Chainsaw Repair

The only reason to make the bottom of the intake port close/open progressively is to take it easy on the piston skirt . Make the intake 70% wide and the bottom of the port totally flat , run it 5 minutes and look at the piston skirts , it sure won't look good !

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.

Also more intake open time(Duration) will help to increase "ram effect" which will occur in a race motor with enough rpm,intake duration,and large enough carb to allow it to happen. Ram effect actually supercharges the crankcase to a certain degree.
I have actually documented ram effect on a race motor by conducting vacuum and pressure tests on a running engine from idle to full throttle. My tests showed that there was constant pressure in the crankcase during ram effect,and there was no vacuum even on the piston upstroke... The fuel/air column reaches enough velocity and energy to overcome blowback from the desending piston,and rams into the crankcase.

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.

Do not go crazy and "square" the port out. Of course you are right,you must,round the bottom edge to help gently guide the piston skirt back into the bore. I.ve seen so many blown up motors(mostly pistons shattering).this is always the cause,guys who think they can "square out" the port for more size,but it's fatal,put a gentle curve on the bottom edge of the intake,then chamfer it very well! Your engine will last forever while the other guys try to figure out their piston shattering problems...