What Dan said and add a bit of 2-stroke (TCW-3 is best) or MMO to the gas so you are t around 100:1. The added oil will get to the cylinders and will often work sticky rings loose
.
If you have an EI open the gap to about .045. This will create a bit more heat around the tip of the plug and aid in not fouling. If points try around .040. While you are doing this identify the fouled plug and place it in another cylinder. This will confirm a bad plug or bad cylinder.
A dry then wet compression check will yield good info too.
Dave Neptune