99 YZ400F, Misc Questions

4 replies to this topic
  • DGXR

Posted June 17, 2010 - 10:28 AM


I have searched the threads in this forum and found some useful information, but still have some questions... bought the bike last week from my bro-in-law, who got it from his insurance agent. So I am at least the 3rd owner. It's in decent shape overall.

Carb/fuel: the tank valve leaks slightly, so while it sat the carb got gunked and reeked of turpentine. Cleaned the carb and got it running, then it died and would not restart. Checked the jets and found the pilot/slow jet is plugged again! While diagnosing, we took the fuel valve apart and the rubber backing on the body inside the valve is kind of messed up (a bit of this rubber crap got into the bowl which is what plugged the slow jet). Cleaned the bits out of the fuel valve and it seems to be closing better than before but still not all the way, still a very slight drip. Can the valve be repaired or do I need to replace it?

Exhaust: the FMF Power Core currently installed is ridiculously loud. It also came with 2 other pipes: the stocker and a FMF Q. I care more about the noise than getting a few extra hp so I am inclined to put the stocker back on. Have not installed the Q, wondering if it's much louder than the stocker? Anyone interested in buying the Power Core? Will post in classifieds - mods please don't move this thread, thx.

Timing chain: this bike always ran strong so we have never taken the valve cover off and we don't know the condition of the timing chain or tensioner. Is this something to worry about? I'll check the valve clearance soon but wanted some tips on the chain/tensioner while it's apart.

Rear suspension: when the bike's on a stand, you can pull up on the swingarm and it kind of "bumps" so there's a little slack at the far end of travel, but when sitting/riding the damping feels fine. Is this a problem with the shock, the shock mounts/linkage, or the swingarm bearings? Or is it normal?

Gearing: I will never take this bike on tight single track (have another bike for that) plus I like to ride fast. Wanting to re-gear for more top end, thinking of a 45 for the rear sprocket and/or a 15 on the front. Maybe that's too tall considering the holeshot first gear ratio? Input?

I ask these questions because it's a good fun bike and I want to ride the piss out of it for years to come. Thx ppl :thumbsup:

  • grayracer513

Posted June 17, 2010 - 10:39 AM


  • You'll be less frustrated if you just replace the petcock. You can rebuild them, but the success rate isn't great.
  • The old Power Cores were loud, but it may just need packing. The OEM pipe wasn't real quiet, either, but it's not obnoxious (admittedly, a subjective value). The Q is easily the quietest of the three.
  • When in doubt, change it. The tensioner is "probably" OK, but they aren't that expensive.
  • The upper shock bearing is a single ball, "Heim joint" type bearing, and a bit of clearance is normal. Should be ~1/4" at the axle.
  • The bike will currently run over 80 mph easily. I'd ride it first and see what you think. You could go a 15 front OR a smaller rear, but I wouldn't jump right out and do both to start with.

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  • phart2005

Posted June 23, 2010 - 07:25 AM


15/45 would be way, way too tall at least it would have been on my 426, which had 14/49 stock. I think 400 and 426 were geared the same both internally and externally. I doubt you could pull that tall of a gear effectively/efficiently in 5th gear. (400s had 5spds, right? Still, that'd be a lot even if it was a 4spd)

15/45 sounds like supermoto gearing, again on a 426 anyway. Made for the smaller diameter wheels.

I ran 14/48 always and it was plenty fast MPH wise and I could still run 1-2 in the woods without too much clutch action.

Geared this way, I would guess the bike did 90 or more, but that was WOT at really high RPM.

  • DGXR

Posted June 23, 2010 - 07:52 AM


Thanks for the replies! Ordered a new OEM fuel cock for $23. Put on the FMF Q, it is much quieter than the Power Core while revving in neutral, we will see what it's like on the trail. Gonna go through the rear suspension anyway as part of sorting out the whole bike, will order swingarm and/or linkage rebuild kits as needed. Fortunately it had a recent oil change, cooling system service, and the forks were rebuilt at Folsom Lake Yamaha. So long as it starts easily and runs strong, I have no reason to pull the valve cover. Yes I will eventually have to check the valve clearance, at that time I will order any shims, and also a TC/tensioner and gasket(s). I have ridden the bike several times before buying it and I don't like the close gear ratios... what's the point? This motor pulls very strong from any rpm so the ratios should be wider, right? Then I remember this is a green sticker race bike. Anyway I wanted to spread the ratios out with higher gearing but 1st is tall already. Since it tops out over 80mph stock, I'll just get stock sprockets and also a 15t countershaft just for giggles. Also gotta get that seat padded up better! Finally, I found out the insurance agent bought it from the original owner, so I am the 4th owner. Never had a Yami before, looking forward to this summer...

  • grayracer513

Posted June 23, 2010 - 09:02 AM


... I don't like the close gear ratios... what's the point? This motor pulls very strong from any rpm so the ratios should be wider, right? Then I remember this is a green sticker race bike.

Exactly. When you run it up in the rev range, the spacing gets wider the faster you go. Plus, the close spacing increases the likelihood of having the right gear for every spot on the track in MX, which is what it was built for. You won't find too many places other than very low speeds where it won't work well, but if you come a cross a used WR400 gear set, you might want it.

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