Basic Carb question and rough running question

Two questions:

1. What is the adjuster for located at the bottom left side of the FCR39, it has the black plastic knob on it? My manual makes no reference to it whatsoever.

2. At mid-range type RPM's my bike will not run smooth if I try to keep the speed constant, it sputters. So far I've moved my DQR needle clip from position 4 to 3. Based on my searching, I'm considering moving the clip even higher but have not tried it yet. I have a 158 Main and ride at about 5000-9000 feet. Stock uncorked US pipe.

Should I try other needles/main jets or can someone recommend some simple adjustements before I go out and start buying jets/needles and the like?

1)Idle speed adjustment.

2)Try clip #2.

Compared to a stock '99YZ400 needle and clip: DVR#4=DTR#3=DRR#2=DQR#1.5. So, clip #2 is still in a range used previously and your altitude would indicate a need for leaner jetting.

Steve,

While I am not a jetting pro, nor am I familiar with high altitude jetting. The popping is an easy one.

The jetting on the pilot screw is too lean. This lean condition may also be causing uneven running at a steady throttle.

Unscrew the pilot screw until the decelaration popping stops. What pilot are you running and how many turns out on the screw do you have. If you have to turn the pilot screw out past 2.5 turns to stop the popping, you may consider a richer pilot jet.

Steve,

While I am not a jetting pro, nor am I familiar with high altitude jetting. The popping is an easy one.

The jetting on the pilot screw is too lean. This lean condition may also be causing uneven running at a steady throttle.

Unscrew the pilot screw until the decelaration popping stops. What pilot are you running and how many turns out on the screw do you have. If you have to turn the pilot screw out past 2.5 turns to stop the popping, you may consider a richer pilot jet.

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2000WR - EKN#5, 172MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

Airbox lid on or off?

I test rode with the airbox lid on and off and saw no difference. I will try changing the pilot screw setting tomorrow.

But, I did one thing that was VERY interesting. I put the Euro pipe back on it (the one it came with). It is likely much more restrictive than the uncorked US pipe. With the Euro, it runs steady, around my block anyway. I will go to the track tomorrow to further verify.

So, to me this means that my jetting for a free flowing exhaust is completely off and I will have to start from scratch if I am to use a less restrictive pipe. Agree?

If you want to see a pic of the Euro pipe, go to this website and follow the WR426 links and look at the brochure you can download. http://yamaha-motor-europe.com/

Thanks!

James,

Thanks for your reply, just got back from the test ride. Clip pos #2 did not really seem to help. Basically, the bike does not like to run at any constant rpm after low rpm range and up. I could go to pos #1 or #5. The plug was just grayish, not really sooty or anything, but I only have about 80 miles on it. Any other ideas?

I assume these bikes, when properly tuned, do run smoothly at fixed rpms? True?

Thanks Much

Steve

Forgot to mention a couple more characteristics that might help diagnose the rough running problem:

1. From WFO to closed throttle, there is always one very big backfire.

2 From mid-low rpms to closed throttle, there is various little backfiring.

Accelleration is generally always pretty good.

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