal setting) then running the bike. If it stalls out like it's starving for fuel, switch to the prime setting and see if it runs normally. If so you have either a problem in the petcock or in the plastic inlet unit on the inside of the tank.