If it is a 2-stroke Chinese engine, that is normal. After the intake charge is drawn into the crankcase, port open and piston traveling up, the piston has to travel down before the intake port is covered. During that brief time the intake charge will actually get pushed backwards out the port and backwards through the carburetor which will soak the air filter element over a short time. This is but one more reason why a reed valve in the intake is desired. The only way to fix it is to install a reed valve, which some have crafted themselves as none are marketed for these engines yet.