250r Sputtering in powerband

[speeddemon13]

I just recently bought an 87 250r and its setup for flat track racing on the concrete. So it has no airbox at all, just an open filter which I would eventually like to replace with an airbox because I do most of my riding in the dirt. When I took it out today to ride, after it was completely warmed up it continued to sputter like it wasn't warm yet. Mostly when in the beginning gears towards the top of the gear. It didn't sputter every time I got on it, just quite a bit. I haven't checked the spark plug yet because I'm busy with school and its stored somewhere else but what are some other things I should check for? Could it be that because it has no airbox its getting too much air and not enough fuel?

this is the exact atv http://www.youtube.com/watch?v=x9IkpCcY45w

just to get an idea of what im working with

[profab250R]

sputter indicates either a rich mixture or a weak spark. A lean condition creates an eratic throttle control and high poppy (almost back firey) sounds through the exhaust. lean will also typically cause a hesitation in that area of throttle. also check for cracked reeds.

First check you plug. then check your spark. then do a plug chop with a fresh plug. get your readings. and adjust jetting accordingly. If jetting does not change your plug color then you likely have a spark issue.

ps you can have spark at the plug and the spark could still be bad. A purple color spark indicates bad pick up coil, wire, coil, plug, or connection and will cause bad ignition. if you determine it is an ignition issue, make sure the bolts holding on the pickup coil ar goo and tight, your balancer bearing carrier on the mag side is not cracked, and good connections. If all is with in spec, then check your coils resistance to determine if they are good. (helps to have known good parts to swap for diagnosis) or if you have a buddy with a 250r put your parts on his bike one at a time so you dont risk frying their electronics on your bike.

[HAYESTRX250R]

I seen this for sale. You have some changes to make for riding it off road.

Good advice above. I would add pulling the carb apart (easy enough) and see what you are working with. Needle clip postion, pilot and main jet numbers to at least know where you are at. Count your turns out on the air bleed also. Once the carb is off you definitely need to check those reeds out since it is easy also.