The O-RING mod


32 replies to this topic
  • grayracer513

Posted March 17, 2008 - 04:23 PM

#21

Is diaphragm stud length the only method of adjusting duration? All else equal, will different LJ's not also have an effect on duration?

No, to the first question. The total size of all output orifices (LJ included) vs. the size and stroke of the diaphragm, vs. the stiffness of the AP plunger drive spring is what combine to give you the duration of the squirt. The AP discharge nozzles are produced in various sizes, but because of how they are installed (pressed) and their lack of general availability, they aren't a practical tuning tool at all. And as to the second part of that, yes, of course LJ's do affect the total output, as I just outlined.

I’m not aware of any alternative OEM springs for the AP lever. If this was the design intent, it seems they did not intend on making the volume adjustable this way. Besides, Keihin designed the carb, not Yamaha, right? They did, however, offer various LJ's and diaphragms for adjustment.:confused:

Keihin builds carbs to the specs of their customers who have presumably spent a long time in figuring out how they want the carbs delivered. They don't offer the springs as a tuning tool, true. But many other carb manufacturers do, or offer a way to adjust the preload on the spring. Holley, Carter, and Weber all offer selective jets to control not only the leak circuit, but also the output circuit. I know these are automotive carbs, but the car guys have had far more sophisticated carburetors avilable to them for decades than the newest FCR is. Often when a company doesn't offer a part for tuning, it's because they don't see the profit in making them available, or because they just know better than you, so you don't need it.

I re-read your post and think this is where we may have a misunderstanding. I agree that hard wiring and grinding off the diaphragm stud is "caveman" (no offense to any real cavemen:smirk: ). I think, though, that there is merit in the O-ring mod and in adjusting the duration and volume via the available range of diaphragms. Based on this, I also think it is perfectly acceptable to further tune your AP with LJ's, especially when you tune for various seasons, something the O-ring or stiffer spring still do not allow.

I'll half agree with this. However, the O-ring or stiffer springs do not prevent the use of leak jet selection to tune the AP. In fact, because these two approaches allow you to use a diaphragm with a shorter travel, they actually allow more tuning latitude than wiring does. With wired linkages, the pump rod must move through the entire throttle opening, and changing the leak jet is the only adjustment you have left that you can still make.

Whether the LJ is used to reduce or eliminate (or even increase) pump shot or to alter squirt volume and/or duration is semantics.

Not semantics at all, they are two different things. The first modern motorcycle engines to have LJ's were the bikes with smaller, high strung engines with hugely oversized carbs, i.e., the YZ250F's. These engines were had such aggressive cam timing and big carbs that they had to have a really big boost from the AP. (BTW, you know that prior to the YZF and the FCR, there were no AP's on any thumper, and the big XR's didn't have one for years, if it even does now?) Such a massive dose of gas being dumped into the engine every time the throttle is bumped a little, the extra fuel became a problem, and so the solution of a leak circuit was brought over from the car world, where it had existed for years, to dump the smaller, slower deliveries before they reached the air stream.

The YZ450 did not even have a leak circuit until '05, nor did any previous model of the big blue bike, but I'm sure you could tune the AP on one if need be. My point in this is that the leak jet is not a cure-all for everything that ails your throttle response, and a perfectly tuned AP will not make up for incorrect jetting throughout the rest of the carb, as some people (not you, I would think) are now starting to believe.

What I would really like to see is an adjustable stop screw for the diaphragm, and a way to change the tension on the AP linkage spring. Then, the LJ could be used to do the final trim, and would rarely need to be changed more than one size.

  • William1

Posted March 17, 2008 - 04:32 PM

#22

grayracer, bg10459

Terrific analysis. Much clearer than a bunch of the other articles I have read. Hopefully, others will read what you guys have written and understand it.
:confused:

  • bg10459

Posted March 17, 2008 - 04:47 PM

#23

.....as some people (not you, I would think) are now starting to believe.

Thank you.:p

What I would really like to see is an adjustable stop screw for the diaphragm, and a way to change the tension on the AP linkage spring. Then, the LJ could be used to do the final trim, and would rarely need to be changed more than one size.

There you go, thinking outside the box again. :sweden: That's a great idea.:confused: It would be real easy to add a set screw through the bottom of the AP cover and it's not available on any aftermarket ones that I know of.:excuseme: For the record, when Boyesen comes out with one, I want half credit (you get the other half):crazy:

On a side note, why did my 03 WR come with a leak jet, but the YZ didn't get one till 05? (seriously, not playing devils advocate here)

grayracer, bg10459

Terrific analysis. Much clearer than a bunch of the other articles I have read. Hopefully, others will read what you guys have written and understand it.
:excuseme:

And thank you, too.:lol:

  • grayracer513

Posted March 17, 2008 - 06:29 PM

#24

There you go, thinking outside the box again. :excuseme: That's a great idea.:confused: It would be real easy to add a set screw through the bottom of the AP cover and it's not available on any aftermarket ones that I know of.:excuseme: For the record, when Boyesen comes out with one, I want half credit (you get the other half):sweden:

On a side note, why did my 03 WR come with a leak jet, but the YZ didn't get one till 05? (seriously, not playing devils advocate here)

Sealing the stop screw might prove to be the trickiest part. And I want royalties, not credit. :crazy:

The WR, remember, is intended to be driven on the street, and so things like conforming to emissions testing is a more critical issue. Leak jets clean up part throttle driveability by eliminating the pump shot where it isn't needed.

  • Family Man

Posted April 16, 2008 - 06:46 PM

#25

I have noticed some bog occasionally on my 07 yz450, Kind of dangerous when the bike bogs before a jump. Im thinking about trying the oring mod to see if it fixs the bog. I may install two incase one breaks. If one breaks could it cause the throttle to stick on or any thing that may be dangerous, or would the AP just go back to how it was before? How long does the o-ring mod last, if installed correctly?

  • William1

Posted April 17, 2008 - 02:19 AM

#26

Just put in one oring. You want the 'stretch ' to be there, two may be too unforgiving. Oring should last years. If it breaks, only take two minutes to replace on bike, even the alloy framed ones.

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  • henrics

Posted April 17, 2008 - 03:12 AM

#27

what do you think about using and oring from an oring chain, say a regina orn-6?


That what i use works great

  • Family Man

Posted April 23, 2008 - 01:07 PM

#28

I used one from my regina orn6 and liked the improvement. I was looking at the recomended size of #78 and went to the harware store only to find that size is huge, probley 2 inch outside diameter and skinny. I purchased a number 58 instead, 1/4 inch inside diameter and half inch OD and 1/8 inch thick, Im going to give it a try instead. Im not sure that the oring from my chain is stiff enough.

  • Family Man

Posted April 23, 2008 - 06:18 PM

#29

The oring fit great and works, its called a number 58 at my local Ace. The Oring from my chain wasnt thick/strong enough to secure the parts, still had a gap upon opening the throttle. Now no gap, with the larger o-ring.

  • rufusz

Posted April 23, 2008 - 09:44 PM

#30

One, probably dumb question : when you install/change the O-ring with the carb mounted, how do you test it without flooding the engine by pumping the throttle several times (I think this way you're testing if the O-ring is holding,etc.)
:thumbsup:

  • MaxPower

Posted April 24, 2008 - 12:59 AM

#31

One, probably dumb question : when you install/change the O-ring with the carb mounted,

[COLOR="Green"]Not a dumb question if you arent understanding something
You install it just as if it we on a bench infront of you.Pull the cover off and slip the oring over the 2 pieces.Its simple as can be.
I am honestly not being sarcastic, what is confusing you with the process? Maybe Im not understanding your question
[/COLOR]

  • rufusz

Posted April 24, 2008 - 03:37 AM

#32

[COLOR="Green"]Not a dumb question if you arent understanding something
You install it just as if it we on a bench infront of you.Pull the cover off and slip the oring over the 2 pieces.Its simple as can be.
I am honestly not being sarcastic, what is confusing you with the process? Maybe Im not understanding your question
[/COLOR]


There is no problem with the installation, just curious IF do you test it with the engine stopped and pumping the throttle several times??? IN this case how do you prevent over-pumping (flooding) the engine or you're just testing it with a running engine?

  • 642MX

Posted April 24, 2008 - 10:56 AM

#33

One, probably dumb question : when you install/change the O-ring with the carb mounted, how do you test it without flooding the engine by pumping the throttle several times (I think this way you're testing if the O-ring is holding,etc.)
:ride:


2 or 3 twists of the throttle won't hurt anything when your testing it. Just use your hot start when you fire the bike up. :thumbsup:





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