99 yzf idle, carb, pilot screw question

7 replies to this topic
  • espotten

Posted June 22, 2005 - 12:04 PM


I recently picked up a 1999 yz400 and really like the bike. The only problem was when I would turn the fuel on and try to start it, fuel would be pouring out of the carb overflow tube. I took the carb off and found that the o-ring on the outside of the valve seat was leaking and letting fuel by. I replaced the o-ring, adjusted the float to what my manual said, took out the two offset height screws on the inside of the carb to make sure they were clean, tighted them back up and adjusted my pilot screw to 1.5 turns out from all the way tight. Most of these settings I took from my Factory Manual. Put the bike together and the overflow no long leaks however the bike will barely run at idle. If I get lucky enough to get it started, it will only run if I keep it reved up. Motor seems to pop and back fire a bunch. I have tried moving the pilot screw out little by little (1/4 turn at a time) and it doesnt seem to fix the problem. I also checked the plug and it looks okay. Bike ran fine before this minus the leaky overflow so I dont think I need to change any jetting but do need to get it back to normal. Any settings, advice, or tips would be greatly appreciated. Seems to be a lot of knowledgable riders on this site. Thanks, ERIC :)

  • Hick

Posted June 22, 2005 - 12:13 PM


What happens if you turn up the idle speed screw?

Did you by chance take the slide out while you had the carb apart?

  • espotten

Posted June 22, 2005 - 12:27 PM


I turned the idle screw all the way up to try and get it to run and dont know where to reset it because the bike doesnt idle long enough. I only took the bottom of the carb apart and what exactly is the slide. I dont think so.

  • grayracer513

Posted June 22, 2005 - 12:52 PM


The slide is the throttle valve.

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  • espotten

Posted June 22, 2005 - 12:59 PM


no, the slide is still in place

  • sirthumpalot

Posted June 22, 2005 - 02:38 PM


Did this problem start suddenly, or slowly progress over time? If your valves have tightened up then it can do exactly what you describe in regards to the idle. If the carb is clean and the settings are where it has always run fine, the pop off the valve cover and check the valve clearances.

  • espotten

Posted June 22, 2005 - 02:58 PM


no this problem was all the sudden. The valves were fine prior to this. Saddly, the problem was cause by me somewhere along the line

  • dpetteys

Posted June 22, 2005 - 09:01 PM


The first thing that springs to my mind is the accelerator pump push rod. On the 99's it is exposed and can easily end up out of place if you have had the float bowl off. If it is not in place it will cause the bike to run really lean when accellerating, but it will hold a consistent rpm.

Further I have had problems with the accellerator pump diapraghm failing abruptly and causing similar problems. But you will likely see fuel leaking from the boot that covers the push rod if the diaphragm fails.

Then I would go back and be doubly sure all the passages and jets were clean, that the throttle position sensor is connected, and that the hot start was closed.

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