Following is the formula for brass cleaner used by Bud Eiler, restorer of outboard engines:

BRASS CLEANER - ONE GALLON MIX

8 oz. 28% Commercial strength ammonia (Blueprint Supply Store)
4 oz. Oleic acid (Pharmacy)
4 oz. Liquid detergent
2 oz. Acetone

To 3/4 gallon of water, add Olecic acid, mix well, then add detergent and acetone. Slowly add ammonia.
Some "clumping" may result as ammonia is added; shake well.
Allow solution to age 3 to 4 days.
Shake before using.
Pep up aging solution with new ammonia.
Use outdoors or in well ventilated area.

Store in brown glass (glass - not plastic) bottle away from light.

Dip parts to be cleaned in solution for approx. 20 minutes.