I've cleaned carb bodies and a bunch of other stuff in ultrasonic cleaners. Here's what I recommend.
1. Pre clean the carbs as much as possible. If they're covered in oil, grease, dirt, etc. all it does is foul the solution and inhibit cleaning, not to mention that the non-soluble bits like sand and grit can get re-deposited inside nooks and crannies in the carbs. I like to pre-clean in a 5 gallon bucket, using undiluted cleaner and a stiff paintbrush. Knock off the big chunks and then rinse with water.
2. Disassemble as much as possible. Take out the pilot screws, main jets, etc. DEFINITELY remove the rubber diaphragms if these are flat-slide carbs, and remove any other non-metal bits you want to salvage.
3. Ideally, clean the small, critical bits like jets first. You can use a tea strainer to hold them and keep them from bouncing all over creation. If anything in there is really delicate-- a very fine pilot screw, for example-- clean it by hand. Parts can make contact in an ultrasonic cleaner and be damaged, though I've never had it happen to me.
4. When cleaning anything, you're probably going to be using a 10-20% solution of some type of cleaner in water. Get the water as hot as you possibly can beforehand. I boil it in a kettle or the microwave to get it as close to boiling as possible. Hot water cleans much better, especially when you're talking about petroleum-based deposits. Obviously if you're cleaning anything heat sensitive (some plastics, etc.) you may have to reduce the temperatures to what the material will handle, but this isn't a problem for metal carb parts.
5. For cleaners, keep away from anything with lye or any of the "purple" cleaners. These are very caustic and will etch (and eventually dissolve) aluminum. I watched a set of aluminum brake calipers actually froth when I tossed them in a bucket of Purple Power. I knew this would happen so I cleaned them very quickly and rinsed them thoroughly with no ill effects. I would NOT do the same thing with carb bodies, no matter how short the duration.
6. For carbs, I have used LA's Totally Awesome Cleaner from the dollar store, Simple Green, Oil Eater, dish detergent, and Pine Sol, to name a few. Pine Sol works well but it will leave your garage smelling like pine for eternity. I prefer LAs Totally Awesome because it's cheap and works reasonably well if mixed strong (15-20% by eye) with very hot water. Oil Eater seems to remove heavy deposits better but not every place sells it. Common dish detergent has a lot of surfactants in it which are good for removing particulates like dirt and grime, but not as good at dissolving fuel. Dish detergent is really optimized for breaking down things like food proteins. It can work in a hot solution and is fine if it's all you have, but you may have to scrub a little more by hand. Simple Green is OK-- no better than LAs Totally Awesome and more expensive, but works fine if you have it around. Any of these may cause staining of the carb bodies, which will have a mottled, darkish appearance when removed from the ultrasonic. This is just a possible side effect of cleaning carbs in any water-soluble solvent at higher temps. The staining doesn't affect function and is no big deal (to me at least).
7. Clean the carbs in 5 or 10 minute intervals, checking after each one to see how clean it is. I've never had a carb body get damaged in the ultrasonic cleaner but you still need to be careful. You will still have to make sure every small passage is clear because there just isn't enough room in those tiny orifices for solvent to get in and break up hard varnish, etc. An ultrasonic cleaner gets you about 90% of the way there but you might need to chase the passages out with thin wires or tip cleaners. Blow each one out, then blow them out again.
Most ultrasonic cleaners won't handle more than one or two carbs. If you're doing a rack of 4 carbs, an alternative is to dunk them in a Goodwill or yard sale crock pot overnight. Fill it with water and 10-20% of your favorite cleaner, put the carbs and parts in, let it cook on medium until the next day. Make sure all the carbs are completely submerged and make sure the lid is on to limit evaporation. Again, the carb bodies may come out discolored but they'll be very clean.
If in doubt, practice on a junk carb, scrap aluminum casting, etc. until you dial it in. Hope this helps.
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