WOW! You guys are going way to deep with this. Fourtrax650ex- a 145 main jet is not necessarily too small, it all depends on where you are at. If you live in an elevated area, it is highly likely that you could have too big of a main jet. Follow the steps that Dragracer789 gave you. Do not ask anyone what size you should run because it completely depends on YOUR 250R and where YOU live at. I live at about 800 feet above see level and with mild porting, filter, and pipe I run a 156 main. This info is totally useless to you because you do not live by me and you have a different quad. When I go home to Kentucky, I have to run around 148 on my main jet and my quad is slightly modded.
To jet it properly, you MUST take it out and jet it yourself. If you need more info than Dragracer gave you, just do a search on "jetting 250r" or something along those lines and I'm sure you'll find all you need to know right here on this sight.

I think all you will need to do is jet properly and your problems will be solved. Just start with a fresh plug and go from there. Once you get the main jet dialed in, I bet you'll need to play with the needle settings to truly get rid of the stutter.
JP

 

fourtrax,
to help improve your low end buy and install a 89 CR250 head gasket its thinner, and will increse your compression to about 190psi from 120 take a reading b4 and after.I had a 88 250R If I remeber right I ran a 155 Main FMF pipe mostly stock, Its funny you brought this up my brother inlaw bought a 88 and is having the same problem bog off bottem and rips in mid to top. we did the head gasket thing and it did help the bottem, he is running a 38 mm carb so the bottem end might never be good. he needs to do a rebuild get the top end freshend up. depending how many hrs are on your the top end ,you might need to do the same? hope this helps[img]i/expressions/face-icon-small-smile.gif[/img]

 

sam- just to let you know an origanal 250R should have at least 175 pounds of compression. NOT 120!!! My R with stock bore and think head gasket had 190 pounds, and I just put a 0.05 think head gasket on it and now I think shes over 200, but if anybodys 250r had less than 155 then you need a rebuild.

fourtrax650ex- If you need anymore help with the jetting you can either do what boogerbub said and search or you can ask us. Im sure one of us will be around....

 

I was going off of what the dealer told my brother in law they said 120 to 135 per stock, but Ive never put my trust in most dealers, his 88 R tested at 120 ( but not knowing when was the last rebuild was) we put in a 89 cr 250 head gasket and it went up to 190. he did get a noticable improvement but he still needes a rebuild. then Im sure he will be up in the area your talking about.
I would still recomend fourtrax to do a compresion check just for piece of mind and elemante that posiability. [img]i/expressions/face-icon-small-cool.gif[/img]

just my 2 bits I havent had my R since 98 and I do regret geting rid of it. I fell for the hipe of the 400EX, needles to say going from the R to the 400ex I was disapointed, so buying the R fourtrax was a good move.[img]i/expressions/face-icon-small-smile.gif[/img]

 

I will bet that you have a bad ground or your stator is going out. As your stator gets weak you have to go down in jet size to burn the fule. Check your spark see if it is white or yellow. If it is there is your problem. By the way put the K&N in the trash and put a UNI in it.