I have a YFZ450 with the following: Full HMF system with the air box lid on. I am jetted with a 180 main, 48 pilot, 5th clip on stock needle, and 2 turns on the air screw. The thing runs fine, but I get an occasional "pop" and flames on decel. I checked the spark plug and noted it was still slightly white. I was also messing with the choke and found that if I pulled it out half, it made the engine idle higher and then it died. I want to be as fast as possible and I don't beleive I am jetted just right yet. Any suggestions?? Thanks for the help.

BTW....13.5 rules and Punchdrunk's fish taco's are the BEST!!

The slight popping means your lean. You could probobly stand to go up atleast a size on your idle and maybe two on your main. It's very hard to tell for sure over a computer but I'm posative your lean.

Thats what I thought too, especially when my pipe was glowing like a heat shield off the space shuttle!! But everyone else I talk to seems to be running 175's and down....all the way to 158! I will definately try a new needle and bigger jets... thanks.

There is no way you are running lean with a 180 main, and 48 pilot. Especially with the lid on. I think your problem is the needle. Here is what I would recommend. Try going to a 45 pilot, 170 main. 2 turns on the screw, and go get the YZ dirtbike needle. it is called the NCVQ needle. Run that on the 4th clip. That should get you real close.

Just a thought....I'm of the understanding that if the jets are from dynojet, they are different than the factory numbering system..if this was the case, it's possible he's lean with a 180. If these are dynojet parts, I'd seriously consider going back to the ncvq needle, and stock yamaha jets, and starting around where 24v suggested. That would get you into the ball park and similar to what "everybody else" is at.

i was told by a very knowledgeable source that if you mix brands of jets and needles you are tempting fate. try running a matched set of jets and needles from the same kit. was told that mixing parts from different manuf.'s, even if the seating is off by 1000th of an inch, it could absolutely fok up the mix and chuck you lean.

I am using factory jets for the Kehein carb. I am definately going to try the YZ needle and go back a few on the main and pilot. From messing with this thing, I think I have just about every main/pilot combo you could think of. Thanks alot for all of your input!!

24V is right on the money, you are not lean on the main, raise your needle to the fourth clip and that'll get you closer..

I think I am at 175 main, 45 pilot, 4th clip, 2.5 turns out, and the bike freakin rocks, header pipe glows a little on idle, but goes away when we ride, I think I am a hair lean, since it doesnt seem to want to idle, warm or cold, and when I pull the choke out, it doesnt seem to matter, I am going to tear it all apart this week, and mess with it a bit more. I have all Yamaha parts

I did turn out the idle screw and it either idled really high or not at all, when it is in neutral it will idle fine, but plop it into gear and it wants to die, there is no slippage on the clutch, so I am not sure about that, also, if I pull on the throttle cable itself by the handlebars, I can make the engine rev, I am not sure if the cable just isnt tight enough, so the idle screw isnt making enough difference, I am pulling the plug within the next day or so, so that will tell all, its the same plug that has been in there since new years, the bike doesnt pop or ping, is really responsive, hits the rev limiter without hesitating, just cant figure out the idle thing, also havent worked on it too much either, but tomorrow is the day the raptor gets some love along with the YFZ. Thanks 24V,

Your idle screw raises the slide directly. It has no relationship with the cable I believe so something should happen. Are you bottomed out on the idle screw? If your bottomed out and nothing jappens to the idle, you may be running way lean on the pilot. Bascially, there isn't enough gas running through the circuit to light. The choke should make a difference though as it bypasses the pilot completely. The choke is it's own circuit, kind of like a mini carb inside your carb.

I jetted my YFZ last night....What a huge power gain
baffle pulled, lid off
45 pilot, 165 main, ncvq on #5.
2 turns on air screw.
Idle's very smooth and the pipe (last night) was a faint orange glow.
I printed and followed "daddog's" instructions.
YFZ450 jetting info.

put an exhaust temperature guage on it and yuo'll know exactly where to be. Want to be around 1100 degrees. you can buy one and mount it for about 50 bucks. It's also a good way to know when you have been up China wall too many times and need to cool down. Best thing I ever did. It also take the guess work out of the jetting/needle/idle/screw game. Just a thought. 50 bucks is way less than new valves or ceased engine.....