I apologize because this will be a long post but I want to make sure I give all required info. I have a WR 426 w/the air box lid removed, grey wire cut, a B.D. baffle, YZ throttle stop, and twin air filter. The bike ran great on stock jetting for 500 miles. A mix of high speed and tight single track trails. It always started on the second kick when cold and w/the hot start if dumped or stalled. The pilot screw has never made a noticable difference so I have always kept it at 1 1/2 turns. When the weater got colder, in the 50s It began to get a bad sputter at constant half throttle. I thought it would be a good time to take it in the basement for the winter. I tore it down and serviced the whole bike. I cleaned the carb and changed the plug. The carb was very clean to start with. The plug and head pipe was full of carbon. I figured that it was running a little rich and over the 500 miles the plug was just starting to foul to the point where I had the above symtoms. I did the BK mod and changed to YZ timing. I lowered the needle clip to posistion #5 as both mods need richer jetting. I know that both mods need several jetting changes but I wanted to test the bike first. I got everything done quicker then I thought I would and the weater held out. I brought it out of the basement for some more rides. I did some short street runs first in 50 degree weather and it ran good but seemed to take a long time to warm up. I thought this was because it was colder out and running on the street w/few stops was keeping the bike cool. I let it warm up for several minutes and took it for another short ride and it ran good w/no sputter or bog. I then, on a 60 degree day went to do some trail riding. I have to street it 12 miles to the trails at my parents home. About 1/2 way there the bike sputtered so bad it died. I changed the plug and limped to my parents w/the hot start out to get it going. Once there I backed the screw out for the BK mod to elimate that and leaned the needle out two positions, one past stock to the #3 posistion. I then rode about two hours mostly tight single track w/some high speed runs. It ran GREAT, no hesitation or bog and started just as it should. No sputter at all at constant throttle on the 12 mile road run home. I noticed a huge difference w/the YZ timing, the bike just ripped! Yesterday, I was running late to meet some friends at a sand pit 25 miles away. The air filter was still clean so I didn't change it. I road it hard on the street for the 25 miles and it ran great. As soon as I got in the sand pits at trail speeds it began to sputter bad again. I immediatly left for my parents, about 15 miles away. It ran good all the way there as long as I stayed hard on the gas. If I didn't it would sputter badly. I know it was rich, I pulled the plug and it was full of carbon. The electrode was fairly clean though. I cleaned the plug w/a wire wheel on a bench grinder and some fine grit sand paper as I didn't have a new one. I leaned the needle out one clip to position #2. It was way too lean and the pilot screw wouldn't make a difference. I then put the needle back to where I had it in position #3 and played w/the pilot screw again. It was immediatly way too rich and sputtering. So now it seems that I need to be between clip positions #3 and #2. #2 is way too lean and #3 is way too rich. The pilot screw has never made a difference in the idle and seems to make no difference in the way it runs. What has me confused is how it ran so good for more then two hours on the first day I had the clip in the #3 posistion w/the BK mod bypassed. It ran awsome and the engine temp seemed right on the money. It got hot , but not too hot like it would if it was lean. On that day I thought I had it all figured out. One question I have is, on that day I had about three miles on the new plug after I limped it to my parents, where I changed the clip from #5 to #3 posistion. I never changed or cleaned the plug as I only had one spare. Could I have gotten the jetting right but severly shortened the life of the plug on that three mile run? Again, it ran great for over two hours. But yesterday it acted very rich after the 25 mile street run. Can a plug be cleaned and reused like I did yesterday. Would a bad plug make the bike act rich. After I cleaned the plug throughly I had rich symtoms immediatly at clip #3. I have no problem throwing a new plug in and doing some testing. I don't want to be stranded in the middle of no where though after two hours of riding. Again sorry for the long post. I am sure I made several jetting mistakes but I am new at this, especially since the bike ran so good from the start. I would appreciate any advice.