Unless you are the original owner of the bike and therefore know with 100% certainty how it spent the last 35 years, how often the oil was changed, and how hard it's been ridden, I don't see how you can claim whatever problems you are having are with the design. On one side, there is three of you with major issues. Even between the three of you the issue is not the same. On the other side, there is a pile of people all over the world with the same bike that aren't having these kinds of problems.
If the XS 400 was that fragile, there wouldn't be so many of them around still. Yes, they have some problems. Pretty much all of them are age-related - crumbling rubber in brakes, brittle fuseboxes, rotted foam in filters. Carburetor problems are the same as for any old bike - clogged with varnish from sitting, air leaks from old rubber. They were never fancy bikes, nobody babied them.
You lost a gamble on a very used bike. Sorry. Life ain't fair. Blame Yamaha, if you like, but any bike that old is going to have problems, and that kawasuzihonda you're pining for can turn out to be just as much of a lemon. A machine that does not at all break ever has not been invented yet.
If the XS 400 was that fragile, there wouldn't be so many of them around still. Yes, they have some problems. Pretty much all of them are age-related - crumbling rubber in brakes, brittle fuseboxes, rotted foam in filters. Carburetor problems are the same as for any old bike - clogged with varnish from sitting, air leaks from old rubber. They were never fancy bikes, nobody babied them.
You lost a gamble on a very used bike. Sorry. Life ain't fair. Blame Yamaha, if you like, but any bike that old is going to have problems, and that kawasuzihonda you're pining for can turn out to be just as much of a lemon. A machine that does not at all break ever has not been invented yet.