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KX250F popping and backfiring

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Ok so I recently bought a used 2008 KX250F for 1900 bucks. When I bought it, it had 213 hours on the hour meter and I've gotten almost 9 hours on it so far. Everything had been rebuilt for the previous owner of the bike when he got it, he was the second owner and never had any problems with it. The owner told me to get the valves checked after I'd purchased it because it was popping a little bit. I did so and they were all in spec. It still had this same popping and the mechanic at the shop I took it to said to get the carb cleaned as that was probably the cause.

So I took the bowl off the carb and it was clean and changed the Main Jet and Pilot Jet in it. It had a 42 Pilot and a 178 Main, that I changed to a 190 Main and a new 42 Pilot. After changing the jets it had alot more power, but still had a slight popping to it at idle. This was about 2 months ago when the weather was still in the 70's-80's.

Now It's in the 50's here and on certain days the bike doesn't seem to pop as much but today it was popping like crazy after I rode it and had been the worse it's ever been. I changed the Main to a 195 because it's only going to be getting colder out and left the Pilot at a 42. After riding it today the bike seems to pop like crazy whenever i give it gas at idle and when I'm riding it.

The idle also gets extremely high after I pin it and then come to a stop, so I adjust it so it isn't screaming as high. I find myself constantly doing this.

So can anyone help me out with what I should do to stop the popping? I'm thinking about going up to a 45 Pilot... ? Any help or tips are appreciated, thank you

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180 main,45 pilot and try turning the fuel screw out 2 turns ....you also want to check the hot start to make sure its not stuck open...also the silencer may need to be repacked as well...

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10-4...chances are the 45 pilot and the fuel screw adjustment will do the trick...the silencer needing repacked may sound ludicrous but if the packing is blown out and theres not much back pressure it will cause a lean condition resulting in popping etc...

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if it helps at all, i know about 4 people besides myself with an 09 kx250f, all of them pop and backfire somewhat on decel with the stock jetting. A buddy and I both changes our pilot jets to a #42, adjusted the screw and both run better with no popping (weather dependent, end of season up here they start to somewhat, btu it gets quite cooler here during the winter so it explains it)

now if i could just get rid of the black smoke on blip (mostly during warm up, or end of season again when the temp changes significantly)

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Changing to a bigger main jet will not solve this problem.

The jetting corresponds to the degree of throttle opening, not at which engine speed you have a problem.

The pilot jet circuit is active at idle and up to about 1/8 throttle. It is fine tuned by the fuel screw.

So even if you have just revved it up, and the popping comes with the throttle closed while the engine is revving down, the main jet has 0% to do with this. It´s pilot jet territory.

Assuming you have no leaks, you can try adjusting the fuel screw to 2.5 turns out from fully seated. If this is not enough, switch to #45 pilot jet and set the fuel screw at 1 turn out to begin with. Then go out further if needed.

#190 main jet is probably a bit too rich, even in cold weather.

FCR_CAL_CHART-vi.jpg

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Changing to a bigger main jet will not solve this problem.

The jetting corresponds to the degree of throttle opening, not at which engine speed you have a problem.

The pilot jet circuit is active at idle and up to about 1/8 throttle. It is fine tuned by the fuel screw.

So even if you have just revved it up, and the popping comes with the throttle closed while the engine is revving down, the main jet has 0% to do with this. It´s pilot jet territory.

Assuming you have no leaks, you can try adjusting the fuel screw to 2.5 turns out from fully seated. If this is not enough, switch to #45 pilot jet and set the fuel screw at 1 turn out to begin with. Then go out further if needed.

#190 main jet is probably a bit too rich, even in cold weather.

FCR_CAL_CHART-vi.jpg

Ok this is very helpful and clears up my confusion, thank you very much

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I have a 09 kx250f JD jet kit with the red needle set to the 4th grove. 190 main jet. 42 pilot jet. Air screw set at 2 3/4 turns out. Valves are it specs. Runs ok but it starts like it's not getting gas at times. Any help would be appreciated.

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Is the needle in the fourth clip up or the fourth clip down,  it makes a big difference.

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So now we have to define what the top is.  Is the top of the needle the end away from the tapered part?  In which case you are saying that your clip is in the fourth groove closer to the tapered end of the needle?  When you say it feels like it isn't getting gas, what does that mean?  Do you yank the throttle open and the motor doesn't respond or do you roll the throttle open and the motor doesn't respond? 

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Is the bike running okay on the top end?  It seems to me that your needle should be in the middle of the adjustment range and if the bike isn't getting enough gas then you probably need to go up one size on the main jet.

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the non tapered part is the top. Kick starting the bike giving it no gas it starts and when I roll on the throttle it just dies.

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