I've had my 426 for about 18 months with zero problems.
One thing I've found to help greatly with the clutch is make sure you warm the bike up good, and while warming it up I pull in the clutch and hold for about 30 seconds and rev slightly. I believe this helps get the clutch plates oiled a little better after the bike had been sitting a while.
Don't tighten the chain too tight. The manual's spec is from 1.6 to 2.0 inches. I would keep it between 1.8 and 2.1. Some have had problems when the chain is run at 1.6 with hubs.
I'm at about 4000 ft and I run the stock pilot 42, stock main 162, and the stock needle with the clip raised one clip, which is leaner. I think it's the 3rd clip position. The pilot screw is from 1 3/4 to 2 1/2 turns out; further out when it is colder, which richens it from about closed to 1/4 throttle. Mine runs excellent. I run non-oxygenated pump premium for now.
Maybe this will give you a jetting reference anyway.
Other than that change the oil often, I use Mobil 1 15w50 auto oil, and keep the air filter clean. You can reuse the oil filter many times, just inspect it.
For about sixty bucks you can fix the clutch before it requires a new basket. Eliminate the chatter by going with part#'s 5JG-16383-00 cupped washer
5JG-16384-00 flat washer
All of these are parts from the '01 426 and keep the first two plates from sticking. This causes the noise and chattering. I ran this setup all winter on the ice after seeing the setup on my '01 race bike. '00 Practice bike is still going strong with no clutch problems.
The best jetting advice I can give you is to swap the needle for the stock ’01 needle (or this optional ’00 needle: 5JG-14916-EP). Clip #3 would be a good starting point, the stock pilot and main should work okay, but a lot of guys go richer on both.
I am currently running the stock main and pilot at 4k ft., but I’ve made a host of other changes to the carb that really helped. If you are a tuning geek (not that I am) or wish you were post back for more.
BTW stock jetting is:
# 42 Pilot
# 162 Main
# 4 clip pos.
EKR needle (’01 YZ has EJP, the PN above is for an EKP, just a ½ clip leaner).
Some of these bikes, like mine, will eventually eat the straight key that secures the gear that drives the counter balancer to the crank. Don’t get paranoid, this bike has a noisy motor to begin with, but if you hear any strange noises have this checked out. I can’t describe it very well, but I was convinced it was a valve train (i.e. cam chain) noise when I first heard it. Cheap to fix if you catch it before something comes apart.
Also, check and loctite your rear sprocket bolts and lower shock bolt.
I'd like to know what else you did to your bike as far as the carb. Mine runs well, has the fmf exhaust system on it(sure is LOUD!). only complaint would be an occasional hiccup if too aggressive on throttle(jazzing the throttle wide open from idle). Sure is a fun bike, a lot easier to ride than my old CR too. Thanks for your help.
The stumble when you snap the throttle open very quickly is normal (especially noticable while in neutral). I don't notice it while in gear, probably because I roll the throttle on more gradually than some :) .
Either of the needles I mentioned will help the infamous hiccup, easily the best eleven dollar improvement I ever made, and this should be even more significant at your colder temps and lower alt.
Of course, like Holeshot mentioned, a lot of that “hiccup” effect depends upon what you are used to. But I am convinced you can improve upon stock, especially the ‘00. First get the ’01 needle, and see what you think.
Next, the Flat CR has a tab cast into the body that you can drill and tap for a screw to limit/adjust the accelerator pump stoke, it was obviously designed and put there for this purpose. Take off the plastic pulley cover and look just above the throttle stop and you will see what I mean.
I cut the pump stroke on mine almost two thirds and loved the difference. I kept shortening it, throttle response kept improving. I eventually turned the pump off, re-jetted my bike, and then slowly increased the stroke from zero until I was happy with the throttle response (the screw is accessible with the carb on the bike, barely). With the modified pump I ended up a clip (or two) richer on the needle and up one size on the pilot.
I suspect the “redesigned” accelerator pump for the ’01 does less of the same thing, I tried to get the pump diaphragm when they first came out but couldn’t. It is only eleven dollars, so my best tip may be to order $22 of ’01 YZ parts, the stock needle and pump diaphragm. Especially if you’re not into drilling holes into your carb and just want to see if you can fix the hated hiccup for around twenty bucks.