Yup -- little bit of a problem here. There are a few things you can do. If your engine is a new model, there will be a tab on the crankcase on the front and back immediately below the fins on the cylinder. Make sure the retainer spring sits in the groove formed by this tab and the lower fin. The tab keeps the spring from working downward and allowing the muffler to tilt. Also make sure the spring clips have not spread apart -- when attached, the spring should squeeze the endcap and muffler body together.

You must make sure the muffler is correctly seated on the exhaust port of the crankcase -- if tilted up or down or shifted forward or rearward, it has a tendency to move around. Over time, unless correctly seated, the port on the crankcase and the opening into the muffler will begin to round -- this allows the muffler to move even more and you may see dark oil on the crankcase below the crankcase port. This is due to leakage and vibration wear at the exhaust port/muffler opening.

With your spring lying flat on the table, make sure the sides that clip over the muffler are either parallel or slightly bent inward. If spread outward, they will have a tendency to pull the muffler apart. Sometimes, in reattaching the muffler, we inadvertently spread them apart. See image.

You can safety wire it by wrapping light wire around it between the muffler and cylinder to hold the spring together -- to keep it from spreading when the muffler endcap tries to work out.

Once the endcap starts to work loose, it just seems to get worse over time -- I've had them come completely out in flight, but have not lost one yet. The final solution is more permanent. I usually decide about what angle I want the exhaust pipe, clean the cap and body very well with acetone, rough the surfaces lightly with sandpaper and JB Weld it together. Aside from redirecting the exhaust, there is no reason to need to rotate the exhaust pipe or open the muffler.

the "other" andrew