2008 LTZ400 & 2006 LTZ250
New member post but have been lurking around for a long time. I would have added this to other threads but it seems like it's frowned upon to revive older threads so here's a fresh one. Picked up an '06 LTZ250 a couple weeks ago in nice shape for a great price. When I went to pick it up it fired right up and ran great, despite being very loud due to a previous owner removing the exhaust baffle. Ran it up and down the street no problems and got it loaded into the truck and brought home. I ordered a baffle for it and got it mounted up. Upon cold start if fires right up beautifully and idles perfectly but when I got it out in the yard I noticed it started to run a little rough after about 10 minutes and would have issues staying running. I've found several threads here and elsewhere with people having similar issues. I'd have to hold it wide open and crank it for a while to get it to start again like it was flooded. At that point I'd have to keep it running with throttle blips as it would sputter out and die. A previous owner had also replaced the carb with an inexpensive unit that I removed and cleaned, but continued doing the same thing. The owner had explained about the petcocks vacuum hose being pinched off due to leaking. I read about the Raptor petcock mod so I ordered one of those and installed it, also putting a rubber plug on the carbs vacuum port, but to no avail. I ordered another carb for it on the off chance that the replacement had cheap Chinese carb syndrome but the new one behaves very similarly. The spark plug is very black and sooty as it's running rich. The fuel/air screw offers no significant change in running condition. The previous owner had installed a new Uni air filter but it was dry so I oiled it and reinstalled it. He did not have a airbox cover on it (and didn't have it) so I made a temporary cover to see if it was just getting to much air. Still no change. Today I did a valve adjustment and they were very close to correct but I decided to go out just a fuzz to the outer end of the spectrum (.08mm intake- .13mm exhaust) for slappy valves are happy valves mentality. While I had it apart I decided that a possible cause for the rich condition might have been that the choke plug wasn't completely closing all the way due to very tight clearances in there (I've not needed the choke to start it cold since I got it) so I disconnected the plug from the cable and pushed it all the way into the seated position and then reinstalled the spring and choke cable nut into the carb (so basically no more choke and spring pressure holding the choke closed). The other thing I did was to remove the rubber plug from the carbs vacuum port and run a breather hose back into the airbox. My thinking on that is that the hose originally went to the fuel tank where it was pulling vapor to keep fuel flowing so plugging it might not have been 100% appropriate. Put her back together and she fired up very easy and I rode it around the yard for 20 minutes or so and then let it sit and idle for a while, which it did without issue. It idles perfect now but I notice when sitting still that if I blip the throttle it will idle a little high, but if I pinch that vacuum hose for a second it brings it back down to normal (so might need a slight restriction in the end of it). I'm hoping it is fixed and will continue to run well for the kids so we can hit the trails. I'll check the plug after a few hours and see if it has turned a nice brown color. Hopefully if someone else is having a concern like this my steps will be able to help them out as well. Thanks for listening and I'd also love to hear of others experiences who have overcome this issue. Other thoughts were lack of proper airbox cover, float level, wrong jetting, or intermittent electronic components such as CDI box or ???