My guess is that this motor doesn't have a stock cam. You need to turn the idle screw in to much. It is idling on the main circuit. There are 2 ways around this. My favorite is to take the carb off and turn it over. in the baseplate on the secondary side you'll see a set screw that controls the opening for the secondaries. Take a small screwdriver and turn this in around 1 turn. It will be very stiff. You'll probably need to put a wrench or something on the screwdriver to help turn it. Put the carb back on and fire it up. It will probably idle rather fast. Now you can close the primary butterflies with the idle screw. This works the same as drilling holes in the secondary butterflies, but much easier and not permanent. If you need more adjustment go ahead, but don;t go more than 1/2 turn at a time. This has always worked for me.
don
