1. Empty gas tank.
2. Gas tank air vent not open.
3. Fuel lines kinked or severely pinched.
4. Water or dirt in fuel system
5. Clogged fuel filter or screens.
6. Motor not being choked to start.
7. Engine not primed -- pump primer system.
8. Carburetor adjustments too lean (not allowing enough fuel to start engine).
9. Timing and synchronizing out of adjustment.
10. Manual choke linkage bent -- auto choke out of adjustment.
11. Spark plugs improperly gapped, dirty or broken.
12. Fuel tank primer inoperative (pressurized system).
13. Ignition points improperly gapped, burned or dirty.
14. Loose, broken wire or frayed insulation in electrical system.
15. Reed valves not seating or stuck shut.
16. Weak coil or condenser.
17. Cracked distributor cap or rotor or shorted rotor.
18. Loose fuel connector.
19. Poor engine or ignition ground.
20. Faulty ignition or safety switch.
21. Low cranking speed.
22. Low voltage to ignition, when cranking.
23. Low Compression
24. Safety Lanyard/Kill Switch disconnected
Takeaway: If your engines won’t start, don’t just keep cranking the ignition or you’ll drain your batteries and add one more thing to the list above.