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Shwthomas1975

2003 250ex backfiring. Please help

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Bought a used 2003 250ex for my wife. It has an fmf slip on exhaust. I think that’sit for mods. So I went from a 95 main jet to a 98. Still doing the same thing. Anyone have any other suggestions?  
looks like I can’t attach a video. If anyone wants to hear it I can text or email it null
thanks in advance.

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A few things I can  think of  that has made me backfire/spit  , water in the fuel , cracked or hole in the  diaphragm in the carb , bad valve seat and the timing chain 

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i am not much for sport atvs ?, but I can say out of timing would be one of my guesses ?. sucking to much air from unknow source ?. get got some smart folks here, they will be along later to toss in some advice, hang in there !.

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Do you have an exhaust leak somewhere? Are the valves adjusted to spec? Air in the exhaust will do it.... lean jetting can cause it too, especially on deceleration. The first thing that I'd do is adjust the valves, thats routine maintenance and very important. Then seal up any leaks in the exhaust system, then go to rejetting to accomodate your FMF muffler.

 

Welcome to ATV Honda!

 

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Thank you all for the replies. Doesn’t seem to be an exhaust leak or intake leak. Idles isn’t affected when I spray it down with carb cleaner. 
I’ll have to go to YouTube for the valve adjustment. Haven’t done that. Something joe shmow can do with basic mechanical skills?  

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How does the spark plug look? That usually can give a general idea on the fuel mixture.

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Welcome to the forums! That piece your missing is the spark arrestor screen. Which if your not riding at any national forests you honestly dont need it. The baffling is the packing inside the can. Which could be blown out completely. Which is possible to be your problem. I would bet more on valve adjustment. Those plugs look pretty rich! So i wouldnt think lean popping and that generally happens on decel. Start simple like retro said. Maintanance stuff first. New plug. Valve adjustment.

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Thank you. I appreciate all the input. Just ordered plugs and oil. Next I’ll go watch some videos on how to adjust valves. Yikes. Lol. 
I’ll keep you posted. 🙂

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20 minutes ago, Shwthomas1975 said:

Thank you. I appreciate all the input. Just ordered plugs and oil. Next I’ll go watch some videos on how to adjust valves. Yikes. Lol. 
I’ll keep you posted. 🙂

No problem bud! Keep us posted! I havent ever adjusted valves on a 250ex. They are more of a utility quad motor put in the frame of a sport quad. So the utility guys will be better off with the info on that motor. You will need feeler gauges for sure!

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4 hours ago, Shwthomas1975 said:

Next I’ll go watch some videos on how to adjust valves.

 

The service manual covers valve adjustment well in section 3. You can get a free copy by clicking on the service manuals link on the menubar at the top of this page.

 

You'll need some basic metric tools and a set of feeler gauges. Valve lash is .005" on both the Intake and Exhaust valve. Adjust while the motor is cold.

 

valve-lash2.png

 

valve-lash3.png

 

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Well? We are waiting! How did the valve adjustment work? Personally I think it's running lean with the erratic idle........classic lean symptom.

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12 minutes ago, Misterclean said:

Well? We are waiting! How did the valve adjustment work? Personally I think it's running lean with the erratic idle........classic lean symptom.

lol..having atv repair withdraws ?..rofl.

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5 hours ago, shadetree said:

lol..having atv repair withdraws ?..rofl.

Yep.....gotta have my fix

 

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