I have a 1986 250R ATC. My problem is that it loads up really bad on low end, from just above idle to about half throttle. Mods are: FMF pipe, Answer Silencer, Boysen Power Reeds, mild porting, and bored .30 over. I got a 145 main which is a tad bit lean, a 35 pilot, and needle in third position from the top. I run Bel-Ray MC-1 at 1 bottle to 4.5 gallons of 89 octane fuel. I can not run it for more than 15 seconds on low end or it just about fouls a plug. I've ran both NGK B8ES and Champion N3C plugs. It's got 170lbs of compression, I've gotten a new jet needle because mine was visually worn out. I've tried many different jet and needle positions, but I just can't figure it out. The carb is spotless on the inside and none of the passages are blocked because I can blow compressed air and carb cleaner thougth all of them? Are there any parts that are know to wear out on them? Thanks in advance.

 

Sounds excatly what mine was doing...first of all is the carb stock..if it is then check your idle circuit by unscrewing the idle and choke needle assebly from the carb...take a close look at the needle for any abnormalities, you might have to take the carb abart to have a look at the brass seat...Now the bad news..the assembly is replaceable for about 30 bucks, however the seat isn't apparentley and requires carb replacement just like mine...but by replacing the screw assbly it should make a big difference.