Air Cut Valve 2002 bike

[S_Phillips]

All - HELP !!

Something is giving me fits in the pilot circuit

area ?

I am suspecting that darn Air Cut Valve.

I want to completely disable the thing.

I have done plenty of searches for this but only come up with mods for the older carbs.

What the @#$@# did Yamaha put that on there for anyway. I'd rather hear my bike backfiring so I'll know the carb is outta whack!!

Thanks

[MN_Kevin]

try a search on the yzf forums, both 426 AND 250.

[ June 01, 2002: Message edited by: NH Kevin ]

[S_Phillips]

I did a search on all forums.

nothing but mods for the old carbs?

Surely someone with an 02 bike has done this? ?

[CHris_PArks]

what is the air cut valve. is that the thing on the left side that the yz's don't have. and what does it do.

[CHUCKY]

Attached is the info on the aircut mod I received.

While going through the jetting I was really having some problems with the idle and picking the right pilot jet & air jet. Me and another guy on the board made a startling discovery. Our WR's came from the factory with an "air-cut" valve on the left side of the carb. Two small allen screws and a small plate, take them off and you'll see a spring & diaphram in there. This circuit is not found on the YZ, the mold is there in the carb body, but it's not milled. This valve is constantly open, creating a little air leak, whick leans the pilot mixture slightly and requires richer jetting on the pilot...thus the 75 pilot air jet VS. the 100 in the YZ. On hard deceleration, the valve is sucked shut via manifold pressure drop and causes a rich condition, which eliminates the "pop" on decel that you get sometimes. It also makes jetting a nightmare!! I removed the took the diaphram out and snipped off the little stem that holds the valve open, then plugged off the little pin hole (on the left of the assembly), put the spring and diaphram back in. Now it's permanently shut, like the YZ. This will require you to jump to the 100 pilot air jet! My bike runs very crisp and just lights up like a top fuel dragster when whacked. YZ guys have ridden my bike and are stunned by how much better it delivers power. I'll admit, I got lucky. But a few others are running the same setup with similar results.

There has been a few different ways of achieving the above. Some plug the hole (which is the hole that has the round ring seal) by taking out the seal and putting a seal without a hole then just remove the diaphragm and spring, put the outer cover back on and seal it. The later means nothing is altered, just removed. As to the pilot circuit alterations, thats a well talked about subject. Hope this helps.

Chucky.

[S_Phillips]

Chucky-

Thanks for the reply. Nice to know someone else has had the same experience.

Yesterday I did plug the little pin hole off with some teflon. But I left the diaphragm alone.

This did show quite a bit of improvement as for as getting consistant readings. Where as before

I would get readings all over the place, one ride would seem rich the next would seem lean.

It is running rich now on the PJ circuit but at least it's a consistant reading and I will deal

with it.

Debating wether I should deal with the diaphragm.

I was thinking of turning it upside down where the little pin would be pointing inside the spring. Hate the thought of clipping that pin off. This way if I had to go back to stock I wouldn't need to go buy a new one.

BTW-

This will require you to jump to the 100 pilot air jet!

Wouldn't increasing the PAJ make it run even richer? My thoughts being that the more air flowing over the PJ would mean even more fuel being pulled out of the PJ.

someone please correct me if i'm wrong on that.....

Thanks

Shawn

[MN_Kevin]

not pilot jet, but pilot AIR jet. Allows more air flow > leaner.

The pilot air jet is UNDER the air horn on the air filter side of your carb.

[dominator426]

I was successful at sealing off the air cut valve by simply reversing the diaphragm! The pin is now on same side as spring...I only had to lean my pilot screw from 1 3/16 to 1 1/16. There doesn't seem to be a need for a #100 pilot air jet as it used to be too lean with valve open causing an erratic idle and poor off-idle driveability, especially during warm up.

The idle is now actually ajustable and stable...and low speed throttle response is perfect and smooth!! ?

[ June 03, 2002: Message edited by: dominator426 ]

[ June 03, 2002: Message edited by: dominator426 ]

[Hick]

Originally posted by S Phillips:

Wouldn't increasing the PAJ make it run even richer? My thoughts being that the more air flowing over the PJ would mean even more fuel being pulled out of the PJ.

someone please correct me if i'm wrong on that.....

Thanks

Shawn

Opinions vary on this question...

But in my experience, all else remaining equal, bigger on the PAJ is leaner, smaller is richer, especially at higher RPMs.

Hope this helps.

[S_Phillips]

I dunno guys I have found some sanity with the following. Yes I put this another post somewhere to.

I highly recommend plugging off the vacuum hole that activates the ACV.....this leaves the white spring loaded valve inside the carb open at all times.

This has been the only way I've been able to get consistant reading at the bottom....

Some people have plugged the vacuum hole off and snipped the tip of the plunger off. I tried just turning the diaphragm around so it wouldn't push the little white valve inside open. But with my bike if the white valve stayed shut there was still no consistant readings........

terrible design..........

HICK-

Thanks for you input.....I guess there's the Hick theory and then theres the Taffy theory. I remember reading somewhere in I think one of his post. You can only pull so much fuel up through the PJ so anything else bigger than needed to get max fuel capability, would indeed make it leaner.

I think this makes you both right ? ?

[Hick]

Originally posted by S Phillips:

I think this makes you both right ? ?

Whatever works for you bud...

[dominator426]

I richened my pilot screw a bit to 1 1/8 to correct a slight 1/8 throttle cruising poping...It runs well but the idle sounds like it's rich and much less choke is needed to start and warm up now that air cut valve is sealed. I wonder if switching to a #100 PAJ would affect the idle mixture and off-idle transition more than it would the 1/8 to 1/4 throttle cruising...

By the way, have many of you removed the screen on the air filter frame? It increases airflow a lot...I had to switch to a 1 step richer needle (DQQ) and #172 MFJ on mine with lid removed...DQQ needle is the canadian version of the USA DRQ needle. Does anyone know what the difference is?

[S_Phillips]

I wonder if switching to a #100 PAJ would affect the idle mixture and off-idle transition more than it would the 1/8 to 1/4 throttle cruising

I tried the #100 PAJ but Idle on "my bike" was out of control.

If you try that lets us know how you make out?

BTW - The little plastice screen in the air filter is supposed to help keep your oil saturated filter from catching on fire if you engine ever back fires through the carb. Which mine has done a few times during my jetting party ? When I know the carb maybe way off kilter, I keep the bolts out of my seat and a fire extinguisher near by just in case.

[YAMAKAZE]

Read and Learn...Oh jetting Students....

https://www.thumpertalk.com/bike/ubb/ultimatebb.php?ubb=get_topic&f=3&t=008362

Bonzai ?

[Hick]

Originally posted by dominator426:

I wonder if switching to a #100 PAJ would affect the idle mixture and off-idle transition more than it would the 1/8 to 1/4 throttle cruising...

DQQ needle is the canadian version of the USA DRQ needle. Does anyone know what the difference is?

So, was your stock needle a DQR??

The middle letter is the L1 spec., which changes the relative clip position. Things get leaner as you go down the alphabet (A to Z), so DQQ would be a half clip position richer vs. DRQ.

IMO the best way to fine tune your trailing/neutral throttle response, provided you’ve settled on the correct pilot and idle mix, is with the PAJ. So perhaps when you went 1/8 richer on the idle mix you were just band-aiding a problem the PAJ circuit...

So, to answer the question I don’t think small changes in the PAJ make a noticeable difference at lower rpms, where you will feel it is at higher rpms, like where you are getting the surging and popping. The latter (and possibly the former) sounds like a lean symptom, in which case a 100 PAJ would make it worse. What size pilot (fuel) jet are you running? I’ve also seen a small exhaust leak make a KTM (w/ FCR) behave exactly like it was too lean on the PAJ.

Hope this helps.

[dominator426]

My PAJ has not been changed and I think the stock is #75...It is running very well now and I like the way it readily idles when cold. Maybe I'll just get used to the slightly "POOF,POOF" idling sound. I do want to try my #38 PFJ though with this setup and will let you know what happens...Stock needle was indeed a DQR...Thanks! ?

[ June 05, 2002: Message edited by: dominator426 ]

[S_Phillips]

Yamikaze-

Read and Learn...Oh jetting Students....

Yes there is a lot of good info there and I have learned a lot (Thanks Taffy, JD, and other contributors) but I still think there is a big difference between the 2002 carb and the earlier ones........Just my .02 cents worth

Though my PJ/PAJ, needle, and MAJ are same as Taffy's the MJ is 160 cause of stock open pipe.

I can't get it to run properly w/o disabling the ACV and also giving it more APJ it just won't do it. I know Taffy's cringing on the APJ but hey

the bike runs "like snot" as he put's it ?

Starts on about the 3rd kick at 90 degree days(no choke)and has been starting 1st kick everytime while hot(no hot start). That I really like a lot. Nothing more embarrassing than to be out on the track and stop to chat with friends they leave and you sitting there kicking your buns off

[dominator426]

Success, my friends!!

Switching to #38 PFJ at 1.25 turn, from #42 fixed my rich idle, low speed, and overly rich choke...While I was there, I timed my acc pump at 1.25 turn (from touch point) with 0.4 sec duration.

Great response improvement from idle and up! Very crisp and smooth and more tractable! ? Easy starts at any temp...Fuel mileage should be back up to normal too.

[S_Phillips]

Dominator!!

What exhaust are you running??

[dominator426]

I have the stock Canadian exhaust...