Suzuki GSX650F Forum - www.GSX650F.biz

after about ten miles or so the bike gets a lag or surge and a rough idle gas is fresh there has been other post but have not seen or heard the fix anybody know

sorry its an 08 with 5000 miles

I'll bet you your gas tank is pressurized. pop the lid on your gas tank, does it gasp open?

I have the feeling that mine is; the bike always run smoother after fueling. How come the gas tank is so closed; is it to avoid moist/water in the gas?

The tank has to be pressurized. That's how the fuel is pumped. Air is pumped into the tank, and fuel comes out of the tank.

Bikehigh, motorcycles don't have gas pumps?

The way it was explained to me when I bought the bike was that the system on our bike uses air to push the fuel out. I was told this as part of a warning not to overfill the tank. What I was told is that the system needs some minimum air volume in the tank to work, and if the tank is overfilled and there's no airspace, the pump can't put pressure on the fuel and the bike won't run. That's what I was told anyway.

Interesting as fuel injection in cars requires pressures higher than traditional fuel pumps could offer. I can see the pressure thing working though as the tank is above the engine so you also have gravity working for you. I'll have to read the manual on the fuel injection, but I would be surprised if there wasn't something upping the pressure....

I think it sounds weird? I mean, if the fuel supply relied on pressure only then there's only gravity to do the job after putting on gas... of course there's also the added gravitational pressure from the higher amount of fuel. But still. I fill up the tank quite a bit but have never experienced any problems what so ever afterwards; on the contrary, it runs smooth as ever.

If you don't get the fuel line reattached properly when putting the tank back on, whatever is supplying the pressure sure suppies enough that it has no problem blasting that fuel line right off, and the fuel shoots out at a pretty high rate. Whatever the mechanism, it's hard to figure how a bit of extra vapor pressure in the tank would make a difference to the fuel injection system.

Fuel weighs what? 6 pounds per gallon? 5 gallons of fuel should weigh 30 pounds. Spread out over the area of the bottom of the tank, that's way less than 1 psi of pressure that gravity can provide.

I've never been able to overfill the tank either, but I only fill it till the fuel is just barely intruding into the filler neck. The way the tank is shaped, there's certainly room for vapor/air in the upper part of the tank above the bottom of the filler neck.

Bikehigh almost had it, but then went off into some fairy tale I don't quite follow...

Whether the petrol is pumped, sucked, or falls out of the tank doesn't matter.
What does matter is that as petrol leaves the tank, the same volume of air needs to be allowed to enter to replace it. This happens through a vent in the cap.
If that vent gets blocked a vaccuum forms in the tank - eventually the pressure of the vaccuum is stronger than that forcing petrol down into the injectors, and the flow of petrol stops.

It's very easy to diagnose as others have pointed out: when the engine dies, open the fuel cap - you will hear a "hiss" as air rushes into the tank, similar to the sound you hear when you open a soda bottle.

Air is not pumped into the fuel tank to make the engine go.
If it were at the pressure required by the Fi system on this bike the tank would be toast in about 2 minutes.
In simple terms the fuel pump (it's in the tank and you can't see it) sucks in fuel from the tank (it's actually submerged in fuel) then pushes it to the Fi at a high rate of flow (generally speaking) and at high pressure (from memory it's ~ 45psi)
NK is right in that air must replace used fuel or a vacuum will form which can sometimes stop fuel flowing.
When you open the fuel cap have a look around the sealing surface, you will see a nipple looking thing (I think it might be orange), this is the breather/vent for the tank. As you close the cap is squashed a ring on the cap which upens some vents to the nipple and allows the tank to "breath". The nipple leads to a tube that goes down to the bottom of the bike near the rear shock area. If this hose is blocked it could be the problem.

Another thing to check is the fuel filter could be blocked/restricting flow. Do a search and you'll find mucho info on many bandit forums around the place on how to change it.

That's how I figured it had to be. I've noticed a hissing sound when opening the tank like before refueling the bike. So I suspected that there could be a vacuum in the tank, limiting the fuel flow to the engine and, thus, giving me a slightly fluctuating engine going at constant speeds.

Did not think the gas cap fits tight, at least on mine. I actually thought for the first time when I closed down and locked my cap it felt kind of loose, like it was not closing all the way. I do not have any hesitation in my bike, and the gas cap when closed and locked still has a little push down in it. I think that is due to the little rubber piece the cap closes down on.

Just my thought.