Hi Mike, welcome to Outdoorking.
From your description, you have lean mixture at idle only: mixture is OK at higher speed, and the choke works. To verify this, I suggest you try warming up the engine then adjusting the choke at idle. If the problem is indeed lean mixture, you should be able to find a choke setting that gives you a perfect idle, then the idle should deteriorate with either less or more choke than that. Please report back when you have tried this.
There should be no problem using a pre-filter made from urethane foam from Clark Rubber. At worst, it would be restrictive and would make the mixture rich. Since your symptoms suggest lean mixture, the pre-filter cannot be blamed. Incidentally, your wording suggests that this was the only air filter, rather than a pre-filter. The service manual specifies a urethane pre-filter in front of a paper air filter. Please confirm that you have the correct paper filter in place.
