Hi, Liam and the usual suspects are:
1. Fuel tank empty.
2. The fuel tank has old dead gas.
3. Fuel tank bottom contaminated with ethanol sludge, dirt, water, rust, etc.
4. Fuel supply valve/petcock turned off.
5. Fouled spark plugs.
6. Engine flooded as a result of overuse of the enrichner.
7. Vacuum hose to the fuel supply valve/petcock disconnected, broken, cracked, or pinched.
8. Fuel valve/petcock or filter clogged.
9. Fuel line to carburetor or throttle body pinched, kinked or blocked.
10. Carburetor float stuck.
11. Fuel injectors clogged.
12. A severely discharged or damaged battery, you need 12.5 volts or better and be able to pass a load test if necessary.
13. 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.
14. Loose or corroded wire connection at the coil or plug between ignition sensor and ECM module.
15. Spark plug cables in bad condition and shorting check for spark leakage in the dark, cable connections loose, or connected to the wrong cylinders.
16. Ignition timing incorrect due to a faulty ignition coil, ignition module or MAP, CMP, CKP, O2, TPS, ETP, IAC sensors.
17. Faulty neutral, clutch lever, or kickstand safety switch.
18. Faulty fuel pump or fuse or relay.
19. A stuck bent or burnt valve.
20. Tilt sensor needs a reset.
21. Security system not disarming alarm needs a reset.
22. 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.
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Yamaha FZS600 98 Service Manual
OEM parts for Yamaha
YAMAHA FZS600 Owner Manual
Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/gregg_c0ec1df182c7330e