Weighting the fly wheel

[Tom_N]

I have seen many talking about weighting the flywheel... What does this do for you and how do you know if it should be done?

I have also seen lots of post regarding the jetting of the bike. I have a service for my 98yz and have read the sections on jetting moving the clip and all that.. I have to say it reads like stereo instructions.. I would like to try rejetting the carb as I am experiencing that cut out when I roll the gas on at low rpms.. but I don't want to experiment only to make it worse and then pay a shop to undo what I have done.. How hard is this to do and any input or advice on how? any good lit. out there to help a rookie along...

Damn I should have stuck to road bikes, you put gas, oil, tires and breaks on the darn thing and ride for ever...

Oh well the rookie will learn. - Hick posted a great manual to print off listing every part on the bike with the part number. Great tool, thanks Hick!

Rookie-out

[Hick]

Flywheel weights create more tractable power delivery and help prevent stalling. I’ve never used one but I’m now convinced I need one. If I didn’t race occasionally I wouldn’t bother with it though. But they would probably be good for any kind of technical riding. Some kinds bolt on (you must drill holes) and some you must exchange your entire flywheel (I think the heavier Terrycable weight is like this).

If you’re methodical about it you can’t do any permanent damage by changing the jetting. I recommend you ride it some more, make a mental note of how it is running and post your impressions here. Other info necessary is elevation, climate, type of riding, type of rider and current jetting. Or you may just post your location and ask for guys from a similar area to post what THEY are having success with.

Those things most commonly adjusted are:

1) Idle mixture. This is adjusted with a screw that protrudes from under the carb and is accessed via a recessed hole in the float bowl. You need a very short flat blade. In is leaner, out is richer, stock is 1.75 turns out. This circuit modulates mixture at idle and just above.

2) Pilot jet. This jet can only be accessed by removing the float bowl. If you remove the throttle cables, loosen the two clamps and rotate the carb in its airbox and manifold housings you can get at it. It is next to the main jet, comes out with a small flat blade and will have a small # stamped on its side. You can look on the fiche to see what is stock for your bike but I believe it is a # 45. The pilot affects off idle to about ¼ turn (mas o menos).

3) Needle. IMO this is where the carb spends the most time. The needle helps transition from pilot to main. You can change the needle or raise or lower its position to alter when it begins to lift out of the main jet by raising and lowering the circlip that retains it in the slide. To remove this you need to take the top cover off, remove an allen fastener on top of the slide and dump the needle out. I believe stock 98 YZ is OBDVR set at # 4 clip position (counted from the top).

4) Main. This can be changed the easiest, all you need to do is remove the drain plug in the float bowl and stick a 6 mm socket in there and unscrew the main. This jet governs WOT only, stock 98 YZ is a 175 I think.

There is a wealth of good information sitting on the Thumpertalk server right now, waiting for you to discover it. I searched for the words “DVR” (your stock needle) by user “James Dean” (resident FCR needle guru) and got this:

98 YZ400 Throttle hesitation question

98 YZ400 jetting problems

If you make note of all your current settings you can just return to them if you inadvertently confuse the issue and your bike runs worse. To cure “that cut out” I would try richening the fuel screw first. Make sure you note its current setting by screwing it all the way in and counting the turns, then try it ¼ turn richer.

Hope this helps!

[dirtdad]

Hick as usual gives very good advice. With the carb try and keep it simple. They really are not that complicated. As he said, ride it some more and see where and if you want the change/improvement to occur. Do one small change at a time and ride it again. You should then be able to see if you're headed in the right direction. Hope this helps and good luck.

[Tom_N]

No doubt great advice from all of you guys, I already owe Hick lunch for putting the parts info on here for me. I will give it a try, I think I will print off everything I can find here on the subject and read it about a dozen times before doing anything.

I have learned more on this site in the last three days than I have in months of talking to the dealer guys and reading the shop manual. The problem is I am not getting any work done, I have been spending all my time on this board! Its fantastic how much everyone knows and how eager you all are to help out! I hope many of you can meet up with us in Colorado, "see colorado thumper trip" for more info.

Thanks! I will take all the advice I can get, I need to out perform my buddies KTM 400 4stroker.. sweet bike but I have more torque! Go Blue!

[FOX426]

I went with the 10oz weight from Terry Cable. I'll tell you how good it is, the first time I rode with it installed it was the first time I had ridden all day and never stalled once. It depends on what kind of riding you do. On a motocross track the only time I really stall is if I come into a corner to hot and stab the rear brake.On the trail however it is a different matter entirely and worth it weight in gold. The only thing you give up is a little of the hit, which on the trail is good but up to you on a track.

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The world loves talent but, pays off on character.