You would be amazed what can cause a trim change. Last race season I lost a stripe of covering off the bottom of my wing. This left a bare wood area about 7 inches long and 3 inches wide. The airplane took half of my available up trim. After the heat I covered the section with packing tape. Next heat I had to take the up trim out. A cowl will certainly cause a difference in trim. Then again the OP doesn't even state what airplane let alone post a picture with cowl off.