Gotta first 4-stroke newbie question for you guys... I am just now mounting my TT F90S 4-stroke engine in my 4* 60 size and noticed a breather fitting on the lower front of the engine beneath the crank shaft. I've identified the fuel inlet (carb) and the muffler fitting for fuel tank pressurization but what is this fitting for? The TT manual doesn't explain it as far as I can tell.

It's the crankcase drip. Simply run some fuel tubing from it down to the oval notch in the fuse that's just under the engine. That's what I did. If you need some pics of it, let me know. I'll be happy to post some.

Cruz is correct, it is a crankcase vent. You don't HAVE to do anything with it, but some black oily residue will leak out of it, so I usually stick a piece of fuel tubing on it and vent it outside the cowl or engine compartment.

Do not under any circumstances close off the crank vent opening! Vent it to a location outside the airplane.

Thanks guys. I'll put tubing on it and run it down and outside the fuse.

Also that is the vent you'll add afterrun fluid in after each day of flying if you want the engine to stay rust free and live to be an old age.