I looked closer at how intake on my old Husq 288 and 266 saws seem to be designed. Fascinating...
Flywheel turns, centrifugal fan, as mentioned before. I think this DOES create positive pressure on the side of the engine because the fan is offset with a larger gap on one side than the other between fins and housing, how a typical centrifugal fan works. I think it forces air up and under the front section of the cover. This fan pulls in air from around the pull start cover and flows it up and over the cooling fins and out over the muffler toward the front.
However, there is also a divider under the cover which essentially seals, separating this forward section from the carb and air filter, which are located under the cover in the rear.
BUT - there is also a small hole or opening on my saws in the divider near the bottom. This is obviously where air enters the rear chamber for filtration. I think the hole is low to keep the intake air as cool as possible. If the hole were at the top, more heat would come in from the cylinder.
Also, dirt and dust and sawdust particles and chunks blow past this hole from below at high velocity due to the fan. The larger chunks have too much velocity to make the turn and go into the hole so therefore they fly on past the intake hole, keeping the rear intake region around the carb cleaner.
It stands to reason if positive pressure exists near the fan on the forward side of the top cover divider, then this pressure helps force air through the hole on it's way to the carb. It may actually help with air flow to the carburetor. I know the carb works on vacuum, but pressure on air on it's way to the carb may assist with intake and reduce the suction required to get the air in.
Mods:
By placing holes in the rear chamber where the carb is, the carb will suck air from atmosphere without the extra help of the force-feed from under the front portion of the cover. But, the air will likely come in a few degrees cooler, so maybe it's a wash?
Also, my saw has another hole with a plug, I think for snow operation, which pulls in more heat from the top of the cylinder. Remove this and get more air, and more dirt I guess?
Also, if the normal intake hole in the divider is enlarged, or more intake holes added, it seems this may allow the carb to breathe easier, BUT it also seems to me it would serve to circulate more flying debris and dust around the carb. If the divider hole is too large, I can see circulating more air around the carb than the engine can draw in, which would just add more dirt to the filter and around all the carb linkages.
It's got me thinkin'.
Overheating is more likely due to lean mix or overworking the saw under high load with low RPM, right?
