Hi, John and the usual suspects are:
1. Severely discharged or a damaged battery, should have 12.5 volts or better and be able to pass a load test if necessary.
2. Check battery terminals for damage or corrosion, check the battery cables at "BOTH" ends for loose, corroded, or broken connectors, "INSIDE" and outside the cable harness, perform connector wiggle test and check cables with an ohmmeter if necessary.
3. Spark plugs in bad condition or partially fouled.
4. Spark plug cables in bad condition and leaking check for spark leakage in the dark
5. Spark plug gap too close or too wide.
6. Faulty ignition coil, module, and or sensor.
7. Loose, dirty, or corroded ignition module connector at crankcase.
8. Faulty CKP or CMP, MAP, ETS, ATS, TPS, O2 sensor.
9. Dirty air filter.
10. Intake air leak.
11. Water, dirt, or rust in the fuel system.
12. Carburetor damaged, contaminated, sticky float, plugged jets.
13. Fuel filter clogged.
14. Old or contaminated fuel.
15. Fuel tank vent system plugged or carb vent line closed off.
16. Gas cap diaphragm valve faulty.
17. Vacuum line from intake manifold to fuel valve is broken, cracked, pinched, or missing.
18. Carburetor controls misadjusted.
19. Incorrect valve timing.
20. Valve springs floating, weak, or broken.
21. Damaged intake or exhaust valve.
22. Incompatible performance parts.
23. The control module may be in "LIMP" mode.
24. Check for engine trouble codes.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
91 XJ600S misfire at idle on 2 cylinders
Yamaha XJ600S 1992 Service Manual
OEM parts for Yamaha
Yamaha Diversion XJ600N Owner Manual