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03 YZ250F erratic idle solved

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Our stock bike started having trouble with what seemed like a sticking throttle. So I cleaned pilot/ carb & adjusted the idle and it seemed ok enough to ride at the time, but it still didn't seem right. Later, it started running hotter than normal and with slight coolant overflow. I eventually replaced a bad w/p seal, but the idle and hot running temp were still an issue. So I took a deeper dive into the erratic idle issue and eventually checked/ cleaned all: pilot, slide, air mixture screw, throttle assembly & cables, float height, TPS, CDI voltage to TPS, valve clearance, valve timing, engine timing, intake boot for vacuum leak, hot start assembly & cable. I got to the point where I knew the carb was immaculate with "stock" jetting & settings for needle & air mixture; CDI/ TPS voltages were all good; no vacuum leaks; and all valve & engine timing issues were good. Yet, I was getting nowhere with air mixture/ idle adjustments. If I left the TPS connected and revved then let off, it would take 3-5 seconds for idle to drop down to normal (sometimes longer.) If I disconnected the TPS the idle returned to normal at a normal rate, but the idle was still rough. (In hindsight, this was a real distraction that made it harder for me to rule out TPS/CDI issues as a possible cause.) Also, no matter how fast or slow I pulled-in the hot start, the engine would die instead of increase in rpm. With this last item about the hot start behavior, William1 and others will instantly recognize what I now know. My brand new, stock pilot jet (size 40) was too small. By this time, I had previously tried a 42 and put it back to the 40 after the 42 made no difference. So now, I tried a 45 and presto!! With the TPS connected, the idle still had cleaned-up, it returned normally after revving, the hot start would now cause the idle to rise and the headpipe wasn't glowing red as soon or as intensely (ie; running cooler.)

In hindsight, I have to conclude that for some reason I had changed out a 45 pj and baselined it to the stock 40 pj (as per Clymer) just prior to when the idle problems started. Then when the problems started, I forgot it ever had a 45 in it. (We have four bikes and my CRS is gettin' worse...time for a log book.) Going by William1's carb tuning info, I was always able to adjust the air mixture down to the point of stalling the engine and back it out to an improved idle so I stopped considering the pj when the 42 made no difference. The key for me was the hot start behavior and the info in Thumpertalk that confirmed I had to be running lean at idle, despite my previous pilot jet testing. I was about to take it into the shop. So Thank You to everyone on Thumpertalk. I wouldn't have figured it out on my own.

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