?? I would have never guessed this... I recently got a newer style carb from Juggalo, Thanks buddy! And I stuck the bottom end on my carb for my 99. I gave it a test run, for the first time in a month. I noticed a heavy bog when you get on it from idle. The motor would just sit at the same rpm, and then go, and hit hard. I cleaned the carb atleast 3 times. No luck. So I got pissed, and I plugged up my filter on my lid. Ran much better, but didn't have as much snap. So I figured it was lean. I go back to the shop, and put a 165 I had laying around, in my 400. It only has a hmf pipe, and vented lid. I gave it a run, wow!! The bog was gone, even off from idling in 3rd. I haven't tried this before, so I never noticed it. Also, I was riding in some weeds, which gave it more drag than a gravel road. I would never have thought that my 155 was lean, but it was. I'll have to see if the plug changes color. I doubt the temperature difference would make that much of a difference, Since it did the same thing in 85 degrees as it did in 70. The motor seems to run cooler too. I'm going to the dealer to get a few jets. I'm going to try to go up until it starts to sputter lightly, and go down a size or two.