98 yz400f idle issue

3 replies to this topic
  • SC_lurker

Posted September 22, 2011 - 10:08 AM


I've had this bike since the beginning of the year and it always started and ran pretty good. I got a 2003 yz450 exhaust cam really cheap for easier starting and it went in with just a valve shim change. The valves were in spec on the intake and exhaust. 1.76 and 1.78 to 1.95 and 2.00. Got it together and checked timing and then checked again. When I started it up with the choke, it would run but never could get it to start or idle without the choke. Maxed out the throttle stop screw and still dies. I checked the jets and cleaned them. 45 pilot, 178 main and 65 starter. The pilot screw was already at a bit over 3 turns out. Pulled it out and checked to see if the o ring and washer were on. Checked out ok. Accelerator pump looks good. The hot start button is somewhat bent near the end of the shaft so I pulled it off and tried it with the hose blocked. I couldnt blow through the valve with it closed anyways. Unplugged the TPS. The thing doesn't want to idle.
Any suggestions? A new plug and probably going up a size on the pilot would be where I'd go next.
I ride at sea level and around Hollister Hills. It has a white brothers e-series header and silencer. Just wondering if anyone has any ideas. I've been enjoying reading the tech stuff here for some time now but never had a reason to post before now.

  • grayracer513

Posted September 22, 2011 - 11:16 AM


I'll assume that you understand that a cam made for a 450 has to be timed by an alternative process, and the marks on that cam cannot be used as the OEM 400 cam would...

The most likely is a film of varnish in the pilot. Even a jet that "looks" clean can be obstructed to the point that it won't run. Your pilot has a metering orifice only 0.45mm in diameter. It doesn't take a huge drop of dried varnish to turn it int a #35 or even smaller. When that happens, you can still see light through it and all that, but it won't work right. The cross holes are important, too.


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  • SC_lurker

Posted September 22, 2011 - 11:38 AM


Thanks for the reply grayracer. I did time the intake cam as per the manual and the 450 cam 14 pins between timing marks. The thing is as snappy as ever it just won't idle so I'll examine the pilot jet again and clean it.

  • SC_lurker

Posted September 22, 2011 - 04:58 PM


Well I took it all apart and put it all back together again and it fired up after I held the throttle open half way, choke off, while kicking. Cleared everything out and settled into a nice idle after a minute or so. Thanks for the tips.

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