Spark plug gaps

I tried opening the plug gap on my 410 as Bewy recommended to 50 thou! It idled a little bit better and was a tad more responsive down low but on full throttle @ over 6000rpm it would backfire badly. I did not cure that back fire until I closed the gaps up to under about 40 thou. My 410 has 11/1 compression. And runs NGK plugs!