I have a 1990 DR250, my piston is hitting my spark plug. I am working with the jetting, trying to get it right. I pulled the plug to check the color and noticed the gap was wrong. I gapped it and put it back in. Started it, it ran, then when I took the plug out to check it again, the gap was to small again. So I figured the piston must be hitting it. I verified, it is the right plug for that bike. The only way I could get the gap to not be affected was to put 3 spark plug washers on it? I dont get it, could someone before me have put a dome top piston or something? What should I do? Any help?