Leaky valve cover on a 4 mix.

[PTmowerMech]

Makes sense.

To skip to the important part, skip to the 2:00 mark

 

[Hammermechanicman]

The valve cover is part of the crankcase primary compression.

 

[StarTech]

And you got to remember the 4 mix is a hybrid engine using both 2 and 4 cycle designs in one unit; hence the use of two cycle fuel mix. It is one of a very designs that hold up to use.
Makita and Shindaiwa are the only true 4 cycle designs that I have seen to hold up to regular use.