[img]i/expressions/face-icon-small-frown.gif[/img] I have an 06 YFZ450 Quad. I put an HMF slip on on and did the Dyno jet kit perfectly, (new needle on 3rd groove, 48 pilot, 170 main, mixture screw 1 turn out) but at 40 degrees it bogs from 1/4 - 3/4 throttle? It only runs right with the choke on? I am at sea level. Should I pull the mixture screw out more.

 

Usually one jets by going to big, then working down until you are just a little lean, then going back up a little to make sure you never run lean. Everybody thinks you can just buy a jet kit, follow their recommendations and have everything be perfect...it rarely works out that way! You have to change jets, then go ride it each time.......

The pilot jet (and fuel screw) only effect 0-1/4 throtle. You are lean in the middle, which is mostly controlled by the needle, and the main jet. I would try a larger main, and see what happens. Sometimes a different shaped needle is what is needed.

 

But i used the only needle they provided. I dont think it is the main. The air screw stock was out 4 1/4 turns and now with the jet kit it is only out 1 turn, should i pull it out one more turn making it 2 turns out for this cold weather? When the bike was stock it ran fine at 40 degrees but at 30 degrees stock it did the same thing? now with the kit 30 and 40 degrees is no good? now It stalls/hesitates at 1/4 - 1/2 throttle. It runs great with the choke half on?

 

<div class="FTQUOTE"><begin quote>Originally posted by: reconranger

Usually one jets by going to big, then working down until you are just a little lean, then going back up a little to make sure you never run lean. Everybody thinks you can just buy a jet kit, follow their recommendations and have everything be perfect...it rarely works out that way! You have to change jets, then go ride it each time.......



The pilot jet (and fuel screw) only effect 0-1/4 throtle. You are lean in the middle, which is mostly controlled by the needle, and the main jet. I would try a larger main, and see what happens. Sometimes a different shaped needle is what is needed.</end quote></div>

But i used the only needle they provided. I dont think it is the main. The air screw stock was out 4 1/4 turns and now with the jet kit it is only out 1 turn, should i pull it out one more turn making it 2 turns out for this cold weather? When the bike was stock it ran fine at 40 degrees but at 30 degrees stock it did the same thing? now with the kit 30 and 40 degrees is no good? now It stalls/hesitates at 1/4 - 1/2 throttle. It runs great with the choke half on?

 

Hi, I bought a 08 yfz450 and from stock "i mean everything including spark arrestor still in the muffler" runs way lean, bad enough that header glows red to the slip on part of muffler.
I bought a HMF slip on pipe, dynatek fs rev box, and a k&n filter kit.
The only thing I got is the pipe and the other will be in about 2 weeks so i wanted to put the pipe on and jet it, but I want to jet it way rich so when i get the other stuff I should only have to rejet 3-4 times to get it right.
So what i was wondering is if anybody has installed the same stuff and has a baseline to go off of? like maybe &lt;pilot=48&gt;&lt;main=175-178&gt;, I don't care if it runs a little rich i'd rather buy sparkplugs than valves and pistions.
As for your yfz running lean, it sounds like you need a bigger main and move needle clip down some more.
When jetting your bike start with 1/4 trottle then 1/2 throttle, 3/4 and then fullbore, when you do this make sure that when comming down on throtlle to do it fast to get on idle circuit and shut off fast to get good plug readings, after that run it up in high gear and from a low to mid rpm full trottle it and then slowly back off throttle, if it seems to get more power when you relesed the throttle you'll need to rejet &lt;via needle clip adjustments&gt; most likely.