Stihl 031 Misfire sound?

[gravydude]

I've been working on my 031AV for weeks now. The unit didn't want to start at all. After checking the gaps on the coil, points and installing a new carb rebuild kit, the saw finally runs!! At least for about five minutes.

The saw runs great until under a load. I can rev this saw up with ease, but when I put it on a log, it loses power and sounds like the plug is misfiring.

Any suggestions?

Don't know if it could be bad coil, condenser, or something else.

[Cornishman]

Plug misfiring. You did ask ! Presumably you have changed it.
Seriously though it could be anything. Weak spark prompted by some electrical fault such as a faulty coil that gets worse as the saw warms up or under load. Mixture wrong. My favourite would be a leak in the impulse tube  and you are running short of fuel.
Fuel pipe(no holes) and filter OK?
Compression OK?

[jerry sundberg]

I've found that crankshaft seals will do this.