Throttle needle clip position....

9 replies to this topic
  • kizzle426

Posted October 06, 2005 - 09:59 PM


A little while back, my 426 was fouling plugs pretty bad. So I took the carb apart and checked all the jets and cleaned them. Also I made sure what size jets they were. They were all stock. I read through the manual and it said what the effects of moving the little clip were. So I moved it up one on the little ribs. It's been running great ever since with almost no dying bike problems. It pops a little bit on decel, just a couple, and I have terrible muffler packing. All I'm wondering is what exactly that movement of the clip did beside lean it out a little. Also, it's starting to get cold here in Texas. It's running good at 80-100 degrees. This is my first cold season and I'm just wondering what to expect for the 40-65 degree temperatures. Any advice and information is greatly appreciated.

  • Hick

Posted October 07, 2005 - 11:03 AM


The needle closes off the flow from the main jet when the throttle is closed. As the throttle opens about 1/3 of the way it eventually contacts the clip and lifts the needle, opening the main. Notice that the needle is tapered, so as it lifts higher it allows progressively more fuel to flow.

Moving the clip up one spot means that the throttle needs to be open a little bit more before the needle begins to allow fuel to be delivered through the main. It affects the timing of the main circuit w/ respect to the slide.

IMO moving the clip is the most noticeable jetting change w/ this carb, changing the pilot size being the second most noticeable, changing the needle type third, and main jet fourth. This is because you spend most of your riding time somewhere on the tapered portion of the needle, and although the main size affects this also when compared to most two strokes changing the main isn't as noticeable for whatever reason (maybe due to the needle being much longer and more tapered on the FCR vs., say, the PWK).

The popping on decel is not related to the needle, or should not be, because the throttle is closed/mostly closed (so the needle is all the way down regardless of clip position) and the bike is on the idle circuit/pilot circuit.

As it gets colder you may begin to notice a lean spot when you open the throttle due to your leaner clip position, then again you may not. Other things that affect jetting besides air temp are humidity and altitude (more of these makes your bike run richer).

Hope this helps.

  • yamarider426

Posted October 07, 2005 - 11:09 AM


First I would adjust my fuel screw and re-pack cannister to try to richen it up a little (to get pops out) you may have to adjust it after temp changes like that. Muffler packing helped mine out alot when It was doing the same thing. More packing will increase backpressure a little also causing it to run a little richer. Try fuel screw adjusting and packing cannister first and if that dosent work try going 1 size bigger on main jet. Your poping is because you are lean between 3/4 and WOT. (wide open throttle)

  • Fastest1

Posted October 07, 2005 - 11:47 AM


Your buddy is wrong! The slide closes completely when the throttle is against the stop regardless of rpm's, It has to since it has a return cable. Also a popping on deceleration is (almost) always too little fuel in the pilot circuit. It is not really running when decelerating just coasting down to idle where it will maintain itself fine. "Hick" has the carbs down pretty well.

  • Fastest1

Posted October 07, 2005 - 11:48 AM


BTW I think your buddy is thinking about a CV carb.

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  • Fastest1

Posted October 07, 2005 - 02:54 PM


In fact it does move by cable. With the carb off of the bike as the slide lifts the accel pump squirts just as the slide goes up, if it is timed correctly. How did you measure your accel pump duration when you had your carb off to do the BK mod?.

  • Fastest1

Posted October 07, 2005 - 03:00 PM


Look here
http://www.ncyyamaha...tion_detail.asp :banghead:

  • yamarider426

Posted October 07, 2005 - 03:26 PM


Sorry man
I am an idiot
I guess I was thinking of another carb
I have worked on about 5 diffrent ones in the past couple months
I believe the one I was thinking of was one off a Polaris Predator 500 sport quad
...had to go take mine off to make sure!
You are deff. right!
My apologies

  • Hick

Posted October 07, 2005 - 03:31 PM


:banghead: The slide is not controlled by the cable!
The slide is controlled by vac. :banghead:
...dont believe me? go take your carb off, open the throttle and see if the slide moves!

Fastest is right, it sounds like you are talking about a CV carb. You would be correct if you were talking about my old CBR 900 (or a DRZ400S), but the 426, and just about every other 4 stroke MXer (even the Husaberg now), comes w/ the Keihin FCR, which is not a CV carb.

The FCR is a flat slide carb, and when the throttle is off, the slide is always down regardless of vacuum, rpm or whatever. There is a return spring on the slide pulley, and also, as stated, a return cable on the throttle.

Most CV carbs only have the one throttle cable, whereas all the FCR applications I've seen (RMZ, CRF, KTM RFS etc) have two throttle cables.


  • Fastest1

Posted October 07, 2005 - 08:06 PM


We all still learn here on TT :banghead: :banghead:

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