Is it safe to use a muffler when breaking in a lapped piston engine? I have some nostalgia-legal K&B Greenheads and Johnsons (.15 to .32) that I plan to use in NOS free flight. A couple of them (the Johnsons) are still a little tight and may need additional running with castor-based fuel before mounting but the noise might be a problem for my neighbors. A muffler should quiet them down to at least the string trimmer level.
Ideally, it would be best to break the engines in without a muffler especially if you are in a high humidity area. However, as noise is a factor you do not have much choice. Just use a low restriction muffler. You may have a problem finding a muffler for the Torp and Johnson engines. A fellow out here in Calif. named Scott Dinger makes custom mufflers that are light weight and not too restrictive. Hyflo Mufflers, 1367 Patricia Ave., Simi Valley, CA 93065 (805) 526-9074