Summer's here and all of a sudden your motorcycle engine is making a new sound. It reminds you of marbles dropping in a coffee can, a rattly mettallic noise that sounds funny but you aren't laughing.
What you're hearing is "pinging" or detonation. It can be caused by a couple of different things.

Using fuel that is too low on octane can do it. Make sure you're getting good hi-test. Sometimes the pump hose has as much as a gallon of low test in it if they all come out the same hose and the guy in front of you used low test.

An air leak in the manifold will do it sometimes because it makes a lean mixture at low speeds. Also when the motorcycle gets really hot (like here in Tucson!) it will ping.

If you have good gas and the motorcycle is in tune, it may be necessary to back the timing off a degree or so to get rid of it. There are other things as well, like the ignition or voes switch malfunctioning too.

One more thing I'd do is to vent the right side gas cap by drilling a 1/16" hole in the plastic motorcycle part inside, in one of the 4 bumps surrounding the spring vent. This keeps the carburetor from starving for gas when it's hot. Be careful not to drill all the way through the cap, only about 1/8" is neccessary.


Source: http://www.directparts.com