We need a description of what happens when you open the throttle. If there is an increase in the roaring sound from the air filter, but no change in engine speed, the problem is almost certainly fuel starvation: the fuel system is only providing enough fuel for the initial engine speed, and opening the throttle just leans out the mixture.
If fuel starvation is occurring, there are several likely causes: restriction in the fuel tank vent, or in the fuel line, or in the fine filter usually fitted to the carburetor fuel pump's intake cavity. Beyond that there are other carburetor malfuction possiblities, all of which require a full strip and clean of the carburetor, almost certainly with a replacement of gaskets and diaphragms.
It usually makes sense to follow a simple process:
1. Does the air filter noise increase greatly when you open the throttle?
2. If you answered yes to Q1, inspect and clean the fuel system upstream of the carburetor and see if the problem is cured.
3. If you answered yes to Q1 but the problem remained after you performed the cleaning in Q2, remove the cover from the carburetor fuel pump input chamber and observe the micro-fine metal mesh filter there. Remove and clean it very carefully, reinstall it, ande see if the problem is cured.
4. If you answered yes to Q1, and have now performed the cleanouts in both Q2 and Q3 but still have the problem, you should strip, clean, and re-kit the carburetor, following the instructions from the carburetor manufacturer's technical manual (usually available on line).
