I've recently built a derbi varient sl. After I put it together it ran at about 40mph and was very powerfull off the line. Then after about 2 hours of riding the speed dropped to around 10 to 15mph. I've done everything I can think of and everything seems relatively ok, but the sparkplug keeps getting blacker and oilier. I' ve tried tuning, I can actually run it lean and the plug stays black, I've even cleaned the gas tank to no avail. Does anyone have any other ideas why my spark plug is black and oily?Could it have some thing to do with timing or points?