Sounds like a carburettor issue.
Remove the carb and take it completelyto pieces.
If you don't have a manual, make a note/draw sketches of where all the bits go.
**** out all the passages with compressed air, especially the idle air bleed under the idle mixture screw which sticks out the side of the body at an angle and has a spring under the screw to retain the setting. When reassembling turn this screw in until spring just starts to contact body, then screw in one more complete turn.
Check the fuel level float setting by assembling the float and fuel needle. Then turn the carb upside down and the fuel float and "hinge" arm should be horizontal with the fuel needle on its seat.
Put it all back together and refit to engine. Make sure that carb/manifold gasket is in good condition, add a little joint compound. Pump fuel up to carb from tank and start engine. You will need to run it at high idle while you adjust idle mixture screw in or out to get engine speed to increase. When it increases, back off on throttle stop screw to reduce idle speed, then fiddle with idle mixture screw again (in or out) until you get highest speed, then back off throttle stop some more until you get nice steady tick over.