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O-Ring Mod; Why should it be necessary?

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In this thread, William1 explains that the o-ring mod (also can be done with Zip-Ties, I guess) basically removes the free play from the AP linkage, as you can see in this video as well.  I understand that well enough, and see why it works to eliminate the bog and lazy throttle response.


 I'm wondering though, if the mod is so successful at improving performance, by just eliminating the slow movement of the AP Cam, why did Yamaha design in with that slow response in the first place? What's the point of having the AP cam moving slowly? Alternatively, what's the down side to this mod?

Edited by AeroSigma

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The accelerator pump does not travel as far as the rest of the throttle linkage.  To accommodate this, AP links on most modern bikes and all carbureted cars/trucks have the linkage separated into a driving and a driven section.  Between these two elements is a drive spring that will allow the linkage as a whole to compress when the pump plunger bottoms out so the throttle can continue to open the rest of the way. 


The "O-ring mod" simply bolsters this spring on the often erroneous assumption that the drive spring is too weak and is compressing at the beginning of the opening motion, rather than driving the pump plunger downward, thus not sending the fuel soon enough.  The stiffer "spring" that results pushes more fuel through the pump sooner, but will still allow the plunger to bottom without jamming the linkage.  The downside is shorter pump diaphragm life and a sometimes noticeable stiffening of the feel of the throttle grip as the plunger bottoms. 


If wire or a zip-tie is used, then the stop button on the AP diaphragm must be shortened so that the linkage can move through its full range to reach WOT.  The downside here is the same shorter pump life, but also a pump squirt that is way too long.


Either mod is a "caveman" or "gorilla" approach to curing a tip-in stumble.  So, likewise, is the tendency people have for bumping the size of the pilot jet to some huge level.  Both are shortcuts to actually tuning out the problem, if it truly exists in the first place.  Pump timing, diaphragm button length, and needle selection (NOT needle position) are better means of addressing this.  (note that the reason that the BK mod works so well on a 426 is that it reduces the amount of fuel the pump delivers) If the bike is equipped with a leak jet (YZ450's had these beginning with '05), it can be used to tweak the matter, but the leak jet was installed for an entirely different reason. Read:




Then, these two:





What it comes down to is often that the bike is either tuned wrong or being ridden wrong, the second usually being a result of unrealistic expectations, such as the desire for the bike to be able to take a full sweep idle-to-WOT instant snap without gagging.  You CAN actually get them to do that, but they almost always display some very bad habits as a result of drowning in their own gas during lower speed/lower load operation.  To address the rider part of the problem, you need to learn to "roll" the throttle, not snap it.  This technique starts opening the throttle a bit more slowly the first 1/8 or so, then progressively faster as the engine catches up.  It becomes so reflexive after a bit that you just don't realize you're doing it, and the engine will respond in a way that seems instantaneous.


If, on the other hand, you find the while underway and opening the throttle at a reasonable RPM in a reasonable manner that the engine stumbles, This is best sorted out by making the adjustments that were intended to correct this.  You'll have a much sharper running engine for having done so.

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