possible vaccuum leak-flammible carb spray will raise engine speed nonflammible will lower or do as you stated . another good test is to limit the ari into the carb ,see if the rpm raises or lowers . if rpm raises you have a major vaccuum leak . popping exhaust minor fix is usually exhaust leak , burnt or corroded plug wires , idle mixture lean , fauled plug or burnt , carbonized cap or rotor , basic tune up stuff . put a vaccuum gauge on it , ported vaccuum if it hasnt any reading and you cant make any adjustment a vaccuum leak you may have . if this dosnt correct anything them you need to test further in other areas. start with basics , make one correction at a time and move forward until repair is made .
