You might give this a try, it was posted by Aerosplat a while back and worked like a charm on my 1.60.

If you have a pump or Cline system on the engine, (and it is working properly) fuel delivery should be consistent independent of Muffler pressure.
First, open up the muffler and let the engine breathe. If you had some of the muffler outlet plugged before Pump / Cline, remove the restrictions.
Second, I say again, LEAN THE LOW SPEED NEEDLE. This particular OS carburetor seems to have more than its share of interaction between low speed needle and high speed needle settings. PEAK the high end, don't back off rich. Go to idle. If engine idles well, for 15 - 20 seconds, lean the low speed needle 1/4 turn. Go back to top and re-peak. Go back to idle. Same routine, if it idles, lean the low speed screw another 1/4 turn. Continue this process until the engine will NOT sustain an idle. It should speed up and die within 15 - 20 seconds. Now Richen the low speed 1/16 turn at a time until the engine will again sustain an idle. After each adjustment on the low speed, go to top end and peak the high speed. AS soon as it will sustain an idle, leave the low speed alone. Each flying day, just re-peak the top end and back off 150 RPM rich for flying. You HAVE TO GO LEAN TO THE NO IDLE POINT on the low speed, then come back to get it set right. Transition from idle, midrange, 3/4 throttle, or anywhere on the stick should now be crisp without sputtering. When tuning the top end for flying, peak the needle and back off no more than 150 RPM rich. More than that will cause the midrange to load up and sputtering transition is back. If you run this engine with rich high speed, you will have lousy transition, and midrange running.

Addendum: Idle speed should be 1900 rpm or less. If you are above 2000 rpm at idle your are still interacting with the high speed setting. Get the idle down to 1900 or less to set the low speed. My OS 1.60 will idle at 1800 - 1850 for over a full minute and still go to full throttle without sputtering.