The most probable cause is plugged/restricted idle passages in carb body due to dirt/gum.
Remove carb from eng,disassemble carb,remove hi/low speed needles,main jet nozzle(if removable)and remove any welch plugs in carb body(if used)soak carb body & all parts in carb cleaner overnight,run a fine wire thru ALL holes/passages in carb body & blow out with compressed air,if carb body has welch plugs & same were removed you'll probably have to buy a rebuild kit for carb,as plugs are usually damaged/destroyed removing them.
You might try a quick & dirty fix 1st before removing/dis-assembling carb,goto a local auto parts store/walmart & get a can of seafoam carb cleaner,add 1-2 oz to gas tank & 3-4 oz to gas container,run several tanks of gas thru eng & see if that cures problem,if not dis-assemble carb,clean & rebuild or replace carb.
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