Chainsaw Repair

The link below is a short video clip showing 2.5s of audio track spectrum analysis from 4-stroking chainsaw.  Each frame looks at 380ms of data. 

The saw was someone else's Echo 590 with a muffler mod that was pig rich.  It's 4-stroking like crazy until it get to the widest part of the log, then it stops.

Starting at 6.5s the saw is very rich and 4-stroking.  At 9s the saw has reached the widest part of the cut.  As the wood gets wider the load gets higher and the rpm drops, and at 8s the 4-stroking stops.

After it begins firing every stroke the rpm rises, but not enough to initiate 4-stroking again.

In the first frame the saw is running at 8950rpm (149Hz), but there is a smaller spike at 1/2rpm (75Hz) that is the 4-stroking signature.  It disappears as the 4-stroking stops.