Carb Help 98 YZ400


13 replies to this topic
  • YamahaWR450FMike

Posted November 19, 2005 - 10:28 PM

#1

I have a 98 YZ400 that I bought new back in 98. I have to say this bike has been amazing. Over 10,000 miles and not one problem with the engine or transmission. Been very low maintanance. Haven't ridi'n it in quite sometime and I think it's in need of a jetting change and carb cleaning. I can't get it started and if it does start it runs terrible. Very hard to start. This bike is stock and has had no mods to it. Just wondering what I can do to get it going again. What kinda carb is on a YZ400? I would like to start out by changing the pilot jet and main jet. What sizes should I use. I'm in the AZ desert ride in about 70 degree temps and elevation is between 500 and 1000ft. I'd appreicate any feedback. Thanks.

  • mxmark

Posted November 19, 2005 - 10:38 PM

#2

Mike, In Ca the gas will last about 4 weeks before it starts to clog up the jets. The pilot jet sound like it is clogged. Pull the carb off and clean all of the jets and the passages that the jets go into with contact cleaner. Be carefull the ruber carb parts will be useless after the contact cleaner get on them so pull the ruber parts off first. Or just pull the Pilot jet out and clean it. Next time you need to use some fuel stabilizer and drain the carb befor you store it. Thanks Mark

  • YamahaWR450FMike

Posted November 19, 2005 - 11:02 PM

#3

Mike, In Ca the gas will last about 4 weeks before it starts to clog up the jets. The pilot jet sound like it is clogged. Pull the carb off and clean all of the jets and the passages that the jets go into with contact cleaner. Be carefull the ruber carb parts will be useless after the contact cleaner get on them so pull the ruber parts off first. Or just pull the Pilot jet out and clean it. Next time you need to use some fuel stabilizer and drain the carb befor you store it. Thanks Mark


Thanks Mark,
What kinda carb is on a 98 YZ400? I have a 04 WR450 and it has the Keihin FCR carb. Thinking about going with a new 48 pilot and 168 main for the YZ400. Have the same sized jets in my WR450 and runs good and strong.

  • mxmark

Posted November 20, 2005 - 08:37 AM

#4

Mike, When I had a YZ400F in 1998 I remeber the jetting was spot on except the fuel screw adjustment under the carb. The Idle circuit (fuel screw) is most likely clogged with the pilot jet. If the bike ran well before storage then don't mess with different sizes on the jetting. Thanks Mark

  • Shawn_Mc

Posted November 20, 2005 - 12:05 PM

#5

The carb on the 98 was the initial FCR-MX. It looks different than the later ones because the later ones have the slides squared up in the flow and the bowl haning directly below it.

I think the early ones were adapted from a big twin like a Suzuki TL or something.

Just because the thing sat it shouldnt need to be rejetted, unless you mean to simply replace the clogged jets with the same size new ones. Carb jetting doesnt change for no reason. Fuel differences, or a mechanical change like a pipe or different air filter or something would cause it, but simply sitting doesnt do it.

  • YamahaWR450FMike

Posted November 20, 2005 - 04:02 PM

#6

well I have decided to re-jet it anyway. I needs it. So does a pilot jet that fits a 04 WR450 FCR carb will that be the same for the 98 YZ400 FCR-MX carb?

  • YamahaWR450FMike

Posted November 21, 2005 - 11:15 AM

#7

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  • Shawn_Mc

Posted November 21, 2005 - 02:21 PM

#8

The jet will fit, although the size may be different. The FCR jets are all the same except for the Primary air jets from the Hondas are smaller in dimension than the YZF's. I know this because Ive got a handful of PAJ's left over from my YZ and they dont fit my CRF.

  • YamahaWR450FMike

Posted November 21, 2005 - 11:22 PM

#9

ok great thanks for the info

  • rdefonce

Posted November 22, 2005 - 09:39 AM

#10

well I have decided to re-jet it anyway. I needs it. So does a pilot jet that fits a 04 WR450 FCR carb will that be the same for the 98 YZ400 FCR-MX carb?


Hi Mike,

I've got a '99 YZ400F (bought new in 99). Not sure about jetting differences between 98 and 99, but this worked good for me (I'm at 600 ft altitude, riding temps between 50 - 90 degrees):

48 pilot (45 is stock), fuel screw 2 turns out (1.4 turns out is stock), 180 main (175 is stock), needle position one clip richer than stock (stock is middle position.)

Good luck!
Ron DeFonce

  • YamahaWR450FMike

Posted November 23, 2005 - 01:50 AM

#11

Hi Mike,

I've got a '99 YZ400F (bought new in 99). Not sure about jetting differences between 98 and 99, but this worked good for me (I'm at 600 ft altitude, riding temps between 50 - 90 degrees):

48 pilot (45 is stock), fuel screw 2 turns out (1.4 turns out is stock), 180 main (175 is stock), needle position one clip richer than stock (stock is middle position.)

Good luck!
Ron DeFonce



great thanks for the info on that. I'll go with those numbers once I tear into the carb this weekend. 175 for a stock main? My 2004 WR450 had a 165 stock main. Wonder why such a rich jet for a 400?

  • sirthumpalot

Posted November 23, 2005 - 04:09 AM

#12

Check out the edlebrock carbs for that bike. My buddy rode a (I think it was 2000?) 400 and had the bog issue. He finally ended up switching to the edlebock carb and was extremely happy. Search for posts by YZFan about the edlebrock.

I'm not saying that your issue is not fixable; I'm just saying this might be a good mod if you feel like and upgrade. Good luck!

  • YamahaWR450FMike

Posted November 30, 2005 - 11:59 AM

#13

well I got the 98 YZ400 started after taking apart the carb and finding nothing wrong. It was fairly clean. Just a little rust in the accerator pump assembly. Had to have the bike pull started about 1/8 mile and it came to life. It still runs hard even with 10,000 miles on it and no valve adjustment. It's almost as fast as my moded 04 WR450.

  • rdefonce

Posted November 30, 2005 - 04:32 PM

#14

well I got the 98 YZ400 started after taking apart the carb and finding nothing wrong. It was fairly clean. Just a little rust in the accerator pump assembly. Had to have the bike pull started about 1/8 mile and it came to life. It still runs hard even with 10,000 miles on it and no valve adjustment. It's almost as fast as my moded 04 WR450.


Hi WR Mike, Good deal that you got 'er running! The old steel-valve motors are stone-ax reliable, more so than the titanium valvers. I just checked my valves last year and they were within spec; they've NEVER needed shimming. :applause:
Granted I don't ride every weekend though.

You questioned in a previous post about why stock main for 400 is larger than for 450. I was wondering that too. I talked to knowledgeable guy about this, he said that often a larger engine sucks gas thru jets harder than smaller one. (The "air pump" is larger!) so smaller jet is necessary.





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