I've got a 625 SXC, and a 690 Duke. The 690 is magnitudes smoother than the 625, at lower revs. I think the biggest problem with the older LC4 engines, was that the owners wanted to gear them tall, and try and ride them around like an old school Thumper. Lug them around at low revs. They hate that, love to rev. They're one of the very early short stroke/big bore high performance 4 stroke singles that are built to rev. Roller bearing crankshaft, roller bearing big end, roller rockers, roller bearing cam, everything designed to spin hard. You had 2 choices if you wanted smooth. Either ride around a gear lower, or gear down with a larger rear sprocket. I'm personally happy just to cruise along at about 120 km/hr, where it's rather comfortable, and we're both in our happy placeI am not sure how the new ones are, but the old ones were a bit of a cement mixer, they would shake quite hard until you hit about 4000 rpm, so you usually had to ride them hard to be comfortable.