YZ400f Won't RUN "RIGHT"

3 replies to this topic
  • bigmack91

Posted October 01, 2011 - 11:20 AM


I've been messin with this bike off and on for 4 months now, playing with it trying to get it to run "RIGHT":banghead:. I bought a new (used) carb and cleaned all jets and passageways before installing.. Bike will NOT start without the HOTstart ON. Turn the choke on it dies, and if you keep the bike running(which is not idling) exhaust heats up REAL quick and glows...(too lean !)

Stock exhaust, stock air filter, stock jets(I think... they look exactly like my other carbs jets)
Mixture screw is out 4 full turns,
The needle is on the bottom clip(max fuel), -How can it still be LEAN?


  • grayracer513

Posted October 01, 2011 - 12:20 PM


OK, let's look at a few clues here. To start with, it's a misconception that a red hot header means the bike is too lean. My 450 runs perfectly well, thank you, and has a heavier than stock stainless header, but I can make it glow inside of 3 minutes just by holding it at a fast idle. The things that heat the exhaust are things that cause there to be combustion IN the exhaust, which are the late closing of the exhaust valve related to aggressive cam profiles (or a cam that's timed too far retarded), retarded ignition timing, and an excess of fuel in the exhaust. In other words, it's partly a normal thing, but being timed wrong or too rich can aggravate it.

Now, you've stated that the bike won't start without the hot start. The hot start does nothing more than create an air leak to make things leaner, so that suggests the bike is rich. Further, it will not run if the choke is used. The choke makes things richer, and since the bike won't tolerate it, that suggests again that the bike is rich.

If your pilot jet is varnished completely or partially shut, the fact that your fuel screw is out 4 turns (which is well beyond the accepted "limit" of 3 turns) won't really matter. If the pilot air jet is obstructed, your idle mix may be richer than hell.

Look at the jetting sticky for the 400 and try one of those setups as a baseline. Check your cam timing, and try it without the TPS connected. See what it acts like then.

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  • Thepowderblue

Posted October 01, 2011 - 12:24 PM


dont they hide the fuel mixture screws on our carbs. I cant find mine and I was told that Yamaha presses a plate over them that I will have to drill out

  • grayracer513

Posted October 01, 2011 - 12:27 PM


dont they hide the fuel mixture screws on our carbs. I cant find mine and I was told that Yamaha presses a plate over them that I will have to drill out

The WR450's had a plate covering the screw for anti-tampering reasons related to emissions laws. Earlier models and YZF's don't have that.



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