Hi, David 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 choke.
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. Fuel injectors stuck open.
13. Quick disconnect check ball stuck.
14. Compression below 75 PSI.
15. A stuck-bent-burnt valve.
16. Improper valve clearance (too tight)
17. Severely discharged or a damaged battery should have 12.5 volts or more and be able to pass a proper "LOAD" test if necessary, you may have a cursory reading of 12.5 volts or more but little or zero amperage the battery is faulty and must be replaced, AGM batteries fail in this scenario more so than lead-acid batteries.
18. 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.
19. Loose or corroded wire connection at the coil or plug between ignition sensor and ECM module.
20. Spark plug cables in bad condition and shorting check for spark leakage in the dark, cable connections are loose or connected to the wrong cylinders.
21. Ignition timing incorrect due to a faulty ignition coil, ignition module or MAP, CMP, CKP, O2, TPS, ETP, IAC sensors.
22. Faulty neutral, clutch, kickstand safety switch.
23. Faulty fuel pump or fuse or relay.
24. Faulty or corroded run/stop switch.
25. Tilt sensor needs a reset.
26. Security system not disarming alarm needs a reset.
27. Check for engine trouble codes.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day. https://www.motosport.com/blog/help-my-dirt-bike-wont-start https://www.youtube.com/watch?v=4ifXLvkB0RU http://www.partsforscooters.com/Dirt-Bikes-Parts/Dirt-Bikes-Repair-Manuals https://www.orionpowersports.com/pit-bike-dirt-bike-parts http://www.ssrmotorsports.com/store/ownersmanual/pitbike/Pit%20Bike%20Owner's%20Manual.pdf
Hi it sounds to me that the teeth on the front chain sprket could be worn so the chain locks,best thing to do is to take the side caseing off and check.(if it needs replaceing its always best to replace the back sproket and chain aswell,wish you all the best.
You can check ur values but some bike do make a we ticking from the values and it will back fire when revved up because of the extra fuel once revved that hasn't brine burn as it revs back down to idle speed.
Check the wiring on the connector plugs make sure all the wires are the same color on both sides, red to red blue to blue and so on. I had the same problem After replacing a stator and just plugging it in,after two weeks a new cdi and coil i noticed there were two wires transposed.
If youre talking about the spring in the carbs. throttle bodythe answer is yes. Inside the main chamber is a cylinder with a long pin ,this pin or needle goes to the main jet which governs the gas flow. if the spring is missing the needle will be in the wrong position . as far as the floats are concerned they teater on a set pin and are designed to float on top of the fuel in the float chamber.make sure they move freely and that there not rubbing on anything or bent.you might want to try some starting fluid to get things started then make your adjustments,after starting aduust the idle screw first. a good formula for idle screw is turn the screw all the way in . Then turn it out 3 and one half turns out, good luck,Gary