Yz 400 1998 carb issues

4 replies to this topic
  • Highrider211

Posted August 18, 2015 - 02:46 AM


Hello I just got the old beast about 3 weeks ago. Ive been all over the carb about three times. Ive checked the cam specs. Had to reshim one lobe on the intake side. Checked the timing. Checked all the jet sizes to oem all good. Put a new pilot jet #50 due to lean popping even when the air fuel screw almost all the way out. Ok so the new pilot jet fixed the lean popping. The real issue is since day 1 the idle scew has been all the way in or up. Anotherwords there is no more adjustment left. The bike runs good if you can get it started and warmed up. After you get it warmed up it will fire right up first kick. The bike has full white brothers exhaust not sure what model but its like a supertrap with baffles. The airfuel screw is about 4 turns out. But its an aftermarket. Please help with more knowledge on this issue thanks

  • grayracer513

Posted August 18, 2015 - 06:32 AM


The pilot is too big, for one thing, considerably so.  You probably had an exhaust pop caused by an air leak in the exhaust that you have now "covered up" by making the engine too rich.  Besides, YZ400-450's will always pop off on decel a little when they are tuned correctly.  Jet to make it run right, not to control popping.


The old FCR carbs on these can be a huge PITA when they get old.  Sometimes, if you can get a main body center gasket (not sold by Yamaha or Keihin, but JD jetting has them), you can find that there is either an air leak at that joint, or a bunch of goop blocking one of the passageways.  Don't separate the two halves until you have the gasket. 

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  • Highrider211

Posted August 18, 2015 - 06:49 AM


Hmmm. The stock pilot jet was a oem #45 I only went 1 size bigger. And the after market air/fuel screw was just for the ease of adjustment. As far as splitting the carb I havent split them. And its possible I might have very little exhaust leak due to its aftermarket. But on that note it ran way worse oem jetting. And the pilot jet is the low and idle circut. Yes? So what do you think the deal is with the idle being all the way in? Like I stated befor after she is warm she runs and idles fine. Just getting to that point is the problem.

  • Highrider211

Posted August 18, 2015 - 06:54 AM


Just to add to the info I know that the air fuel should be around 1.5 to 2.5 out and with oem #45 pilot it wouldnt even start thaught I was goin to break a leg to get it to start. With the #50 it was better but not like it should be. I also tried the air fuel at the same settings with no luck.

  • grayracer513

Posted August 18, 2015 - 08:30 AM


#50 is not one size bigger than #45, it's at least two.  Bear in mind that the number is the actual orifice size in a 3 digit metric measurement,  Jets marked with two digits are assumed to have "0." in front of them.  Thus, a #45 is 0.45mm, or 0.0177" in diameter.  If you dry a film of varnish in that jet only .001" thick, that will take it down to the equivalent of a #39, which is over three sizes smaller than normal, and way too small to run right, IF it would start.  That's probably what was wrong with the original jet.  All in all, you are quite often better off to find a used carb from any YZ450 from '05 to '09 with as much of the OEM cabling as you can get with it, and set that up in place of the OEM carb.  It's a near bolt-on, with very little modification of any kind required, and a huge improvement in overall operation and performance.

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