I wasnt ready to post this yet, because I am still struggling with new jetting, but here goes.
My bike is a 2001 YZ426. I live at 6500ft and ride from my house to 9000ft several times a week, until the snow gets to deep.
I try to ride mostly single track mountain forrest, and some desert. The tougher/tighter the better.
After experiencing the "no compression when cold" thing, I did some research, and was directed to Zip Ty Racing.
They (Ty Davis and Jim Hardiker(sp)) explained the problem to me and also explained their solution. It involves using the stock valves, but different springs and retainers. They did replace one valve (center intake). They also did some porting (cleanup) on the head.
Then I went one step further. I had them install HotCams.
First let me say that my butt is NOT a dyno. BUTT, what a difference!!!!
I was trying to avoid the cliche "this thing rips!" but I can't. It pulls harder and soooo much smoooother right off idle. Easily a gear higher on single track and out of corners. I drag raced (rolling) a KTM LC4 640, and Honda XR650R, on a fire road and easily pulled them both, right to the rev limiter.
And I know my jetting is not right. I changed needles, clips, and pilot jets and could'nt get it right. So off I went to my WOT testing site, a .2 mile dam. (level pavement).
I was running a 160 main. I did my first pass, ( I come on to the straight in 3rd, quickly shift to 5th and pin it until I hit the limiter, then kill button and clutch).
The plug was white on the insulator, but had soot on the steel around it. I guessed it was not warmed up enough.
Because it was so white, I skipped to a 165 and did another pass. The insulator was still very white, but no soot. I made one more pass with a 168 and it is still close to white on the tip. That is my biggest main jet.
One major thing I noticed was that previous to the head mods, when doing these WOT tests on the dam, after getting to the rev limiter I could not stop before the pavement ends and where the dirt starts. (where the truck is parked).
Yesterday I could stop long before the truck, without any hard braking!!!!!!! Much better acceleration than before, but it is so smooth, you almost can't tell it is stronger.
Now the suspension. I have a MX Tech off road setup. I thought it was good. Then I changed to a 18" wheel, and added the Zip Ty Magic Link. This bike is now MAGICAL. The steering is unbelieveable, and it is much better in the rough stuff. I chased a 20yr old kid (I'm 47) on a KTM 200 on one of our tightest single tracks, and was able to stay close to him until I was exhausted. (Maybe 2 miles)
The combination of the smooth low end, and plush handling makes this bike a whole new experience, and it is AWSOME.
Thank You Zip Ty Racing.
I may post an update when I finish jetting. By the way, don't forget; all this report IS with incorrect jetting, and it STILL runs awsome!!