I just picked up a 97' formula 500. It runs great for most of the day...but then starts sputtering and blatting. If I change the plugs it goes like hell again. I have been running br9es plugs because that is what the former owner had in it. Are these the right plugs. Also the mileage is terrible on this thing. My buddy 01mxz800 and I ran 50 miles and we both fueled at the start of the day. When we topped off our tanks....my gauge was showing empty.We both fueled and he put 7.85 in his 800...and I put in 13.50 in my little 500. Is it just me or does this seem really horrible mileage.My old 91 polaris 650 never fouled a plug and I could get right around 100 miles on a tank. Any suggestions would be appreciated.