Carb Needle Question


3 replies to this topic
  • iveyyam450f

Posted December 05, 2011 - 08:20 AM

#1

I bought a used carb that has been bored and has a velocity stack on it for my 08 YZ450F. The carb has a NCVP needle in it. I am not sure what all the letters mean. The manual shows optional needles for richer or leaner and this not one of them. I know the last letter is richer or leaner and I believe the 3 letter is where the taper start but i am not sure about the rest. I am also trying to find a air intake boot that will work on the velocity stack. I was running PVC pipe to a universal K&N to get me through the end of the season and that the guy I bought it off of used when he ran it. I have seen other bikes with a velocity stack that have a OE style intake boot but i cant find what they come off of. The OD of the velocity stack is 62mm. This bike is used for flat track racing.

  • grayracer513

Posted December 05, 2011 - 09:42 AM

#2

The N is a series indicator showing what family of carbs it will fit.

The C should be the taper (D is 0º 45', G is 1º 30'). Based on the increments listed in the Sudco catalog, it looks like C should be 0º 30'.

The V should be the length from the top clip groove to the start of the tapered section. V would be 82.75mm. The "larger" the letter (P is larger than L) the longer the needle is, so the larger letters are leaner at any clip position.

P is the diameter of the non-tapered upper portion of the needle between the clip grooves and the top of the taper. P should be 2.735mm. The larger the letter the larger the diameter, and the leaner the needle.

The way it works is that the diameter of the straight part controls the mixture from off idle to 1/4 throttle, mostly at 1/8. From about 1/8 to 1/4, the clip position becomes more influential, tapering off to around 3/4. The taper begins to influence things at around 1/4, becoming most influential at around 3/4, then becoming less significant as the throttle is opened further.

In general, each incremental letter is about 1/2 "step" richer or leaner. For example, an NCTP is a full step richer than the NCVP, exactly like lowering the clip one step. An NCUP would be halfway between.

I can't give you any more solid info than that, but that and a look at the Sudco catalog should help you figure out what to do with the needle if it doesn't run right.

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  • iveyyam450f

Posted December 06, 2011 - 01:45 PM

#3

Your help is greatly appreciated. This will help figure out what needle to try first. The bike runs good on the topend with the modified carb, about 1hp more than the stock carb, but it has backfire on decel that im trying to work out. It also kind "cuts out" at steady throttle, probably about 3/8 throttle, between 6k and 8k rpm on dyno. I am going to try some different needles to try and help this.

  • grayracer513

Posted December 06, 2011 - 03:41 PM

#4

Backfire on decel is usually not related to the needle, and teh only way to get rid of all of it is to make the bike too rich. It should pop a little some of the time. If it's excessive, don't try to jet it away until you're sure the exhaust has no leaks, and if it still does it once the pilot is set correctly, just don't worry about it.

http://www.thumperta...613#post3350613

If the cutting out at part throttle is erratic, like a misfire, it's probably lean, but the Pilot Air Jet (aka Slow Air Jet) may be at fault:

Read:

http://vma.cape.com/...brm/carbkei.htm

or download it as a PDF: (below the movie ads):

http://fliiby.com/fi...1f4quybz1e.html





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