Running out of Fuel Sooner After Replacing Pump?

[danath 34]

Hey guys, was hoping someone could shed some light on a weird fuel issue I'm having...
The bike sat for about 6 yrs, and when I went to get her back on the road a few months ago, I found the tank, fuel pump, and injectors all gummed up with varnish and rust... Queue derusting the tank, cleaning the injectors and fuel rail, and replacing the fuel pump.
I replaced the stock pump with one from Quantum: https://www.highflowfuel.com/quantum-intank-efi-fuel-pump-w-tank-seal-for-suzuki-hayabusa-gsx1300r-efi-2008-2012-replaces-15100-15h00/

This is the pump only, you still use the rest of the stock sending unit. While I was at it, I bypassed the stock fuel filter and installed an inline filter as per this thread:
Fuel filter mod | Maintenance and Do-It-Yourself

After doing all of the above, she fired right up and runs great! That is, until the tank starts getting lower. Twice now it's started sputtering and dying like it's running out of fuel sooner than I expect. Both times I managed to get to a gas station in time and fill up only to be able to put in about 4.6 gal or so. And after filling up, it runs great again! What happened to that extra gallon or so of capacity?? My only guess is the aftermarket fuel pump is only able to provide sufficient pressure when there's at least a gallon's worth of fuel providing pressure to the inlet? Then as the level drops below that, the delivered pressure drops off...? Anybody seen something like this before? You guys think I'm on the right track? Or could it by chance have something to do with the extra volume added for the external filter and longer lines, etc?

Any thoughts or advice are much appreciated.

 

[Hayabusa Wannabe]

I am wondering if it is developing a vacuum and thus holding back fuel due to a vent being blocked. I myself do not know how air is replaced into the Hayabusa tank on each generation...fuel cap, vent line? Possibly a blocked line due to the tank being reinstalled on top of it?

 

[danath 34]

I believe there is a vent line. And I thought that at first as well, but its at least venting well enough to get through 4.5 out of 5.5 gals... I would think if it wasn't venting, it would cause an issue after a gallon or so? But I could be wrong. Maybe I'll open the cap next time its close to empty and see if that helps.

 

[danath 34]

update: I think you're right. New gas cap doesn't seem to be venting. I opened it up when it started doing it again, and she would come right back... Seems the lower the fuel level gets, the worse it is, I guess because there's less pressure from the fuel to offset the vacuum. Venting it a couple times I was able to burn most of the gas in the tank - refueled at 5.3 gals.

Now to figure out why it isn't venting...

 

[macka]

Have you checked your rubber vent hoses are not blocked or kinked