I just picked up a 400EX with a bad valve cover gasket and am in the process of getting it back together. The previous owner put an HMF exhaust on it and it sounds pretty bad *** in my opinion. I asked if he jetted it or not and he said he never touched the jetting on it because it ran so nice. I'm just wondering if it's a must to rejet it or not. It sounded fine when I test drove it. Am I risking anything if I don't?