I modified my OS 61FS, 2- OS 120FS, and my OS 120 Twin. In each case a severely restricted nipple was necessary in order to not upset the idle mixture and low end acceleration. This had to be determined thru trial and error and was quite time consuming. The 61 required a restrictor of .0145", the 120's a restrictor of .020" and the Twin a restrictor of .040". The intake pipe on the 61 and 120 FS is too thin to tap so to provide a nipple, I soldered a piece of 1/8" copper tube containing the restrictor, to the intake pipe. On the twin I tapped into the manifold tee.

The 61 proved to be sensitive to temperature change adversely affecting the idle and midrange. The 120 FS engines were not similarly affected. The 120 twin suffered from engine failures on approaches when the fuel level was less than about 1/4 . All engines had approx double the rate of carbon accumulation in cylinders and on exhaust valves as compared to when the breathers were vented to atmosphere.

After experimenting for a couple of years and many hours of test flying, I returned all the engines to their stock configurations, and all engines perform better.