Four-stroke engines tend to run a bit better if you're using fuel with all-synthetic or mostly-synthetic oils. That's because castor will tend to varnish and gum. If you disassemble your engine a few times per season, it's not a problem, otherwise go with lower-castor oil content fuels. That also means you'll have to flood your engine with oil at the end of the day to prevent rust.

For the O.S. FT-160 twin-cylinder engine, you should start out with the high-speed needle at about 2-1/2 to 3 turns out from closed. That's a very rich setting, but is a good starting point. The air-bleed screw should be set so that it blocks the air-bleed hole 1/2 way across. Start the engine at a part throttle and advance it to full. Then get the high-speed needle set. If one cylinder is leaner than the other, set the engine to the lean cylinder. Once the top-end is set, you can adjust the idle.

The air-bleed screw works opposite to a needle valve. You turn it clockwise to richen the idle mixture, and counter-clockwise to lean the idle mixture. The FT-160 should be able to be set so that you get an excellent idle.

If one cylinder tends to quit at a reduced throttle setting, but not at a true idle, or one cylinder won't idle for very long, then you may have an ignition problem. Use brand-new O.S. Type F glow plugs. If new plugs don't work, try adding an ignition battery to the plugs that will heat them when the throttle is below 1/4 or 1/3. A last resort would be replacement of the piston rings, cylinder liners, pistons, or all three. Bad compression will keep any engine from idling well.