I have a short back 1977 k2 that I purchased last year. I use it for corn and soybean research plots. The machine runs great, except it has a hopping vibration that I have isolated to be the shoe (chaffer/sieve assembly). I just replaced the rubber bushings - only 1 was 1/2 worn thru rest were not too bad. I also changed the eccentric bearings. It still shakes. I have run the long back K2s (1978-1980's) and have not notice this problem. Is this the reason Agco built the long backs or is there other reasons for my problem? The vibration really plays hob with my weighing system. Thanks, Eric.