Just to help you understand what's going on... The engine revs to the max due to a lean fuel mixture. The reason you can't stop it normally is because the spark plugs are actually glowing red from heat at that point which is what is sustaining ignition even though there is no spark. Fresh fuel actually cools the plugs and pistons before it ignites. When you pull the throttle open after pulling the lanyard, you are in fact cooling the plugs which kills the ignition source. Putting your hand over the carb actually pulls more fuel into the carbs which does the same thing but in a different way... Only difference is it's really hard to put your hand over the carbs when the flame arrestor is in place.
Like said before, you have an air leak somewhere which you HAVE TO fix before you even think about hitting that start button again. If you don't know how to do a pressure check or leakdown check, do some research on here as it's been discussed multiple times. If that's a new top end, you've already done some pretty serious damage to that engine that will inevitably shorten its life.
