Help! 2004 KX250f almost never starts.

Hey:

I have a 2004 Kx250f that is a real pain to start. Sometimes it will fire over after you kick it a million times. It will usually bump start. Once it's running, it runs fine.

So far I have done all of this stuff:

Leak down test (less then 2%)

compression test

Valves are within spec

valve timing is set perfectly

new ignition coil

new plug

totally cleaned carb and messed with the choke jetting a little

The bike has a high compression piston and was tough to kick, so I added the manual decompression kit and disconnected the auto decompression.

checked the stator coil (resistance test)

disconnected the neutral switch (was actually like that when i bought it)

When i hold the plug up to the motor and cycle the kick starter there is spark.

The bike mostly just backfires when I try to start it.

I even tried to spray ether in it and it would just backfire.

After a round of trying to start it the plug is wet.

It's acting like it has low compression, but it the compression test and leakdown test results are both totally fine.

Someone told me that i might have "worn out valves," but i don't understand how that could be if the leak down is fine?

Do I have a bad CDI box?

Any other suggestions?

I am at my whits end with this thing.

thank you

pat

are you sure the valves are within spec? have you cleaned out the petcock on the tank? That gets gummed up some times... Are you using the right starting method? turn the gas and choke on give 5 kicks slowly with the throttle open. get the piston to TDC kick it over hard though the full stroke with no throttle. If it back fires give it a tad of throttle but not much. hope this helps.

Both exhaust valves are within spec, one intake is within spec, and the other intake is .001 loose.

The petcock is clear, but thank you for the idea.

I've tried clearing it out before kicking it, no luck.

thank you

richen the pilot jet 42 or 45 air screw at least two turns out. see if that helps.

I've tried richer choke jetting (starter jet), and that seamed to help a little. When i say richer though i mean i went from a 60 to like a 120 (drilled out). When i first bought the bike, it had wierd jetting in it. 160 main, 58 pilot (stock is 178/40) and the accelerator pump screw was adjuster very far in. The bike rode well with that jetteing, but would only bump start. I put it back to stock jetting except the choke jet and it started easier. It seams like ultra rich choke/pilot jetting is compensating for the real problem.

I'll try the pilot jet.

It's just weird, my YZ400 starts first or second kick almost every time. Why is this thing so difficult?

Are you 100% sure on your valve timing? Is it backfiring through the carb or is the exhaust popping?

With the ether, it backfired through the intake. on gas it may have been through both, but definitely from the exhaust.

The timing was one tooth off when i bought the bike, i put it back to spec per the manual. I will check it again tonight.

Do you have an '04? Where are you located? I'd like to find someone with the same bike to test my CDI box on.

I had about the same problem...my valves were in spec.....but come to find out....when we took the valves out of the head....the exhaust valve was worn out.....it was just seating enough to run.... Like you....I would kick the bike to death.....could bump start it and it would fire up........just something to check.........good luck....

With the ether, it backfired through the intake. on gas it may have been through both, but definitely from the exhaust.

The timing was one tooth off when i bought the bike, i put it back to spec per the manual. I will check it again tonight.

Do you have an '04? Where are you located? I'd like to find someone with the same bike to test my CDI box on.

I'm in Ontario about an hour from Buffalo NY.

Yeah, the more people i talk to, the more likely it seams that the valves are bad. I am going to pull it apart this week and check it out.

Are most people going to SS valves or are you staying with Ti valves and using Beryllium guides?

pat

Yeah, the more people i talk to, the more likely it seams that the valves are bad. I am going to pull it apart this week and check it out.

Are most people going to SS valves or are you staying with Ti valves and using Beryllium guides?

pat

I replaced all of my valves with the Kibble White SS.....I know the Ti valves are lighter.....The tech that done my head work said I may lose a few rpm's but I would never be able to tell it......I guess if I was a pro racer I would want every rpm the engine could put out. This tech has built some thumper engines that run the baja races....He said he has seen over 50 hours on some of the KW SS valves..... they still had the same valve clearence from when they were installed.......that sold me......:thumbsup:

Zook1 ..... You might want to check out Dave at MXTIME.com. I had him rebuild my head and he also hooked me up with all the other parts for my rebuild. You can't beat the price and service. Search the forum and you will find many comments.

There was tons of discussion around these back when...do a search. The 04's two piece flywheels had issues often as you describe. In 05 the flywheel (one piece shell), stator, and CDI were re-designed. Save yourself and just upgrade to the 05 and up parts all together.

I pulled my head today. The piston is brand new just like the previous owner told me. How can I tell if the valves are bad?

I'm begining to think he was having problems with the bike and being a two stroke guy, thought that new piston and rings would solve it.

Is there a way to tell if the 2 piece flywheel has gone bad? (i would assume the two pieces and come out of alignment).

pat

Edited by zook1

You will need to get a valve spring compression tool and remove the valves from the head.....

The outer shell of the flywheel distorts...it is riveted. The 05-up one piece is fully machined and does not distort. The stator also was changed after 04. You can use parts up to 08. The 04 ignition/cdi was also completely remapped in 05 for better starting and power. High compression pistons under fast rider would take the cranks out as well...both rod and crank halves were subject.

I had the same problem on my KX250F. Drove me nuts. Want to know what fixed it for me??? OK, I will give you a freebie on this because it drove me absolutely nuts for months. Ended up being the position of the clip on the needle. I set it back to spec (if I remember was 3rd position, maybe 4th). Whatever the spec position is, put it there. When I did this, damn thing drove like a champ and started right up. I had to get all of my jetting back to spec after I changed that clip position. Great running bike after that.

Another thing that can happen is the flywheel woodruf key can shear or just become partially broken. This will throw off the ignition timing enough to do exactly as you describe. Easy check, just pop of the flywheel and check the key. Also inspect the taper of the crank and flywheel.

Another thing that can happen is the flywheel woodruf key can shear or just become partially broken. This will throw off the ignition timing enough to do exactly as you describe. Easy check, just pop of the flywheel and check the key. Also inspect the taper of the crank and flywheel.

Agreed its an easy fix.. It's not a very common thing with the kx's actuly I've never heard of it happening on the kx but it could have.. Or possibly it was not installed when the previos owner changed the piston.

The flywhel must be installed completely DRY on the taper, or else it can slip.

Also, check if the camshafts have spun in the sprockets.

The hole in the outer lobe should align perfectly with the hole in the cam sprocket.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now