Today, after four perfectly fine flights, as I was putting the model away, I noticed that the muffler seemed very loose and figured that was probably the reason that I was getting a couple hundred less RPMs today. So when I got home I took the cowl off to tighten the muffler and found the engine and cowl covered in "silver slime." The only thing I can imagine is that this is metal filings mixed with the RCV91cd's usual copious oil residue. I cleaned it all off and tomorrow I'll run it again to see if it reappears...but really what is this???

Note: The engine seems to be running very well.

It's fine alluminum powder that chaffed off your loose exhaust and mixed with the oil residue . It is either from the muffler threads were vibration from the looseness just filed the threads away and made power out of them ,or it is from the muffler body possibly rubbing on something when it was loose. If the mufller was loose at the cyl. head ii may be from the exhaust threads in the cyl. head.

That was my guess. The threads on the muffler seem ok (though possibly a bit worn). I was able to lock it tightly against the nut. We'll see what happens when I run it tomorrow. But it might be a good idea to order another muffler (and maybe also an extra needle valve).

Luckily it was tight on that end.

Why not just use a sutable lock washer?

Dennis

I hate when that happens.

that is the threads vibrating away...sounds like you caught it in time..

But, unless you do something different, it will happen again, somewhere....some time....

locktite is your friend..............

You might try wrapping the threads in teflon tape.

I've had luck using a real flare nut wrench (almost a full circle) on the locknut and tightening it HOT after a run.
I can't tell you how many times I've seen that slime, ignored it, and been sorry.
Finally wore the threads right out of my Saito muffler- cost me $35!