air cut off valve

[bdtoady22]

what exactly does this do and what problem does it cause? when i am in the throttle no matter were and let off it has a flat spot getting back in to the throttle, if i try to clutch it and spinn the tires while moving it just cuts out please explain and any ideas would be help ful

[justin1022]

you ever work in the oil patch or pipe lines? my dad explaned this to me on back hoes and cats they must have a air shut of valvue because if some type of gas is leaking it wil get sucked through the carb and it is bassicly gas so the engien will rev right up til it blows even if the key is shut off. so the shut off cuts of the air so it cant run. i would imagine it did the same thing on a quad just stopeds it from running.

[bdtoady22]

it runs fine except for the hesitation, starts good revs good its all when you let off and jump back into the throttle it hesitates no matter at half full 3/4 it happens every where so i cant pin it down to a circut runnin a 152 main 38 pilot hmf slip on stock filter and air box air screw at stock setting needle at stock

[justin1022]

Originally posted by bdtoady22
it runs fine except for the hesitation, starts good revs good its all when you let off and jump back into the throttle it hesitates no matter at half full 3/4 it happens every where so i cant pin it down to a circut runnin a 152 main 38 pilot hmf slip on stock filter and air box air screw at stock setting needle at stock

k weight do u even have a air shut of valvue?

[bdtoady22]

its a 99 400 ex yes it does it is under the throttle cover it is where the vaccum lines go to its 50 bucks to change it so its not really that cheap to just try and see if it fixes it you know?

[justin1022]

instead of changing it why dont you jsut pulll it out. there isnt really a need for it anyway.

[bdtoady22]

from what ive read it is in there to stop the motor for backfiring under deceleration it is vaccum operated annd it tells how to check it so i guess i just need to take the carb off tomarrow and check it out, would too lean of a piolet cause this? lots of guys are runnin a 42 im still runnin the stock one?

[justin1022]

nope a pilot jet only goes up to i think like 1800 rpm. it wont make you back fire the reason why they put 42s in is because of cold starting it starts better and alot of people have the choke removed for better air flow so it helps out. yours is a 38 i am pretty sure. the back fireing is caused from the main jet. and if it back fires from the air box it gots not enough fuel and if it comes from the pipe it has to much. if i was you i would take that air cut of out and jet it properly.

[bdtoady22]

i appreciate the help but i dont think you understand what i am trying to explain if you were here in zanesville oh i would let you ride it and you would see what i am talking about. it is jetted properly plugs look good starts great runs mid range great top end great it is just when you let off lets say you are climbing a hill and there is a little dip in it and you back out just a touch im not saying clear off the throttle just enough to keep the front end down and the power to the ground you then you hammer back into the throttle that transition point from letting off to back on is when it does it. and it does it anywhere in the throttle range . it does not back fire at all

[78ta]

brad, just thought of something else to check, check accel pump make sure it is spraying good. just a stab in the dark though

travo

[justin1022]

Originally posted by bdtoady22
i appreciate the help but i dont think you understand what i am trying to explain if you were here in zanesville oh i would let you ride it and you would see what i am talking about. it is jetted properly plugs look good starts great runs mid range great top end great it is just when you let off lets say you are climbing a hill and there is a little dip in it and you back out just a touch im not saying clear off the throttle just enough to keep the front end down and the power to the ground you then you hammer back into the throttle that transition point from letting off to back on is when it does it. and it does it anywhere in the throttle range . it does not back fire at all

but will it eventully climb? once you let of and crack it again. k so its jetted right everything is good . exept that air cut of you think its cutting of the air or not letting ot past fast enough? for when you crack it? well i would still remove it just because i dont like it but since you dont seemed to be wanting to do that maybe someome else can help you out. i havent eveer messed with them sorry.

[78ta]

just thought of something else, does it do it all the time or only once it gets warmed up?

If you can't find anything in the carb messed up, we'll swap coil and cdi from my bike

[justin1022]

Originally posted by 78ta
just thought of something else, does it do it all the time or only once it gets warmed up?

If you can't find anything in the carb messed up, we'll swap coil and cdi from my bike

thats actully a really smart idea swap cdis might be dirty.

[78ta]

thats actully a really smart idea swap cdis might be dirty

what do you mean by "dirty"


I'm sure mine is just as "dirty" as his


now if you ment corroded wiring harness connector then maybe, but "dirt" my friend, has nothing to do with it

[big95xj]

I have seen the plugs for the CDI get "dirty" well kind of...if you ride in alot of mud then sometimes mud/water an seep into them and do the same thing as corrosion...unplug it and see. It cant hurt...

[GPracer2500]

Originally posted by bdtoady22
what exactly does this do and what problem does it cause? when i am in the throttle no matter were and let off it has a flat spot getting back in to the throttle, if i try to clutch it and spinn the tires while moving it just cuts out please explain and any ideas would be help ful

The air cut-off valve senses the high manifold pressure of closed throttle deceleration and cuts off some of the air to the pilot circuit. This temporarily richens the pilot circuit for as long as the high manifold pressure exists. The purpose of doing this is to lessen or eliminate the closed throttle decel pops that would otherwise be present with a correctly jetted pilot circuit.

As far as I'm aware, the main thing a bad ACV will do is cause more backfiring under engine braking than would otherwise be there. Many ACV valves have replaceable diaphrams but I think the 400EX's is a sealed unit. Whatever the case, the ACV can be checked to be sure it's working.

You could run with it purposely disabled/removed but if that impacted your problem at all it would probably make it worse. And you'd start getting (or get more) exhaust backfiring on deceleration.


It sounds to me like the accelerator pump could stand to be adjusted. It's the AP's job to handle the transition period that occurs when quickly opening the throttle. Unfortunately the 400EX carb's AP isn't readily adjustable. You're pretty much stuck with its stock function (but make sure it IS actually working).

I'd guess a slightly lean pilot circuit is making it worse. Try richening the pilot circuit via the fuel screw if not actually trying a richer pilot jet. That may help. I think a slightly rich pilot is more likely to help than hurt your stumble. Pulling the needle up a notch might help too (if it's adjustable). Raising the idle speed a smidge can help when the problem mostly happens after grabbing a bunch of throttle from idle speed.


In my experience not being able to adjust the AP limits how close to "spot-on" the carb can ever get. Seems like some carb quirks never completely go away and the best you can do to "ride around the problem". If it's bad work to fix it (and in your case I'd start by richening your pilot circuit) but it may never totally go away without a lot of experimentation of your part.

$0.02

[78ta]

I have rode this bike, it does this stumble any and every time you back out of the throtle and open it back up. Idle to wide open anywhere in the rev range and does it hot or cold. Sometimes its bad and very noticable and sometimes it runs fine.

Would a sightly lean pilot cause a stumble even wide open and pretty high up in the revs?

I realize all the circuts of the carb overlap, but at WOT I would think the pilot would have little to do with this prob.

That being said, my very first suggestion to him was to try a #40 pilot , because when I first rode it it felt like to me that it was a light lean spot. But the more i think about and the more I talk to him the more I don't think it is a jetting issue. He said it starts fine cold without the choke and idle like a champ on cold starts also so I don't really think it is a lean pilot. But I have been wrong before!

Thanks for the great info GPracer2500, any other suggestions let us know.

[MtnEX]

Well, I'm surprised, so I'm going to post what I expected to read...

Try cleaning the carb... :D

You could try some Seafoam in the gas, or you could try removing the air filter and shooting some Seafoam deep-creep in there and in anywhere else it can be shot in... or you could just remove, disassemble and clean.

Now... the first thing I would personally check is the fuel line. Let someone grab a fist full of throttle while you observe it. It could be soft or slightly twisted.

For what it is worth, I have a similar problem to a much smaller degree. My line was twisted some. I ended up having to trim it some to make it shorter and make the routing straighter. It helped "some".

Mine studders just a tad right when I get into the throttle. Not bad, but I know it's there. It also sputters on the needle when held at steady throttle in neutral.

I'm running RICH.

I probably know less about these carbs than anyone here, but I don't think you have a lean condition.

Myself, I want to look at the adjustment in the throttle linkage under the cover.... you know, the gap between the link arm shaft & throttle link arm (0.004-0.012) to make sure the piston & butterfly are syncronized properly.

Then I want to look a all the throttle valve parts, the needle, and the accelerator pump diaphram & spring...

And also, something is eating away at me about that tube on top of the carb... the one that I think goes in the tank heat protector... have you had all that off to adjust the valves? I know I have....



Let me know if you find something... because I'm not ready to dive into mine yet.

[bdtoady22]

all right guys heres what i did and it helped tremendously first adjust the throttle linkage to.004 round about, didnt help, tried a 78ta's cdi didnt help and then i said heck with it i,ll try and lean out the needle bingo!! i went to the leanest setting and decided to go to a smaller main i am right now running a stock 148 with the needle in the stock set and runs great no hessitation etc, when i am in town again i will get a 150 main and play with it and see how that goes any i'll get back with you later im not really buying rejetting a carb for a slip on pipe. 78ta has two ex's one with a cobra slippon and stock jetting and it runs good so lets hear some opinions on that subject

[bdtoady22]

thank you gpracer2500 for the great info muck appreaciated

[78ta]

toad, might wanna start a new thread asking that. It would prob get way more responses

[MtnEX]

Hey guys... just digging this back up to shovel a little more...


When I had my carb off, I paid attention to the slide and butterfly syncronization... but I didn't exactly understand how to go about adjusting anything there. I don't understand?... Any insight would be helpful.


Additionally, I wanted to bring up one more thing...

How come we EX riders don't ever discuss the accellerator pump, or the accellerator pump timing?

[78ta]

adjust syncronization by opening or closing slot in the throttle link arm.....

the throttle link arm has the long slot in it, spread it apart with a screw driver, or squeeze it together with pliers....kinda crude, but that the way to adjust the clearance


the accel pump is not adjustable on these carbs, and as far as I know nobody makes an adjustable one for the stock 400ex carb either.

[MtnEX]

Originally posted by 78ta
adjust syncronization by opening or closing slot in the throttle link arm.....

the throttle link arm has the long slot in it, spread it apart with a screw driver, or squeeze it together with pliers....kinda crude, but that the way to adjust the clearance


the accel pump is not adjustable on these carbs, and as far as I know nobody makes an adjustable one for the stock 400ex carb either.

Thanks...

It seems my stuff is syncronized best I can tell by trying to do it myself. But I don't think I have any gap between the two parts.

What kind of condition would this cause (no gap)?