4. Yamaha YZ490
Yeah, sure, the Yamaha YZ490 has a cult following. Whatever. It’s a real stinker among the open-class machines of its time and a worst dirtbikes candidate for sure. Yamaha introduced the YZ490 in 1982, after a solid run with the YZ465. Sadly, the 490 was heavier than the 465, nowhere near as reliable, horribly difficult to start (hot or cold), came with a grim suspension, vibrated enough to bring blisters to your hands and was impossible to jet.
If you jetted a YZ490 rich enough to keep it from seizing, it would blubber, puke, foul plugs and produce no power. If you jetted it to run strong, it would invariably seize. It came with air leaks, a wandering ignition, gimpy motor mounts and the worst case of Yamahop at high speeds since the original DT-1–all at no extra charge to the customer. Yamaha issued a mountain of service bulletins to try to fix the problems. None of them worked.