07 kx250f backfiring problem

Hi everyone, I have a 07 kx250f and I have a major backfiring problem. Well at first, everybody told me that it got to be the carburetor adjustments. So, I tried that but it didn't make that big of a difference. Next, I tried switching carburetors on the bike and adjusting it. Still made no difference. Then I tried switching the coil pack or the rubber plug that goes on top of the spark plug. Nothing different. I switched exhaust systems, spark plug, cdi box or ignitor box. Still nothing changed. There are no intake or exhaust leaks on the bike. We did a leak down test and it was good with only 15% leakage at tdc. We also did a compression test and only got 60 pounds of pressure. But that is because of the compression release on the cam. Could the compression release maybe be malfunctioning? This is what I'm wondering. But if there is anything else that you guys could think of, I would definitely appreciate it. Anybody's input would help!!! Please email me at vwracing56@gmail.com with comments or input.



First, has the bike alway backfired or did this suddenly start happening? If it just started, what did you change on the bike/did you have a crash just prior to the backfires?

Try disconnecting the killswitch.

Well, I bought the bike and it was backfiring when we bought it. I didn't think that it was a big deal, just thought it was going to be simple carb adjustments.

I have a friend that was having sort of the same problem....he tried alot of the same stuff that you have.......Come to find out that the intake cam had a bad wear spot in it and was not letting the valve seat....If this is not a carb issue.....you may have to tear down the top end and take a good look at all the cams and valve train.....Compression and leak down test can miss problems sometimes.........good luck on your project......

If.... the backfiring is abnormal and the bike runs well, first thing to check is the exhaust system. Try a new head to header gasket. Ensure the bolts are properly tightened and the flange is even. Make sure the header to muffler connection is sound and the packing in the muffler is serviceable.

All bikes backfire, the quieter the muffler, the better it quenches these sounds. Non race bikes have a ACV (Automatic Coast Valve) that adds fuel on decel and that reduces the backfires somewhat. Race bikes have neither and therefore have more decel popping. Pro race bikes do not pop because these guys are either on the the gas 100% or on the brakes 100%, rear wheel barely turning often. The engine is never coasting or de-accelerating.

There are things you can do but they come at a cost:

You can add a quiet muffler and loose a little top end HP at 10,000 rpm. If you are a A or B racer, this will make a difference to how fast you are. If you are a C racer, not really. And if you were an A or B racer, you would not be getting decel popping anyways.

You can put in a slow fuel jet (aka the pilot jet) or a smaller slow air jet to richen the idle mixture (popping is most prevalent when the throttle is 100% closed and the only fuel is being delivered by the idle circuit) and or a needle with a step or two thinner on the straight diameter(part that affects operation between just off idle and just under 1/4 throttle). The down side here is extreme difficult hot starting and poor low speed performance, the exact throttle areas most people honestly ride in a large part of the time, the C riders and the trail riders.

Is this a "backfire" or a "pop" on decel?

Is this a "backfire" or a "pop" on decel?

The bike backfires and pops. And the bike backfires and pops all the time, not just on decel

If it runs poorly under load, that is another matter entirely.

I first suggest pulling the carb, carefully cleaning it with brake parts cleaner or electrical parts cleaner (do not use carb parts cleaner, it can ruin internal rubber parts).

Make a note of what jets are in the carb, the needle designation and clip position.

You also ahould pull the valve cover and check the cam timing.

I would replace the spark plug. I will assume you have visually inspected th bike and everything appears correct.

The bike backfires and pops. And the bike backfires and pops all the time, not just on decel

Try this quick check for HT problems:

With the bike on the stand, slowly open the throttle, no quick twists...does it rev without backfire....If it does rev ok, ride it but this time put the motor under load, lots of throttle. If it backfires now, you may have HT issues. Plug cap, coil lead or plug.

The bike backfires and pops. And the bike backfires and pops all the time, not just on decel

You are welcome to try disconnecting the killswitch like I mentioned earlier. It´s a fast, easy and free check.

Crashes and water can make it "semi-ground" and disturb the ignition.

You are welcome to try disconnecting the killswitch like I mentioned earlier. It´s a fast, easy and free check.

Crashes and water can make it "semi-ground" and disturb the ignition.

yup! I say giving DK's suggestion a try.

have you checked the valve clearance?

Yup, checked the valve clearance and it is perfect

I do hope you tried DK's suggestion.. Sometimes the simplest answer is the correct one.. That and DK & my self post good info! Respect the man DK for his help!

After reading that you tried a different carb and it was still doing the same thing...There is a good chance that there is a problem in the engine....Always try the small and simple stuff first....I just have a feeling that you may need to tear the top end down and inspect each part.....You said that it had this problem when you bought the bike... This could still be a valve problem.....I had to replace a worn out valve that gave me trouble...It always was in spec....but after taking the valve out of the head is when I found out it was worn and not seating all the time....Hang in there...you will get it figured out.......

I tried disconnecting the kill switch and it made no difference. But one other question that I have is does anybody know how to test, adjust or do anything with the ignition timing? Thanks

You do not have to adjust anything for the ignition timing. That is not the problem.

You've elimated:

The carb - You swapped it from a bike (intact, not jet swaps) from a good running bike

Electrical - Tried a CDI, Coil, new Plug, confirmed it is not the kill switch

This leaves either cam timing (check to ensure the cam sprocket did not moce on the cam) or a bad stator. I would run the tests from the manual on the stator and crank position sensor. However, even if they return values in an acceptable range, the stator can still be bad. A sign this is the problem is when misfires get worse as the rpms increase.

Finally, just to be crystal clear, this is not a issue on decellerating but under power, correct?

Correct, is backfires when under a load, not when you let off the throttle. And another thing that might make a difference is that the bike does not backfire when the bike is on choke. Thanks

I bought a Polaris 700 once that was doing that and found the flywheel key had started to shear which threw it out of time. It had just moved about 1/8th of an inch but was still able to run but backfired like crazy. Good luck!

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