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.

 

Try a b7es. i wouldent run a champion they miss fire. If that dont work try a denso (proubally miss spelled). I had to move from a 8 to a 7 on my bike it makes a difference.

 

You run 1 bottle of oil to 4.5 gal of gas. How much oil is in bottle? What mixture are you running? 20:1, 32:1, 50:1?
Is that the highest octane fuel you can get from the pump? 92 or 93 octane?

 

It's a 12.8 ounce bottle of MC-1, which I am figuring is somewhere around a 42:1 ratio. I can get up to 94 octane around here, but gas is so darn expensive around here i'm going with the bare necessities.

 

Why don't you try to go down in pilot jet size? Try a 30.

 

Deffinately go down one or ever two pilot jet sizes. My friends 85 250r was the same way always loading up at idle to about 1/4 throttle, dropped the pilot jet size and made a big difference.

'84 ATC 300R, '86 ATC 200X

 

stock pilot on a 86 atc 250r is a 42 so i doubt you have 35 pilot in there. that sounds pretty small. my 86 runs perfect with 42 pilot,148 main (stock main is 145), needle in the 3rd clip positon. it has fmf pipe and silencer, and fmf reeds otherwise all stock.

 

also i run spectro sx synthetic mixed 32:1 with 93 octane

 

It is a 35 because I put it in there. In the mean time I did figure out the problem. I took it to a salvage yard that I'm good friends with the guy there. Plus that's where it was purchased in the first place. He looked at the carb and said that the body itself was wore out. He compared it to another one and it was surly wore out from the slop it was creating. So I guy i'm out for a new carb.