Chainsaw Repair

Lean mixtures are known to burn much slower, but mixtures richer than stoichiometric burn a bit faster - to a point.  Richer than 11:1 AFR flame speed drops like a rock.  The graph attached shows that.  That kind of threshold effect is really what I'm looking for, as it matches the observed running characteristic where 4-stroking breaks out suddenly.

Richer than about 9.5:1 AFR you get misfire. 

I have sometimes been able to see 1/2rpm lines in audio tracks from saws that are 4-strokking, but sometimes it's just not visible.  I've also been able to see 1/3rpm spikes from saws on a rev limiter - it's clear to me they cut out 2 out of every 3 ignition pulses.  Looking at a video from an old Stihl red light when it was clearly on the limiter it appeared to be only knocking out every other ignition pulse, and it didn't work well.  The saw could blow right past the limit. 

Diaphragm carbs are simply missing the entire air corrector jet, emulsion tube and fuel well that normally goes between the main jet and venturi on other carbs.  They had to get rid of that because the fuel chamber under the diaphragm is held below atmospheric pressure in order to keep the fuel from spilling out, and so they could not have an air bleed feeding into it.  Those parts have a function, and it is to make the AFR constant as air velocity varies.  Actually by adjusting the fuel and air jets you can make the AFR anything you want as air velocity varies.  Without that system the diaphragm carb is stuck with one curve - it tries to deliver a volume of fuel that is the square of the air velocity.