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I've noticed a difference that when the fuel in the tank is low, it takes a bit more time for the bike to accelerate to a higher speed than if the fuel tank is mid level or full. Same thing said by a person in this video too: https://www.youtube.com/watch?v=pC1kaBBCC7A

I've noticed this with cars too. After I fill more fuel, the car responds much better to acceleration.

Why does this happen? In the bike, the fuel flows downward, so it couldn't be because of liquid pressure. The carburetor would still be getting the same amount of fuel, so it couldn't be that either.

  • 1
    The only possible explanation would be that a tank at a low level is sloshing about such that the fuel is not being delivered continuously. I can think of no modern vehicle that does not have baffles or "ducks feet" devices which mitigate this possibility on anything but an almost completely empty tank. – SteveRacer Jun 25 '16 at 4:41
  • I think this is because of a weak fuel pump – method Jun 25 '16 at 12:11
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It sounds like you have a motorcycle with a carburetor with gravity fed fuel line. (i.e. Without a fuel pump of any sort). In those conditions I can see that there is a higher fuel pressure at the inlet to the carburetor when the fuel tank is full. It's possible that when the tank is low the pressure is barely enough to meet fuel demand.

With that said, you need to improve fuel flow between the tank and the carburetor. Fuel line and petcock could be too small (diameter). You could have a partially clogged fuel filter or residual crud in bottom of tank restricting fuel flow.

It is also possible that your carburetor's fuel bowl system isn't working correctly. The fuel bowl in the carburetor acts as a little reservoir of fuel to ensure there is always fuel available to meet fuel demand.

I'd start with checking the filter and tank for dirt and crud. Let us know what you find!

Update:

Hmm. I watched that video before I responded.

What you are describing sounds like the newest religion. You just gotta have faith.

I will say I'm not a rookie at this stuff. I worked at a Detroit car company where I managed the sale and engineering of millions of fuel tanks. I'm more than familiar with fuel tanks, pumps, carbon canisters & reservoirs. I'm really good in troubleshooting related issues.

What you are describing on your car is fooey. Facts not in evidence. Pure snake oil. "You can see it if only you are a true believer". Ditto for the person in the video. (Or he just has a clogged fuel filter :^). You probably shouldn't believe everything you read on the internet.

And on your bike if you do have a true measurable difference in acceleration do check for dirt and debris between tank and carburetor. Hint. A measurable difference here implies the use of a dynamometer (or simple stopwatch on a measured and marked distance, given exactly same conditions except fuel level in tank. )

  • Nope. It's not about dirt or crud. This happens even with my brand new car. Happens with the bike even before and after it has been freshly serviced and a carburetor tuning done. In my question, I linked to a youtube video where another biker says he can feel when the bike has low fuel. I'm talking about the same thing. I can feel the difference. – Nav Jun 25 '16 at 16:13
  • Bunk. Facts not in evidence. See my update. And yes I meant millions of fuel tanks. – zipzit Jun 25 '16 at 17:21
  • @Nav When I first answered this I thought you were a total stack rookie. You have a very impressive stack count. Apologies for the 'attitude' in my response. No offense intended. – zipzit Jun 25 '16 at 18:13
  • Hey, we are all here to help each other. I didn't feel offended. Rather, I was happy to receive good info from a knowledgeable person. Thanks for the compliment :) – Nav Jun 25 '16 at 18:26
  • Been examining this phenomenon, and my conclusion as of now would be that the vehicle feels different after a fuel refill because of the added weight of the fuel. In a bike, the center of gravity changes. In a car, it feels heavier and more stable. Recently ran the bike till the tank emptied, and I didn't feel any difference in pull until the fuel ran out. So you were correct, @zipzit. – Nav Oct 29 '16 at 4:08

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