Uninformed
Ultrasonic carburetor cleaning is little understood. You wouldn't think so this late in the game. But what most people think they know is wrong. Sometimes folks refer to ultrasonic units as vibrating machines. YouTube videos showing electric sanders strapped to plastic totes claim to be, by this means, creating ultrasonic machines. No. Ultrasonic does not work by vibration. It works by cavitation. Now, I recognize that cavitation is not a widely know principle, despite its significant representation in powersports. So, think of how a boat propeller wears. As the prop spins in the water, its metal slowly wears away, at the tips of its blades and at its base. Boat mechanics call this "prop burn". It happens because the water around the prop is so violently affected that it cavitates. That is, its oxygen is sucked out. And this action steadily "sands" away the metal. Guess what? An ultrasonic machine works in the same way. The high frequency waves generated at the tank's bottom create tiny cavitations in the water that scrub the carburetor. Kind of implosions. The result is dramatic. The ultrasonic water must be changed frequently, so much muck builds up in the tank. Ultrasonic reclaims consumer electronics from smoke damage. Engine parts and firearms, all kinds of things are cleaned with ultrasonic. Yet it gently cleans jewelry too. It's pretty amazing. And with the proliferation of ultrasonic machines on the market, it helps to know how to choose one. So here's the dope.

Wattage
Wattage is the first consideration because it relates in the most direct way to cleaning power. When looking at wattage ratings attempt to separate the tank heater's (if so equipped) wattage from the transducer's wattage. Manufacturers like to combine them for more impressive specifications, which I consider deceptive. As far as that goes, a heater is an unnecessary feature, as anyone who has been around ultrasonic machines knows. Heat does of course improve the detergent's cleaning ability. But even machines without heaters heat their water just by the ultrasonic waves. In an hour's time you can't put your hand in. Therefore the only advantage a heater brings is it preheats the water. It's for those in a hurry. You can buy ultrasonic machines at virtually any wattage you can think of, all the way up to the megawatt engine cleaning units. But carburetor work starts at about 250 watts. Anything less is wishful thinking. This is a $3000 unit. The level of cleaning provided by a Harbor Freight machine is no better than you'd get scrubbing the carb in your solvent tank. It's just a fact.

Capacity
Capacity is the next consideration. But like wattage, capacity can also be used to deceive. A 10-gallon tank sounds great, but not when you discover that the manufacturer achieved that volume by building the tank tall instead of wide. A too-tall tank does two bad things. One, it places the workpiece farther from the all-important transducers (wave producers) at the tank's bottom, making the waves pass through more water, dissipating their effectiveness. And two, it leaves less room at the tank's bottom for the transducers, meaning too few will be installed, making the tank less powerful for that reason as well. The best tanks are wider than they are tall. But these tanks aren't found in economy model systems. Understand capacity.

Frequency
Then there is wave frequency, and it matters the same way wattage does, because frequency determines two things that are both very important but somewhat at odds: penetration and cleaning aggressiveness. A 45 kHz unit's smaller waves penetrate but are weak. This is what you'll find in a jewelry cleaning unit. It's also the kind of machine the Chinese produce the most. At the other end of the wattage spectrum, a 25 kHz unit's larger waves don't penetrate as well but are more vigorous, making it good for engine parts. Carburetors are best cleaned somewhere in the middle where the frequency is high enough that the waves get inside tiny carburetor passages, yet low enough that there is also good scrubbing. The midrange unit is a good choice. Even better however are machines producing dual frequencies. These are called "sweep" units because they constantly swing back and forth between high and low frequency, thereby offering the best results of both.

The Chinese "standard"
Harbor Freight and other Chinese suppliers have unfortunately defined ultrasonic cleaning for most people. But these units are toys. They typically exhibit all the bad things just enumerated. Low-powered, high frequency, with poor quality electronics including short-lived transducers that are epoxied onto the bottoms of their tanks instead of brazed on as in the better units.

YouTube
Have you noticed that all of the YouTube videos focus on the cheap machines? Sure, in the videos the carbs turn out somewhat nicer looking. But as I say, you could toss the carburetor in your dishwasher and get the same result. For real. Another way to spot an unknowledgeable presenter is when you see them using plastic containers for small parts. Though this will work, the ultrasonic waves are partly absorbed by flexible materials, reducing their effect on the contents. Glass or metal are better choices.

Chemicals
Ultrasonic machine makers promote surprisingly expensive cleaning chemicals. But all of them are basically detergent. By definition, detergents contain not only soap but more importantly, surfactant. A surfactant reduces the surface tension in a water film. It makes water "wetter". Even the most basic detergents work well in an ultrasonic machine. Old-school dishwashing liquid is popular. But beware of any cleaner that has a high PH as that makes it reactive with aluminum, creating white fuzz on the carburetor bodies. This includes Simple Green and its many copy-cats (Purple Power, etc.).

Disassembly
The point of ultrasonic cleaning is thorough, biologically safe cleaning. Not convenience. Not "cheating" the clock. Although there have been some folks promoting it, it is bad practice to put carbs into the tank whole. They must be completely disassembled. For one thing, left assembled, pockets of water will remain inside the carburetors, and this will react with the diecast aluminum main bodies, corroding them. But there are many other reasons to not do this as well.

The cat's meow
Incredibly, a few "experts" have spoken out against ultrasonic cleaning. It's difficult to imagine why. One even insists that wet blasting, which is actually a surface finish technique, takes the place of cleaning. Incredible. Ultrasonic cleans and does so very well. Without it, you have no choice but to expose yourself to chemicals that are either carcinogens or known neurological hazards. Without ultrasonic you must find separate ways to clean plastic, rubber, steel and aluminum parts, not to mention tiny springs and diaphragms, all of which are safely and thoroughly cleaned by ultrasonic. Without ultrasonic, you are avoiding the very best way to free corroded-together and varnish-stuck parts. Without ultrasonic you have very limited options for removing minor oxidation. And without ultrasonic you miss the pleasure of seeing freshly cleaned carburetor castings glisten prettily almost as when they were newly made. Great stuff, ultrasonic! Be informed.