2006 trx500fe

I bought this bike a month ago and have been in the process of rebuilding it. I have put a new jug and piston, new valves and seals as well as reseating them and set them to 006.. four wheeler is back together and runs great when it starts. The problem I am having is the motor acts like it has to much compression starter turns engine but not over the top, pull start is vary hard to pull but will start bike. When you take the spark plug out the engine will cycle smooth but once you put plug back in it doesn’t turn well. Is there a way to adjust compression? I’ve read up that it’s possibly built in to the cam but I’m unsure what route to go on this issue. Thank you in advance for any and all help!