could be too rich on oil, yes. but, are you too rich on gas? also, to check plugs good, you gotta run wide open down the road in at least 5th gear and hold it there for 6 sec or so, then kill the motor, but hold the throttle still wide open, till you come to a stop, then release throttle and take plugs out. check them now for that carbboard to chocolate brown. even better yet, if you can see the tops of your pistons, you can check the wash, which is a good indicator of how rich you are on gas.