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KX250F 2005 Spluttering and Popping on throttle Help Needed

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Hi hope someone can guide me in solving a problem I have. I'm quite mechanically minded but new to dirt bikes. I purchased this bike in 2012 for my son. He rode a couple of tracks as a newbie and the bike ran fine. Haven't touched the bike since then and now he has joined a motocross school. On the first session he got virtually to the end of the lesson when the bike overheated. At the start of the lesson I did think that the bike was making too much popping from the exhaust but just put it down to lack of use. Before it was parked up in 2012 the carb was trained and run dry and the fuel tank emptied. I have cleaned the jets out, checked the exhaust packing and kicked it over and its popping badly up and down the rev range. The engine seems to be solid and revs hard not like its struggling. Im ok on the spanners I just need some knowledge to know what to check and in what order and what to look for.  Cheers

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Assuming proper maintenance has been done.. Oil, new oil filter, radiator is full, running the correct spark plug.. Ect

Running the same fuel as before?

Assuming since he's in training, was he hard in the rrrrrr's or hitting the rev limiter to long/to much?

Could be running lean, like he said but if it was running fine before and your still riding at the same elevation you should be fine..

Open the top end and have a peek. Check compression,(before opening) check your valve lash, check the valves, valve seats, check the piston and rings..

Popping could be something stupid simple as someone removing the spark arrestor from the silencer or poor packing.

If it's running lean and you never changed the air screw, you may have done an upgrade such as a different exhaust ect. Which you will need to adjust the air screw. Some how your getting raw fuel in the exhaust..

Good luck.

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The bike is most probably running lean, so its getting too much air in somewhere or too little fuel, either its as simple as adjusting the fuel screw out a turn or the fuel screw oring has fallen out when removing the screw.

 

The pilot circuit may be blocked, there is 2 fuel passages that the pilot fuel travels up, one comes out directly under the slide and the other goes past the fuel screw were it is metered and then towards the front of the carb. The passage under the slide is normally the one that is missed when cleaning.

 

You may also find the slide plate oring is worn and air is getting past it, or in more serious cases the slide is worn or even the carb housing where the slide moves in.  

 

Also check that the inlet manifold between the carb and head is sealed and the exhaust header is sealed to the head, as these will also cause popping.  

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Thanks will take on board all your advice and start going through it bit by bit

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