Very hard to start 1991 yamaha fzr 600,

check for fuel to the carbs, pull plugs and check for spark in the plugs while cranking over. if no fuel check the fuel shutoff, if no spark, check for +12v at the coils and also the c.d.i. box.

how long has the bike sat? it sounds like your haven a fuel problem. i would start with the simple first fuel filter, fuel lines, vac. lines and intake boots. If you have a vac. leak it could cause it run funky. Check your spark and spark plugs, plug wires and boots. if you haven't found the problem yet you should look into your carbs, you could have trash from your fuel filter or gas tank if your lines are old they could have started to brake apart. If your ride has been sitting for awhile you could be facing clogged jets. I don't know how you are with taken things apart and putting it back together with no left over parts, but i will say that the carbs have A LOT of small parts. If you don't have any luck with simple check list and would to take a look into your carbs ill be more then happy to help.
Good like
Mark Woodring