Four stroke newbie
 

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

Vent
 

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

Four stroke newbie
 

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

Four stroke newbie
 

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.