Timing on 1971 18hp

[mab867]

Is there a timing mark on this particular model. When I cleaned the carb, I had to remove an "arm?" from the carb. This "arm?" has a set screw on it. When I put it back together, it just has not ran right since then. At first it would not even start until I adjusted the set screw. I can only think of it as some kind of timing set screw. The motor now starts fine, but keeps fouling the plugs and I can only get about 40-50% rpm out of it. When the motor is running at full throttle, you can hear and feel that it is trying to fire, but not at a constant rate. Hope someone can help...thanks