So I've finished rebuilding my stock 450R into a full out racing quad, minus new suspension and a steering stabilizer, I've just about nailed it. I rode it around with a Yoshimura Full Pipe while my HMF was being sent off to get the header pipe to match the slip-on. I was using the Yoshi to seat my rings, and it worked well. So I put my HMF on yesterday along with my axle, hood scoop, and new MXR6's (front and rear), and hooked up the Dynatek box as well. So I went riding today since I got off of work early to see how the bike performed. Everything was ok, and I noticed a rough idle, and it didn't quite sound like what I was used too, it actually sounder a little louder and more "wild" (only thing I can think of to explain it). So I get the bike back and decided to change jets. While doing this, I noticed that the plug and top of the piston were beginning to look a little dark like I was running rich, so I decided to lean it out some. I went from a 175 Dynojet to a 165. Hooked everything back up and noticed that a hose was disconnected from the carb!! So I reconnect everything and it starts on the first kick and the pipe sounds normal, like I was expecting. I let the bike idle for about 5 minutes, and went inside while waiting. When I came back out, I noticed that the header pipe was GLOWING red, I mean absolutely 200% overheated. I was shocked to the point that I was scared to let it run any longer and cut the bike off. Apparently the hose that was disconnected is a vent hose of some sort and hooks into the carb and was allowing ambient air into the carb rather than what it should have (i'm not 100% on what the hose is for). I restarted it and hit the throttle and noticed a little backfire. I think I leaned it out too much after reconnecting the hose to its proper location, and now I'm really cooking the pipe. Anyone ever run into this issue?

 

Not sure what hose it was, but i cant think of one that would lean it out.
did you put in a bigger pilot jet...and readjust the mixture screw? Or is it stock?

 

I replaced the pilot jet with the dynojet pilot jet and I've adjusted the mixture screw. The hose is on the right side of the carb (near the kick start). It was allowing ambient air in and I think this was causing a rich condition some how becuase the plug and piston were starting to turn black (I've only got about 5 hours on the rebuilt motor). Once I hooked the hose back up, the motor ran smoother and the pipe sounded normal once again. I think I need to go back up to at least my 175 jet in order to cure the problem. I had a buddy that said it wouldn't matter that it was getting this hot and his race ready 426 EX would turn the pipe a bright red all the time with no harm to it (he's an atv/motorcycle mechanic). But I am on the right track with the lean condition, aren't I?

 

Ya, the pipe turning red is normal. About every 450 I have seen will glow bright red after a short period of time. Also, the main jet wouldn't affect/effect it while idling anyway. If you have a dynojet kit I would start where they suggest and ride and see how it does. In my experience they are usually pretty close to what you need.