^this. your shim's just the wrong size.
I've also had a lot of luck making shims. my favorite material is milk jug, it's gas resistant, easy to cut, available and compressible. cut a strip, wrap it as many times as needed and clamp down. electrical tape is NOT a good option though, it's not gas safe, especially not the glue and gets real messy real quick, plus it can flap around or get sucked in places. just a mess.