I just installed a HMF full stainless system on my 250ex 2001 and the jetting recommendation from EX-riders.com as well as my local bike shop said to install a 105-108 main jet. So I installed a 107.5 main jet and now the quad idles good but back fires and sputters under acceleration and back fires on deacceleration. Is there anything else that needs to be done when installing a pipe. The bike has a uni-filter and this pipe that is it. If the A/F screw needs to be adjusted how many turns out should it be about? 2.5 is stock.

 

you most likely need to adjust your a/f screw out. back it out a quarter turn at a time till it gets better. any more than 4 turns out and you need a bigger pilot jet.

 

Agreed and did you install a new needle with the kit or are you running stock...I would do a 1/4 of a turn like mentioned and it should end up around 2-3 turns...Take a ride and check the color of your plug so you know how it is running, but it sounds like you need to adjust the screw which is at the bottom of the carb...any ?'s let us know...And are you currently running the airbox lid?

 

Hi there thanks for the reply's. I have not had the chance to adjust the A/F screw. But I will before riding this weekend. I am running the air box and lid with a uni filter. I still have the stock needles in there and my directions said to put them to the 4th click position, but I have no idea what this is talking about. When I removed the carb and cleaned it and installed the new jet I did not see how the needle could be repositioned or taken apart. It just spins and has a little spring above it in a little plastic housing thing. thanks again for the reply's

 

The needle comes out of the carb slide. You have to take the cable out from the slide than the needle. The needle will have 5-6 slots with a c clip. Most times they are in the center slot. You want to lower the clip to richen up the circuit. If it backfires then its running lean. Moving the clip and adjusting a/f screw will help.

 

Ok I was trying to tune this thing again today and I moved the A/F screw out from 2.5 to 3 turns and installed a new plug and did a full throttle blast 1st and 2nd gear And it was still back firing and sputtering. I removed the plug and it had a bright white insolator and no carbon or anything like that. I then moved the A/F screw to 3.5 turns out and it was the same. So I took out the throttle slide again and figured how to remove the needle. The needle has 3 slots and with the needle facing down just as it would in the carb, the clip was in the bottom slot. I moved it up one slot and tried again and no change. I then went to the top slot and it got a little worse. I realize now that I was leaning it out more with the needle position clip moving up a slot.(thanks 2manytoys). So what should my next step be new needle and bigger jet or what.

 

Well it might be that your pipe doesnt have a spark arrestor or u took it off and that would make it back fire also with no back presure it would probably make it spudder. so just put it back on if its off

 

yea im pretty sure a spark arrestor would fix it because ive seen a bike without a spark arrestor and it was shooting flames

 

Your way to lean!
The stock needle on a 250ex is not adjustable.
Go to your local dealer and ask for a OEM kelihn 110 main jet and put her in.
I'd also vent the air box by adding a few UNI vent so it can breath better and along with the extra fuel you"ll be getting with the bigger jet you"ll need so more air to make more power. If you take the lid off commpletely go with a 112 main jet.
I'd also bump up one size on the pilot jet. You can get all these jets for about $3 a piece. Good luck!