09 yz450 low end power problems


9 replies to this topic
  • Rkumle09yz450f

Posted December 12, 2013 - 01:55 PM

#1

I bought a 09 yz450f the bike is fresh only 3 hours on fresh rebuild and carb rebuild it has the boysen quick air fuel adjust screw and full FMF exhaust and stage 1 or 2 hot cams the bike runs good it just when I'm rolling and in the bottom of a gear and want to wrap the throttle it Boggs and almost chokes but top end is amazing mid range is ok but still hesitant I want to know if it is two rich if I can adjust for it in that quick adjust or what size jets I should run and if a 06 cdi would help! ImageUploadedByThumper Talk1386885319.731987.jpg ImageUploadedByThumper Talk1386885340.060799.jpg

  • Bturner

Posted December 12, 2013 - 02:39 PM

#2

What size pilot are you running? That's what controls the 1st 1/4 of your throttle. Sounds lean if it's got a bog off idle. Maybe go up a size on a pilot & dial in your air/fuel screw with it. How many turns out is your air/fuel screw? If it's under 1.5 you need to go up a size & if it's over 3 you need to go down a size (pilot). What's your gearing & what kind of riding?

  • grayracer513

Posted December 12, 2013 - 03:31 PM

#3

What size pilot are you running? That's what controls the 1st 1/4 of your throttle. Sounds lean if it's got a bog off idle. Maybe go up a size on a pilot & dial in your air/fuel screw with it. How many turns out is your air/fuel screw? If it's under 1.5 you need to go up a size & if it's over 3 you need to go down a size (pilot). What's your gearing & what kind of riding?

 

The pilot does not control the first quarter throttle, only the first 1/8 or so.  It meters fuel for the idle circuit, which controls idle and the hand-off to the main circuit.  The most active element of adjustable jetting at work from 1/8 to just beyond 1/4 is the diameter of the upper, straight portion of the needle.  After that, the needle position up to around 3/4, then the main jet.  The combination of slide cut out and needle diameter have the most to do with the transition from idle to the main circuit, along with the accelerator pump. 

 

The problem of bogging on throttle opening is usually a combination of things, starting with the rider simply snapping the throttle open too fast for the engine to be able to deal with it.  Before it was common for motorcycle carbs to even have accelerator pumps, riders had to learn to "roll" the throttle open.  Here, instead of "snapping" the throttle from closed to WOT, one quickly opens to about a quarter, then "rolling" the throttle the rest of the way open as the engine responds. The whole maneuver takes less than a second, and to stand off to the side and watch, you'd see it as instantaneous. 

 

The situation in the carb under the conditions described is this: First, it must be understood that what brings fuel up through the jets and into the air stream is intake manifold vacuum provided by the air moving into the engine.  At idle or near idle, low RPM and the throttle almost closed, the vacuum is high enough to raise fuel up into the airstream.  But at very low throttle settings, there is no fuel flow in the main circuit yet, or if there is, it's fairly minimal.  If the throttle is opened fairly slowly, the air flow increases and brings the main circuit into play.  But if the throttle is opened very suddenly, the sudden increase in intake opening causes the manifold vacuum to drop off, and fuel flow through either circuit actually stops.  If there is nothing available to add some fuel to the engine to get it past that sudden failure of the intake signal, it can stall completely. 

 

Enter the accelerator pump.  Its job is to physically force fuel into the intake tract on quick throttle openings to give the engine something to burn while the vacuum has fallen off so that it can accelerate itself to a speed high enough to establish or re-establish flow in the main circuit and continue running. 

 

Jet the pilot circuit for the correct behavior of the idle circuit.  Then jet the carb for the best response possible across all throttle positions with minimal accelerator pump assistance, then tune the accelerator pump to make up the difference.



  • Rkumle09yz450f

Posted December 12, 2013 - 04:39 PM

#4

I'm not sure what's done to the carb haven't opened it up yet, I'm not quite sure on the gearing but it seems to be geared more for torque then speed but I haven't fully went threw and documented what gears and all that! The air fuel screw is where the quick adjust screw is at the bottom of the carb correct, what size is stock for the pilot jet and what size is stock for the main

  • Rkumle09yz450f

Posted December 12, 2013 - 04:46 PM

#5

I also get some decel pop from the exhaust idk if that matters at all

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  • grayracer513

Posted December 12, 2013 - 04:51 PM

#6

Stock '09 jetting: http://www.thumperta...52#entry5850952

 

This is the one I'd recommend.  I've seen the bike run: http://www.thumperta...99#entry4459899

 

Also read: http://www.thumperta...ircuit-setting/



  • Rkumle09yz450f

Posted December 12, 2013 - 05:15 PM

#7

Thanks a lot I appreciate it, what about the decel exhaust popping?

  • Rkumle09yz450f

Posted December 12, 2013 - 05:17 PM

#8

What about the decel exhaust popping what is that caused by??

  • grayracer513

Posted December 12, 2013 - 10:19 PM

#9

Some of it is normal if everything's right.  Excessive decel popping is either too lean on the idle circuit or just an air leak at either end of the header.

 

http://www.thumperta...t/#entry2983539



  • Rkumle09yz450f

Posted December 13, 2013 - 06:05 PM

#10

Thanks for all the insight I really appreciate it!!!





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