Hi,
I have a used OS .91 four stroke and can't figure this out.
I see the fuel fitting on the carb. I see the pressure fitting on the muff. I now see a fitting on the bottom side of the engine. What is this for???
Also, will this engine work well inverted???
Thanks.
Matt

Crankcase vent to atmosphere, make it about 3 inches long or to bottom of fuse.

The fitting on the bottom of the crankcase is the crankcase breather. Either leave it alone or if you are compelled, put a piece of tubing on it and run it outside the cowl.

Four strokes present a couple of problems being inverted. One is tank height. It must be centered on the needle valve. The second is that you no longer have an updraft carb but a down draft one. They will often flood easily when inverted.

Other than that run it just like your 2 strokes with the same fuel.

Use a 14-6 or 13-8 prop and a OS 'F' plug.

Enjoy,

Jim

Quite a bit of oil will be ejected from the crankcase vent, that is why it is a good idea to put a line on it and get it outside the cowl. This is also where you inject any afterrun oil in a 4 stroke. Putting after run oil in the carb or exhaust does little good on a 4 stroke. You need to get the oil into the crankcase where it will coat the bearings on the crankshaft.