Chainsaw Repair

OOps I made a math error,intake port open time in duration is .00021 seconds. Increasing intake duration to 185 degrees =.00023 of a second actually increases the port open time by 9%,-a possible 9% power increase. I will take that anytime,in fact I will do the mods needed even for a 1 or 2 percent increase,it all adds up quickly.

Off by 10 - at 13k a 185deg duration is 0.0024sec, or 2.4ms.  It's still a damn short amount of time and it's amazing they run up there at all. 

I'm sort of skeptical the carb actually responds to each revolution/intake pulse at all at those rpms, rather I'd bet it responds to the average.  The outlet check valve is probably fast enough, but I'm not sure the metering diaphragm/lever is.  What I was thinking about is that these carbs to not give a constant fuel/air mixture as air velocity varies (because they have no air corrector jets), instead the volume delivered goes up as close to the square of the air velocity.  This is why a saw 4-strokes - with only a small increase in rpm (=air velocity) it gets so rich it misfires.

Given that, I was wondering what those very fast air pulses from the flat lower port edge do in regard to metering.  The short intake can't help in that regard.  Still, I guess if you can tune it then it must all work out.

Also, the carb venturi shape is optimized for flow in one direction, but it is still going to add fuel in the opposite - so those strong pulses may contribute considerably to spitback.  Whether that is a problem might depend on the air filter, etc.