I bought a 81 650 special recently and it had some work done to it like new tires and seat and chain as well as a tune up. However once in a while when I drive it the right side cylinder will slightly backfire. Is this just a spark plug problem? I did check the plug and it is not black but golden brown but there is some oil that drips once in a while above the plug, does this mean that the oil is going into the cylinder? And lastly do all 2 cylinder bikes have alot of engine vibs when the revs are up?