i have a 2001 R50 4x2, 215K miles I would start with fresh tune up including fuel, air, in-cabin, and oil w/ synthetic blend oil. If smooth idle, just the plugs. Then clean out ALL passages that the air gets in using soap & water. Most IMPORTANTLY, CLEAN THE MAF SENSOR !! Make sure that it's kept clean every 5K miles... I'm driving 45 miles each way to work/home. 1st 15 miles each way is traffic. I get a solid 19.6 using premium @ 80