Mystery Adjustment Screw.

My 99 yz400 was running weird so I pulled the carb apart and found that the float valve on the valve assembly (what I would have normally refered to as a slide) was cracked. I bought a replacement ($136 + $8 for the gasket = &%$#@!?!?!) and reassembled everything. However, I have a question that I am having a very hard time finding an answer for so hope you guys can help...on the rod that holds assembly mechanism (fork) that lifts the valve, there is an adjusting screw and crown nut. I can't figure out what that adjusts or what the proper adjustment for that would be. Any ideas??


The manual is not at all clear that there is anything to be adjusted in this setup, and simply says to install it. I haven't fiddled with one of these older FCR's in some years, and I don't remember it precisely, but I believe that there's pretty much only one way it can be set. If the screw threads into the throttle shaft, run it in until it bottoms and tighten the lock nut down. If the screw threads into the lifting cam, there should be a flat on the shaft. In that case, tighten the screw against the shaft firmly and lock it down with the nut.

Thanks gray. I have the manual as well and searched all over the magical internet, but like you said, everything just mentions to install it and then install the lock nut. After further inspection last night and out of frustration at midnight, I finally just did what you mention. Screwed it in until it stopped and loaded the lock nut on top of it. Started it up this morning when the rest of the neighborhood was awake and it ran like a charm. Bless Yamaha for all the innovations they've pioneered in the dirtbike world, but along with that comes these unnecessary little complications. Does a lot to keep D.I.Y.'ers like me on our!! :-)

The newer FCR-MX is much simpler as to that particular part of it, so it looks like they pay attention.

Ok, got home from work today and ran the bike a little longer until it got nice and warm. So much for the running "like a charm". :-( . It still has the original symptoms for which I tore it down in the first place. The problem is an erratic idle. Sometimes it idles just fine. Sometimes it take about 3-5 seconds to come off of throttle onto idle (this is what it was doing before and makes it VERY difficult to trail ride - lol, I'm sure you can imagine). And on occassion, it just won't idle at all and dies out. Any ideas? Some of the posts I've read point towards a lean mixture as the reason for an idle that won't come down immediately. I guess I can mess with the idle mixure screw since I know the jets are fine.

Any ideas? My gut tells me I'm looking at an idle position sensor issue. How can I test that?

The test procedure is in the manual, but the first thing I suggest is to check the valve clearance, then clean the pilot:


As far as the TPS is concerned, if you suspect it of being a problem, you can unplug it. It will run reasonably well without it.

Thanks again. This is a used (and kind of neglected) bike that I bought a few months ago. Fun bike, but obviously I have a lot to learn about it. I'm sure you'll hear from me again...

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