Woody426's awesome discovery!

I felt like this was important enough to everyone here to start new thread on this... After having trouble getting my pilot jet adjusted (not much response to turning it, hanging idle with any jet I tried, etc.) Woody wanted me to try taking the spring out of the deceleration valve on the left side of the carb. I also read a thread over on the 250 board that a guy named Larry had posted about this too (155 main jet or something.) Over there I was told the blocking off the small hole, where the little 1/8" o-ring is, would do the trick. So I first cut a small round piece of rubber about the thickness of the o-ring and just blocked off the hole and got rid of the o-ring, but still giving an airtight seal. That helped but not much. I then went back and pulled the spring (as Woody had wanted me to do) AND left the rubber plug, put the diaphram back in without the spring. WOW!! The sweet sound of popping on decel was back and so was the adjustability of the pilot screw, plus it slams back to idle quickly after a rev, no hanging at all (well some, until I tweeked it richer smile.gif )

This valve is only on the WR carb and is there obviously to eliminate popping on decel (or to make jetting a f-ing nightmare.) James said he did not have a problem with idle & adjustments on his, so perhaps this is bike to bike. The fact remains that this is NOT on the YZ carb. And it's such an easy mod it's worth a try. I am going to take the diaphram completely out and make another rubber plug and just cut it all off completely. There is another small hole next to the diaphram that is still open as it is, not sure where this goes, but it's worth some more experiments.

Woody...YOU ROCK! (Jetting) life is good!

Air Cut Valve- (ACV) is the name for it in the manual.

Thanks for posting the results and sharing a tip that may help LOTS of '00, '01, and '02 WR owners.

Again, this is the small cover on the left side of the carb with a diaphram under it. You could describe it as "ACV blocked off" or "ACV off".


[This message has been edited by James Dean (edited October 21, 2001).]

Scott in KC,

Glad you tried this mod. I found new info with diagram of how this valve works. My brother has a XR and purchased a Honda common service manual.(not to be confused with year/model specific manual) This manual lists different types of carbs and goes step by step of how each circuit works. According to the book when you whack the throttle shut you get a temp lean condition. This is opposite of what I thought, but whatever. During normal operation the area under the diaphragm acts as an air bypass. The pintle on the diaphram pushes the port open due to the spring pressure on the diaphram. It draws air from the port just to the left of the pintle through the check valve port that the pintle is holding open.(basically a vaccum leak). Now on the other side of the diaphram vaccum is not able to overcome the spring until you close the throttle. When you close the throttle engine vac goes up and overcomes the spring.This in turn pulls the pintle out of the check valve allowing it to close and shutting off this controlled vaccum leak and makes it richer.


Anyway... If you just block the port where the o-ring is and leave the spring, the controlled vaccum leak (if you will) under the diaphram is always occuring. This is why you noticed little if any diff.

But if you take the spring out and leave vaccum applied to top of diaphram (where the o-ring is), it will pull the diaphram pintle all the way out of the check valve, and shut off the controlled vaccum leak on the bottom if it.

To any one who has not pulled one of these apart and looked...this may make no forking sense.

More later..gotta go


[This message has been edited by Woody426 (edited October 21, 2001).]

okay, so when the spring is removed, the bypass is always closed and will make the jetting more consistant. Why can't you block the hole where the o-ring is and remove the whole diaphram and spring? this way there is no pintle to operate the check valve. Is this wrong? i just wanted to do it this way so there is no way the check valve can be operated.

can you just remove the diaphram and block the o-ring hole?


I think that should work fine, the only reason I put the diaphram back in was to get the seal around it. With the spring removed & the vacuum (o-ring hole) sealed I don't think there is anything to open that valve. My plan is to take it all out of there and just make a larger rubber "plate" or plug to fill the void and keep it air tight. There is also another small hole open on the diaphram side, just to the right of it. Probably not doing much if the valve is closed, but for good measure it would be nice to have the whole thing out.

Yes you can do that with the same results. The reson I left the diaphram is that it is the seal for the cover. You could find an o-ring the size of the diaphram to seal it, or you could just snip the pintle off.

With the spring removed, when the bike starts the vaccum will immediatly pull the diaphram back and should give you no problem at all as long as the engine is running.

Doc, let me know how this works for you.


Doc, any luck yet?

Woody, I guess I'm a little stumped on why just plugging the small o-ring hole didn't do the trick? But, yet there seemed a significant difference when the spring was removed and the valve shut. This has me wondering how that valve could be affecting performance at all if the passage between the slide & engine (small hole) is totally shut off. Is the valve in anyway connected to the pilot jet???

Good day gents. I removed the diaphram, spring and o-ring seal. I plugged the hole where the o-ring was. I then applied blue forma gasket to the outside lip of the cover to seal the system (used blue to make sure it would be colour co-ordinated. very important!)

Anyway, the bike ran fine after i went to a 42 pilot. There wasn't any difference between this method and just removing the spring (vacume holding pintle out of valve).

Took it for a quick ride through the local pine forest. Was riding down a rough track like i have done before, and then suddenly i was on my face. A deep hole hidden in some grass caught my front wheel and down i went. No damage to me, just a few dents in the bike. no worries! rode for another 5 minutes until i felt a tingle in my right knee. It progreessed to a pain so i thought i better go back to the car. Once there, i removed my dacks to find my knee had exploded to twice the size! on closer inspection, it appeared to be filled with fluid. If you pushed it with your finger, it giggled like Homer Simpsons belly. So i went to see my friend who is a nurse and she took a look. She says it was probaly filled with blood and wasn't a real concern for now, and i should see my GP the next day (it was 7:30pm at this stage).

So i wake up this morning to find it is even bigger and is turning black. About to see my GP in a few hours, so i hope it can be fixed without too much pain. My nurse friend says they drain the blood out with a needle, but now its really painful whereas last night it didn't hurt that much.

Judging by the bruises on the inside of my leg, i think the bike dropped onto it, but it happed so quick, im not sure. Three weeks earlier i flipped the thing at 80km/h. A wrecked pipe and many cuts almost had me selling the bike. But now, im out.

Sorry to hear about your misfortune DOC...hope everything goes well with your doctors visit and wishing you a speedy recovery. Been down that road before only it was a stump instead of a hole. Torn ACL, MCL and meniscus faster than the proper length of an accelerator pump squirt(.3-.5 secs.).

Heal quickly.

Hey guys,

Doc sorry to hear about your spill. I hope you heal quickly and get back to riding. I have only recently picked back up on riding. March 2000 to be exact. I went out and bought a new XR600 only to find that the beast would literally kick my a$$ while trying to keep up with my buddies 98 KTM 200. So I decided to sell it and pickup a new 426 this last May. What a bike! Anyway the last thing I owned before the XR was back in 92 and it was a stock 81 CR250. blush.gif ...and I thought it was fast. That was the last time I piled it up. I have not wrecked since I have taken it back up. Must not be riding fast enough... but I know it's coming.


Looks like Larry has answered your question about how the ACV works on the WR/YZ 250 thread. I still have some final cleanup on jetting/needle to do. I am going to dive into the 40 pj club now that I have pulled the ACV spring and its running richer. I am playing with an EVQ needle now. I am looking forward to trying Taffy's lean pj/rich needle (E-N) approach. I have not had much time to play lately. I am going down to the Yucatan Pen. of Mexico next week so it will be awhile until I get to play with the bike again.

More later.......Woody

Woody, I was trippin' (once again) on the valve and the function. Got it right in my head now. smile.gif Yes, leaner is better with this mod! I'm back on a 42/75 pilot, ELN #3 and temps are even cool, so this is indeed leaner. For some reason, I think the YZ pipe I added wants it leaner again than the WR pipe. At any rate the bike is just freaking dynamite...literally. Gotta love those 3rd gear power wheelies while sitting on the tank!! smile.gif Hopefully get to ride a YZ or two this weekend to see how it stacks up.

One downside to this mod, my cold starts are weird. It only wants to start WITHOUT the choke, on 3rd kick...once it starts it will take the choke but not for long. Hell, I could STILL be a little rich. I did reach down and pull the hotstart out a little while running at like 1/3 throttle briefly and it got even cleaner sounding. May pop that 38/75 combo for $#its & grins and just see....

"Yes, leaner is better with this mod! I'm back on a 42/75 pilot, ELN #3 and temps are even cool, so this is indeed leaner"

Not sure what you mean, but generally lean jetting causes higher engine temps.

I think he means ambient air temp. The lower the temp, the more oxygen is in a given volume of air. It is condensed.


Yes, cooler air temps.

soory, the drugs i was given are too strong.


Anymore testing with the ACV disabled??

I'm still rich with 42/75 pilot & PAJ. The weather was warmer yesterday and actually had to use the hotstart to get it started COLD! And I'm having to use the hotstart when riding probably more than normal. I'm going back to the 100 PAJ to test.

has anyone tried this on the 2005 model? I removed the pin,spring,daiphram and used and used an inner tube for a gasket. Along with cutting the airbox fins out. what an improvement. but looking back thru these archives. I think Yamaha closed the hole in the ACV cover in 05. Will the mod still increase my performance? And shoould I still try the 100 PAJ? Thanks for the input.


OK Guys,

I have read this whole thread and am unclear on one thing. Do you guys think this would be a worth while mod on the 02 426? I had discussed this with James Dean once before and we decided to leave it alone. However you all seem very pleased with it. I too notice very little change with the fuel screw, and am currently running a 168 main, 42 pilot, JD blue needle 3rd position, and the fuel screw is at 1 1/2 I think. So, should I try the ACV mod? And if I am understanding from earlier, I can remove the whole diaphragm, o-ring, and spring and replace with a piece of inner tube for a gasket? Thanks,


I have an 02 426 and I just flipped the diaphram over. it seems to work great for me....no more hanging idle and my jetting is now close to being perfect. I left the spring in and just flipped the diaphram so the little do-dad on the daiphram doesn't engage the other little do-dad on the carb. Works for me and sorry for the do-dad language.....I couldn't remember the names here at work. :D

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now