Why an ultrasonic cleaner? Generations of mechanics got by without one. Can you buy carburetor cleaner at your auto store? Find a suitable metal container, take the carb apart, soak it in that. If you don't own an air compressor, go to your ironmonger, DIY store, or home improvement store* and find a spray can of compressed air to blow out the various tiny openings. Reassemble and give it a try.
Incidentally, here in the States, several companies sell a sort of super-stabilized petrol (TruFuel is one company, among others) that lasts years in the unopened can and two years once the can's opened (although, if your garden shed is damp, you may get a shorter life). Besides its stability, it seems to do a good job at keeping small motor carbs clean. Ask around in the yard equipment aisle at the appropriate stores. It's more expensive, but I find it well worthwhile.
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*Some of us here in this ex-colony call them "Big Boxy Stores."