CB free play between gears

I pulled the right cover off last night to check the CB gear. The gear with the key in it was absolutely tight on the Crank. I could not move it in anyway at all. There was a little bit of play between the drive gear and the driven gear that sits on the shaft of the counter balancer but it was just play between the 2 gears not the drive gear on the crank, I assume this is normal. I'm going to check the valves tonight when I get home and inspect the chain. This bike is a 99' yz400 and I'd never heard of the CB Gear coming loose on this model in these forums but the noise that I was hearing sounded like what was explained in past posts.



When in doubt, GAS IT!

Originally posted by sirhk100:

There was a little bit of play between the drive gear and the driven gear that sits on the shaft of the counter balancer but it was just play between the 2 gears not the drive gear on the crank, I assume this is normal.

I guess it depends on what you mean by “a little bit” but mine won’t budge.

But, like you pointed out, the drive gear to crankshaft play (which would indicate a worn key) is what we’re lookin’ for. If you’re confident that all is well here then I’m sure this isn’t the source of your noise.

I wonder why this only happens to 2000 models? Maybe they put the wrong key (too long) in some of them…

These things do make a racket, hopefully what you are hearing is just the normal amount of clatter.

S100, mine did the same last fall. Decided to have a look at counterbalancer and all seems fine. I do have backlash between gears however and this in most likely normal as there is no provision for adjustement on this type of build ( it is a motorcycle ) HOWEVER i took the top end off and found the culprit for all the racket. Worn piston / rings / bore. It is now a 420! Some alu flakes from my scored piston ended up in the oil filter. May of been a cold seizure as everything above the rings seemed fine. ( first and last used biker I buy ) Take care!


As for normal racket, I'm pretty noticable as to things that are going on with my bike and I don't remember hearing this before. Since the radiator is drained I'm going to check the Valves tonight and all that stuff up there. It was the first time I rode it in the dunes with a paddle so if I can't find anything wrong on top I'll put it together and run it again this weekend. Maybe it was just something to do with the load on the engine in the sand.


I did notice a little more material in my oil filter but nothing to brag about. I took a magnet to it and all of it was magnetic. What made you decide to go to the 420 kit instead of just rebuilding it? I think the power delivery is fine for me and I would assume that a normal rebuild is cheaper. I'm curiious if you had other reasons besides just the want for more power. (A decision based on your Opinion I'm assuming) Did you have any problems tuning the bike after the kit? I could do most the motor work and suspension work on my bike myself but when it comes to carberation I'm pretty illiterate.



When in doubt, GAS IT!

S100, my bike is on the shop floor right now with a fresh coat of PJ1 paint drying on it, so I haven't even jetted it yet. I will probably use the '01 needle and then take it from there. Please see older posts from about a week ago on this subjet as well as on the WR side. James Dean ( the engineer ) has a diagram on site, you've probably seen it. As for the 420 the deal is that I sold some machinery to a high performance motorcycle engine rebuilder ( J. Precision, the best in the Montreal area if you ask me )and we upgraded an eprom chip on his machine for free so he basically thanked me with a 94 mm piston ( 420 )in return. The scoring in my opinion was too deep for a simple hone / replate. Regardless of this, cost for rebuild would have been pretty much same. Bonus is I have bought a used e-series pipe with high boy header and some UNI-Filters, so it will be kind of neat to see what this thing does when finished. Also helps keeps my mind off women until I get married.

[This message has been edited by Hugh LePage (edited 02-21-2001).]

[This message has been edited by Hugh LePage (edited 02-21-2001).]

Khris, I just repaired my friend's '00 YZ426F today. It had the same banging noise mine did, and sure enough, when I tore down the right side, the CB drive gear had about 1/8" of rotational play. IT SHOULD HAVE NO PLAY!!! I removed the clutch and crank gears, and the key was mashed just like mine was.

I made a new key for it, put everything together, and no more noise.

I have a noise in mine too. I have not ridden it in 3 weeks due to the noise. It runs ok, but it has a knocking sound that is not random. It does it for about 5 revolutions, then not for 10-20, then 5 again. The shop has no idea what it is. It sounds like it is in the front of the motor under the top of the skidplate. Does this sound like a CB key problem. I will have them check it anyway, but I am on wits end with this bike. Thanks.

Also, it's a 98 model.

Hey Guys

Is this a big bore kit that is available and what is the costs

4banger: Take off the right side cover and check for play in the CB gear. Just do it! Or, if you are in SoCal, I'll do it for you.

S100, what I meant earlier is visible backlash between gears, not a gear with free play off it's own shafy. There is a difference. What condition does yours have? Don't want top be a party pooper but take a look at your ring end gap if you are so inclined. I thought the racket in my bike came from either timimg chain, valve train or CB gear. Not so! It was a nice and loose piston skirt flailing away with lots of slop!

[This message has been edited by Hugh LePage (edited 02-23-2001).]


Mine had a little backlash between the two gears which I thought was acceptable. Did you do a compression test or anything before you pulled the top off?



When in doubt, GAS IT!

Khris, I got suckered into taking my top-end off. Here are the reasons: a) am obsessive about detail :) lack of experience, I took off the plug with-out using compressed air in the plug blowhole. The guy who had the bike before rode sand. Guess what fell in.

No I didn't do a compression test on it and the bike ran great, no smoking at all. Just a bumch of racket. From what you said in earlier posts about riding a paddle tire and loading the engine you may have an out of spec bore. But don't fret if that is what it is. Quite easy to rebuild. Take care!

ok, I'll just do it.

Ya know, it is amazing that I took the bike to two shops and they haven't a clue what to do with it. They just wanted to start replacing things until it ran right. I am hopeful that this is the problem and will tear her down this week. Thanks.

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