Let’s say you have a typical, reasonable condition, not-seized muscle car Holley or Rochester Quadrajet. The bodies of these carbs are green chromate-finished zinc; the throttle bodies are natural aluminum. In my old carburetor shop (now closed) we would remove the throttle plates, choke plate and all shafts and fittings before cleaning.

For your own sake, remember that I’m presenting this information only for reference purposes – you must use your own judgment and be sure that you are aware of any hazards present if you’re going to try any of these operations. Some of the chemicals and procedures used below are dangerous and are for professional use only. ‘Nuff said.

Here is a typical cleaning sequence for a fully disassembled carb:

1. Degrease: I use lacquer thinner, gunwash grade. This stuff is flammable and you don’t want to breath the vapors. Use caution. An overnight soak followed by a thorough brush cleaning is sufficient. Use a wood-handled bristle brush, not a synthetic one (they will fall apart). Work outside or in a fume hood.You can also use commercial carb cleaner. If your cleaner contains dichloromethane or tetracholoroethylene (2-layer type cleaner) DO NOT soak aluminum parts any longer than necessary. They will corrode if left overnight. Many bike carbs and SU’s have been ruined this way.
2. Air dry thoroughly and blow out with air. Wear eye protection.
3. Acid soak. I use a commercial metal cleaner which consists of mainly phosphoric acid and detergent. Soak for a couple of minutes until fizzing is evident and ‘white rust’ is removed. Don’t overdo it or your carb will be destroyed! If you don’t have this cleaner or you aren’t restoring the chromate finish later, you can use vinegar instead. It won’t attack the finish as fast, but will remove crud with a bit of brushing. Again, don’t overdo it.
4. Rinse thoroughly in hot running water.
5. Air dry and blow out very thoroughly. The carb must be bone dry for the next step. Bake in a 200 F oven for half an hour if unsure.
6. This and the following steps are only performed if you’re going to restore the original chromate finish.
   Glass bead: One way to get the carb down to bare, shiny white metal is to bead blast using very fine (#8) beads. Use clean beads and do the carb inside and out until it’s a shiny silver color. Use a blast cabinet and wear breathing protection.
7. Blow out thoroughly with a high pressure blow gun – a ’safety gun’ isn’t powerful enough. Wear full-face and eye protection and hearing protection and be thorough.
8. Acid rinse: Using a fresh batch of the same type of phosphoric acid/detergent as above, immerse the zinc parts of the carb only for about 30s or so. Aluminum parts don’t need this step, though they can be cleaned this way – no more than a few seconds exposure, or they’ll go dark on you.
9. Rinse very thoroughly in hot running water until all signs of fizzing are gone.
10. Immerse in the chromate solution for the appropriate time (see below). This is the trickiest part, since it depends on the concentration of the solution, the temperature and on the type of carb – not all zinc alloys are the same. I will say more about the chromate solution below, but for now, suffice to say that it’s dangerous, expensive and tricky to use. You can immerse both zinc and aluminum parts for corrosion protection, though only the zinc parts will pick up the green colour.
11. Rinse thoroughly in COLD running water until all traces of coloured chromate solution are gone. Hot water will wreck the finish at this point.
12. Air dry.  Blow out with clean compressed air, thoroughly as before.

If everything is done right, the zinc parts of the carb will be an iridescent olive-green colour, just like new. Be warned though, it’s tough to get it right.

If you’re doing a backyard resto, you can stop after step 5. Usually, it’s best not to remove the finish (step 6) unless you’re going to redo the chromate. So, I would stop after the vinegar rinse if that’s the case. Now you can invest in a can of Eastwood carb finish and spray the carb. It’ll look OK from a distance, and even better when the hood is closed.

About the chromate solution

This consists mainly of chromium (VI) oxide (aka chromium trioxide) dissolved in water with a bit of nitric acid added. These are dangerous chemicals in untrained hands!

If you want to try, you can obtain the chromate mix (solid) from Atotech. Minimum quantity is 100 lbs – enough for a lifetime. Follow the mixing directions and prepare a solution of about 30 g / L of the solid mix in water. Add about 1 mL / L of concentrated nitric acid.

With this solution, it takes a 10 s soak for a typical Holley or Rochester carb to form the chromate coating, OEM style. The picture above shows a 1970 big-block Quadrajet that was refinished using this procedure.
As I said, this isn’t really practical or advisable for most people.