Please post a picture of the rubber.

pic attached

Look in the breather and etc to see what might be missing. If it was run w/o a breather, what could have been sucked into the cylinder? Was this discovered due to the trimmer having problems? If so, what was happening or had happened. Is the trimmer still running OK?

If the air cleaner has always been in place, and it is
a fairly new machine, then it could have gotten there
at the factory. Wipe one of the pieces off with some
solvent to see if it was originally black, or was
carbonized during the previous running of the machine.
Strange. . . very strange.

Ok, I broke down the engine and I found the source of the "latex" It appears to be a gasket on the cylinder cover. See pic. Forgive my ignorance as I've never broken down an engine beyond the carb but is this a gasket you buy or is it one which is self made with some "liquid" gasket material?

The Brand, Model and Parts List/Diagram should answer the questions. Please share what you find. I would say it ran too hot but the other parts are not showing excess heat from what I can see. The type of gasket will be interesting.

My latest crankcase related experiment was interesting. The cyl and crankcase was one casting with a side cover vs Removing A Jug. There Is Some Interesting New STUFF Out There!!

There is no gasket part number for the cover, so I bought some Black RTV silicone gasket but will have to wait on putting it on because I had to order a piston ring. I broke one of them or it was brittle one.