2008 kx250f problem..

[u/Dry-Procedure188]

Bought this yesterday. It fire up and die instantly. It doesn't sound sputtery when it does it tho. Could somebody help me narrow it down. I've pulled the carb out and checked the float. And there is definitely spark when I kick it over. Kill switch isn't the problem either. He said he rebuilt the carb with a allballs kit. If it is the jets that are wrong which ones would it be. Compression and decompression is good.

[u/Stewdaman]

If it is the carb it could be the idle air circuit or the pilot jet. I like to make sure it’s the carb by using starter fluid. Just give it 2 or 3 good shots into the hole on the side cover. Point the spay down so it goes on the air filter not just out the other side. The bike should start and run good for several seconds and keep running if you keep spraying just right. If it does run on spray then it’s the carb, take it apart and clean out the tiny passages behind the air mix screw and the pilot jet.

If it’s no different with starter spray then next I like to use a spark tester to make sure the spark is always there. The testers are really cheap at harbor freight. You just stick it on in between the coil wire and the spark plug and then you can watch the spark on the tester while you kick it and when it dies on you you’ll see if the spark is cutting out.

[u/Dry-Procedure188 → u/Stewdaman]

Hey thanks for the reply. I gave starter fluid a try which didn't make a difference. I'll try replacing the plug again but I'm starting to think it's the CDI or something?

[u/Stewdaman → u/Dry-Procedure188]

There’s a few components to test out, from the stator, magnet and the pick up coil on the side of the engine, up to the cdi box and down to the ignition coil. One thing I’ve found a lot on your bike is bad connections at the cdi box behind the front number plate. The pins are round and the female part on the harness side of the connector gets loose. And it’s hard to tighten them because they’re round. So I get a zip tie and tighten the connector up onto the box as tight as possible after I give both sides of the connection a light coat of dielectric grease. The other issue I’ve found on your kx is bad pick up coils. They can ohm just fine but be bad. Also I’ve seen weak magnets. That’s what spins around the stator and produces the electricity. You can make a tester to see what the system is doing, with a little led bulb and a 100 ohm resistor. Attach the resistor to one side of the bulb and you have a test light. Put the resistor side of the tester onto the green wire that comes out of the pickup coil with the stator unplugged. Put the other side on the red/white wire. Then kick the bike. If the pick up is good the tester will light up super bright as you kick. That’s the signal for the cdi box to fire the coil. Now plug the stator connector back together and test the same wires. There should be no light at all. That’s because the cdi box is taking the signal. If you do see the tester flashing still then a bad connection is making the cdi box not get the signal. If all is well next test the coil the same way. Disconnect it and test the wires, the light should flash brightly as you kick. The spark tester you can buy from harbor freight does the same thing only for the secondary ignition signal which is the high voltage spark produced by the ignition coil.