To do a leakdown test, you need to remove the pipes, and plug the exhaust ports. Then remove the carbs and plug one side and put in an adaptor with a pressure guage and pump on the other side. Now pump up the pressure to about 5 pounds. Now watch the pressure guage, it should hold 5 pounds of pressure for about 5 minutes. If it drops, you have a leak. To find the leak, use a mixture of dishsoap and water, and run this around the base gaskets and intake boots and gaskets. If you find bubbles, then you have found your leak. You can make one of these tester with parts from your local hardware store, or I think motion pro makes one as well.