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[longlast]

There's been times were I fill the tank, and the bike doesn't seem to run as well. Then I'll top it up sooner at a little past half a tank. After that it runs better yet I'm using the same fuel,...It has happened a few times in the past.
Just wondering what could be the cause.   Altitude isn't a factor in this case.

[phato1]

One thing I can think of that would cause intermittent problems like you describe is variations in the quality of the fuel. Are you getting the fuel from the same station each time? I imagine you don't , I know I don't always get my fuel from the same source. I would make a note of the location of your fuel purchase and how the bike seems to be running to try and see if there is any link to a particular petrol station or brand of fuel, here in the states I have a few places I won't buy fuel from if I can avoid it and I also have a few favorites such as Mobil or Shell branded locations.

Another thing you might consider is if the fuel filter has ever been serviced?  There has been issues with the plastic fuel line inside the tanks developing small pin holes in them which can cause drivability issues. From what I've read on the problem the bike will run fine when the tank is full, but as the tank level gets lower exposing the pin hole/holes the line will draw in air, starving the bike of fuel.

I change my fuel filter and inspect the lines inside the tank about every 20,000 miles, it only takes about one pint of beer's worth of time to get it done.

[longlast]

You're right phato 1, I don't always use the same fueling station I don't know about the US but over here there's all stations are priced different even, lets say shell, one station will be a certain amount then 2mi down the road the next shell will be lower or higher then the one down the road. I use the same primium but keeping an eye out of the lower cost station. I don't believe by paying more you're getting better fuel.

I did a bit of research and kind of answered my own question. I don't think it makes a difference who you get your fuel from. It's more to do with which refinery and from were the distributor get the best price. It appears some refineries are better than others.    Here's a little wright up on refineries.

The result is that no two refineries are identical - although the common factors are similar crude distillation and upgrading units - each taking a slightly different route to a common goal of extracting maximum value from each barrel of crude oil processed. 

I would say that's were fuel quality is depending which refinery has the best deal going on and the distributors will buy from that refinery. Some refineries purduse a better fuel then other's that's noticeable on the motors running performance.

[VaEagle]

Another thing to consider is the type of pump used at the station. If it has one hose for multiple grades of gas you can have the lower grade of gas in the line from the previous person who pumped. No big deal on car gas tanks since that gallon or two of lower grade gas gets diffused with larger volume in the tank, but on bikes it can be a larger proportion of the small total tank so you could be trying to run on low grade gas?

[Road Dad]

The volume left from "other octanes" is not gallons but according to the American Petroleum Institute about 1/3 gallon. Even less in Europe.

Another thing to consider is the type of pump used at the station. If it has one hose for multiple grades of gas you can have the lower grade of gas in the line from the previous person who pumped. No big deal on car gas tanks since that gallon or two of lower grade gas gets diffused with larger volume in the tank, but on bikes it can be a larger proportion of the small total tank so you could be trying to run on low grade gas?

[Twolanerider]

Another thing to consider is the type of pump used at the station. If it has one hose for multiple grades of gas you can have the lower grade of gas in the line from the previous person who pumped. No big deal on car gas tanks since that gallon or two of lower grade gas gets diffused with larger volume in the tank, but on bikes it can be a larger proportion of the small total tank so you could be trying to run on low grade gas?

Even were that so (and it's a minimal difference actually) the higher or lower octane rating would manifest differently than he's describing.  Popular perceptions notwithstanding a higher or lower octane rating isn't a somehow more or less powerful fuel that makes the engine (by the fuel itself) run "better."  The octane rating is just a scale or scales used to measure how much compression a fuel can take before it detonates.  It's a measure of when the fuel will explode.

So a higher octane fuel or a lower octane fuel won't, by itself, make the engine run better or worse.  If you change the tune in the engine to accommodate a higher or lower octane that's a different thing.  If you have to lay off the throttle to avoid detonation due to a lower octane fuel than your tune is is dialed in for that's another thing also.  If neither of those things are done or required though it's just another tank of gas (all other things being the same).

[chaos901]

I try to stop at places that sell a lot of fuel because getting Premium at some of these smaller places can be iffy.  Even if they sell it, they may not sell much so it has been in the tanks for awhile.  Some moisture gets introduced over time, the filters on the tank should get that but again some of the smaller places do not service their filters as often. 

Even riding back roads you can usually find a good place.   

Started doing this after having a fuel filter stop up three miles from filling up in a small town.

[110tHunDer]

I bought one of these kits one day this winter when I was bored.  I had always wondered if the alcohol free 91 at the Phillips 66 station near my house was really alcohol free, when the other 66 about 10 miles away charged $0.60/gallon more for the same thing.

http://www.fuel-testers.com/quikcheck.html

Checks for the presence of alcohol and water.

Now, if only there was an octane test kit.

[longlast]

I bought one of these kits one day this winter when I was bored.  I had always wondered if the alcohol free 91 at the Phillips 66 station near my house was really alcohol free, when the other 66 about 10 miles away charged $0.60/gallon more for the same thing.

http://www.fuel-testers.com/quikcheck.html

Checks for the presence of alcohol and water.

Now, if only there was an octane test kit.

Used the same thing back in the 70s back then it was a small jar of white past that you would smear on a dipstick . It would turn blue or purple on detection.