New to Thumpers, have some questions.

Hi all, I'm new to thumpers and new to this forum, and I'm glad to have found your community. I've been riding 2-smokes most of my riding career and just switched to a 00 YZ426F. I've never been so glad to discard my stroke as I am now, this bike is awesome !!! I picked up my 426 two weeks back, the previous owner rode it twice and it scared the livin' crap out of him. When I picked the bike up it looked like it rolled off the show room floor !! You have to love guys who buy over the heads and or riding skill, they leave great bargains for the rest of us.

I don't race, I'm pretty much a weekend warrior who likes woods riding and hill climbing. I've only put one ride on the bike, and I fell in love with it. The first modification I made after the maiden ride was putting a 13 tooth counter sprocket on. Sheesh the bike was geared high off the floor, but then again it was made for MX. I've only ridden the bike around the house with the new sprocket, but it has made a huge difference.

I have not had the clutch problems I keep reading about, but then again I'm not racing and fanning the clutch alot either. However, I am experiencing the throttle stumple when I wick the throttle with low revs. I've bumped up my idle speed which helped a little, but I'm looking for a better solution. I've heard two suggestions thus far:

1. Turn the pilot screw out. Does this lean out the pilot circuit ??

2. Use the 01' model needle in the same clip position.

Does anyone have any suggestions ??

How often do you guys change your oil ?? How often do you clean out the ail filter and frame filter? I changed my oil tonight but I did't clean the oil or frame filters. What an overall pain in the arse to change the oil on these things. Two drain bolts, you have to move the header to get at the oil filter, and the frame filter nut is supposed to be torqued at 65 ft lbs !!!


Matt W

00 YZ426F

94 YZ 250

"Don't worry about me, how's my bike ?"

If you don't fan the clutch, you will be fine for a while. One way to delay the grabbiness is to hold in your clutch lever for about 30 seconds with the engine running before you engage first gear for the first time of the day. By doing this, it allows oil to bathe the plates. One of the other Thumpertalkers let me in on this and it works great. I can't remember the guy's handle....wish I could give him credit.

Now, about the hesitation problem, I know exactly what you mean. Not only did my '00 hesitate at low RPM's, but it would do it any time I whacked the throttle open quickly. What worked for me was a combination of things. I installed the Factory R&D accelerator pump bottom plate, switched to VP C-12 fuel, and spent some time with the jetting. I think the accelerator plate was the best fix overall. The fuel and jetting was what made the throttle response very crisp. I've found this 4-stroke to be surprisingly sensitive to jetting changes.

The pilot screw is actually a fuel screw. . . not an air screw like a 2-stroke. Turning it out(counterclockwise) richens the pilot circuit. To tune this circuit, use the big black knob to set the engine at a high idle(after a thorough warmup) and then find the setting where you get the highest and smoothest idle with the pilot screw. If it's less than 1 turn out, you generally need to go to a one-step leaner pilot jet. If it needs more than 3 turns out, you generally need a one-step richer pilot jet.

Also, if you are willing to use race fuel and are willing to jet accordingly, the benefits are worthwhile. C-12 usually causes a richer result and requires leaner jetting. Oxygenated fuels such as VP MR2 requires richer jetting. If you ride at varying altitudes, jetting ranges become even more extreme. I'm talking about altitude changes of about 5,000 feet or more. Below these changes, it's barely noticeable.

Change the oil very often!!! Once you get used to the routine, it becomes less of a big deal. I like the Scott's stainless steel mesh permanent filter. It's claimed to filter at a finer micron, plus you just clean it and stick it back in. The frame screen is very coarse and only collects the large junk. Usually, only the first or second times you yank it out will you find anything caught in it. Typically, just paint chips and such. I probably have 150 hours on my bike and the filter has nothing more than a speck or two caught in it..but I change my oil after EVERY ride. I'm very anal.

[This message has been edited by Boit (edited 08-15-2001).]

As usual, Boit gives some good advice in his post.

I think swapping the ’01 needle is something that has a very high return for the effort and expense, so you may just do that first and then see what is what. I know my ’01, bone stock, did not have nearly as much of a Yamabog as my ’00 when stock (and other than Ti valves the motors are the same).

Depending on your riding conditions you may benefit from different jetting altogether. You might consider starting a new thread with your riding impressions, & altitude, temp. etc. once you have some more seat time and a good sense of how it runs now (in addition to the bog/hiccup or whatever off idle).

If you are the mechanically inclined sort you may opt to modify/adjust your accelerator pump yourself instead of buying the Factory R&D part that Boit, and many others, have had success with. Use the search feature on Thumpertalk (the link is at the top right of this page) and try “accelerator pump” or “BK mod” as key words.

A while back a guy named (or claiming to be) BK, Tim Ferry’s mechanic, posted about some of the modifications for Red Dog’s bike, one of them was to drill and tap for a screw that would adjust the stroke (and amount of fuel delivered) of the accelerator pump. Apparently Ferry’s race bike is set up to deliver MUCH less fuel than a stocker when the throttle is opened. The R&D cover also shortens the stroke via a thicker base which the pump diaphragm bottoms out on.

I think everybody who has tried this Tim Ferry/BK pump mod, myself included, has been very pleased with the results (and low cost) of this fairly simple job. Less than a month ago another kind soul here even posted some real good pictures of the finished product (I’m sure that post and many others will return if you just search for “BK” or “pump”).

To summarize then, yes, you can fix the stumble :)

Hope this helps.

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