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I have noticed many times in my Royal Enfield Classic 350 motorcycle that when there is a low quantity of fuel or quantity before reserve is near, the sound of engine becomes noisy and pickup power becomes little bit less.
I want to know exactly what is the effect of fuel quantity on my bike model.
This is just a guess and not specific to your bike. I'm also not sure if the effect is large enough to be noticeable. So, just my theory:
In the bowl of a carburetor, there is a float, pushing a valve up against the fuel inlet pipe. So it closes when fuel is at a certain level in the bowl.
Now, fuel finds its way into the carburetor by gravity only. And if the tank is full, there is more pressure at the fuel inlet, and it needs more force from the float to close the valve. This means, the fuel level in the bowl is a little higher when the tank is full, and a little lower, when the tank is almost empty. And as consequence, the mixture is leaner when the tank is empty.
And a lean mixture could cause the effects you observe, though it's hard to believe that the effect is so strong.
Depending on compression ratio (CR) of any given engine whether it can can benefit from higher octane fuel, regular grade fuel is for compression ratios of 8.5 to 1 or lower, premium grade gasoline is for 9.0-10.0 CR.
If you have 10.5 or higher CR premium will still ping on premium unless you retard the timing 2 degrees or more, which defeats the purpose of high compression and extra horsepower, you would need a fuel additive or racing fuel for these motors to take full advantage of the CR. Engines with aluminum cylinder heads tolerate lower octane fuel better than cast iron heads, less detonation (ping) for any given CR and ignition timing setting.
Yes you will notice less performance on higher CR engines with low octane fuel and they can sound different also.
I definitely notice the bike not running quite right a mile or two before running out of gas or switching to reserve on my bikes. How far in advance are you seeing it?
Check that your fuel tank isn't contaminated with silt / rust particles. It may be worth emptying the tank, removing it from the bike and giving it a good clean. As fuel tanks get older it's very common for them to gather small particles which sit at the bottom of the tank. When the fuel level is low, the ratio of fuel to particles increases so can choke the amount of fuel being pulled from the tank as the particles restrict the flow. They can also be pulled through the fuel system and cause poor running.
A good way to check this is to stick a magnet wrapped in some white cotton securely attached to a stick into the tank and swoosh it around the bottom of the tank. If the cotton comes out with particles attached or dirty in any way, that would indicate that the tank needs cleaned.
