Once the mechanical pinch is gone is a safe time to remove the shim. Mine was never installed to begin with but at approx 1 gallon I removed the thickest of the 2 installed shim. This incresed the compression thus increasing run time and performance. I didn't see the need to install the extra shim because there isn't enough pinch to warrant it IMO. I'm also down to a 7mm restrictor but thinking of going down to a 6.5mm.