If it isn't burning oil now, there should not be much carbon in the muffler: accumulated carbon from smoke emissions prior to a piston ring replacement usually burns away fairly quickly. If it is burning oil now, there would be blue smoke - especially at start-up.
If it steadily emits sparks without smoke, and puts out a flame-ball when you shut the throttle, I think Joe is right as always: you need to take a look at the exhaust valve seating. This needn't be a big job, but do take care to end up with the correct tappet clearance or your work will be wasted.
