Air mixture screw

[PJultra]

98 Ultra, CV carb. I have a CV Performance air mixture screw on my carb. 175/45 jets. I really don't get any idle (900 rpm) reaction when I turn the screw in or out.
It did the same with my other carb on this bike. I can turn it in all the way (gently) and the motor keeps idling.
I have sprayed carb cleaner around the intake manifold with no reaction. so I don't have an intake leak.
Any ideas on what else could cause this???

[WML57]

Not missing the o-ring and washer are you.

[thumper 823]

Prolly been apart and the little washer/ "O" ring things are missing -real common

[PJultra]

Not missing the o-ring and washer are you.

I need to double check... I was just looking at the "packing kit" at the  CV performance sight and started wondering the same thing.

[PJultra]

I'm wondering if maybe there wasn't already an O ring stuffed up inside, meaning I have 2 O rings in there.
I don't know if I can get the O ring out with the carb on the bike.
I'll look later.
But the spring, washer, and O ring are all there and in the proper order.

[thumper 823]

Then you prolly have a manifold air leak.

[JW113]

Need to know more about the bike. Is it bone stock, highly modified, something in between?

As you describe it (turn idle screw all the way in, no effect), it sounds like the throttle plate is opened past the transfer ports. If that is the case, it renders the idle jet and mix screw ineffective. At idle the motor is pulling fuel from the transfer ports.

-JW

[PJultra]

Need to know more about the bike. Is it bone stock, highly modified, something in between?

As you describe it (turn idle screw all the way in, no effect), it sounds like the throttle plate is opened past the transfer ports. If that is the case, it renders the idle jet and mix screw ineffective. At idle the motor is pulling fuel from the transfer ports.

-JW

Engine is stock other than a Ness air box, EV13 cam, and Rinehart 2into1.
You may have a point there. That would explain it.
Thanx for giving me something else to look at.

[JW113]

This is kind of a PITA, but pull the carb as is, look into the back side (throttle side). If you can see exposed transfer ports, there is your answer. Typically stock-ish motors don't need the throttle plate opened behind the transfer ports. Hot rod motors do, though. The solution is to drill an air bleed hole in the throttle plate to let air through but not expose the transfer ports.

-JW

[Racepres]

This is kind of a PITA, but pull the carb as is, look into the back side (throttle side). If you can see exposed transfer ports, there is your answer. Typically stock-ish motors don't need the throttle plate opened behind the transfer ports. Hot rod motors do, though. The solution is to drill an air bleed hole in the throttle plate to let air through but not expose the transfer ports.

-JW

This^^^^
Unless the idle mixture screw, was Gorilla-ed into the seat... crushing the seat and business end of the idle screw is Not difficult... and More Common than you might think

[Evo160K]

Might see if a 42 low speed jet makes a difference?

[Scotty]

I'm wondering if maybe there wasn't already an O ring stuffed up inside, meaning I have 2 O rings in there.
I don't know if I can get the O ring out with the carb on the bike.
I'll look later.
But the spring, washer, and O ring are all there and in the proper order.

I was working on a used carb last year and found 2 o-rings and 2 washers up in the carb. Of course it was of the bike but took me quite a while to fish them out as I didn't want to damage the threads.

[nmainehunter]

Bring it down to a slow idle about half way to where it normally runs or to a spot where it won't die. You will get a reaction from your jet if it's close. Start about 2 1/2 - 3 turns out and bring it down from there. You should get some action before you get to 1 turn out. Below that and you need to go tho a larger jet.

[Racepres]

Bring it down to a slow idle about half way to where it normally runs or to a spot where it won't die. You will get a reaction from your jet if it's close. Start about 2 1/2 - 3 turns out and bring it down from there. You should get some action before you get to 1 turn out. Below that and you need to go tho a larger jet.

Assuming the needle/seat were Not Damaged by too much pressure "seating" it!!!

[PJultra]

JW, this is my first bike with a CV carb, so forgive me for my ignorance, but......
I have another, broken, CV carb that I was looking at, and if I understood you correctly, if the butterfly is not closed all of the way, it is exposing those tiny holes at the bottom og the butterfly.
And the only way to close the butterfly all the way is with the idle adjustment screw.
But if I back off of the idle adjustment screw, the idle drops way down. Then should the air mixture screw bring it back up?
The instructions say to set idle at 900rpm with the idle adjustment screw first, then start turning the air mixture screw to find the smooth spot.
Am I doing something wrong? Or do I blow $600 0n a new S&S...lol

[thumper 823]

S&S? 

Anyway yes- back throttle speed off and keep adjusting the A/F mixture.

[PJultra]

S&S? 

Anyway yes- back throttle speed off and keep adjusting the A/F mixture.

Sorry guys, I had just gotten home fro work and didn't even realize there were more replies.
And the dumbass of the week award goes to.......

[PJultra]

This is kind of a PITA, but pull the carb as is, look into the back side (throttle side). If you can see exposed transfer ports, there is your answer. Typically stock-ish motors don't need the throttle plate opened behind the transfer ports. Hot rod motors do, though. The solution is to drill an air bleed hole in the throttle plate to let air through but not expose the transfer ports.

-JW

Can you please give me more detail about the air bleed hole?

[JW113]

Sure. There are four holes in which fuel can get into the motor.
1. Idle port
2. Transfer ports
3. Needle jet port
4. Enrichener circuit

The idle port supplies fuel when the throttle plate is essentially "closed", as in this condition there is not enough air moving through the carb to pull full from the other ports. The manifold vacuum is high on the back side of the closed throttle, and this is what is pulling the fuel from the port, not moving air and good old Bernoulli. The idle port is fed from the pilot or low speed jet circuit.

When you crack the throttle open from closed/idle position, there is still not enough air moving to pull much fuel from the needle jet port, and the enough would lean stumble. Therefore, they put a series of little holes just on the front side of the closed throttle plate. When the throttle plate moves over them, the high manifold vacuum on the back side begins to pull fuel from them, keeping the mixture correct. As the throttle plate is opened even further, air is now flowing over them and the moving air will pull fuel. These ports are also fed from the pilot jet circuit. Also note that at low throttle positions, these ports are the dominate fuel path and controlled by the pilot jet, while a lesser amount coming from the needle jet port.

Past about 1/3 or so throttle opening, the needle jet port, i.e. main circuit, is the dominate fuel path and is controlled by the jet needle circuit. The more the throttle is opened, the higher the slider raises and increased the fuel supply to the moving air. The taper jet needle is what controls how much fuel is pulled.

Near the last 1/4 of throttle position, the fuel is still coming from the needle jet circuit but the main jet is now what limits fuel flow. It's like the taper of the needle hits it's maximum flow by the restriction of main jet.

Back to your issue. At idle, the only source of fuel is supposed to be the idle jet and port. If fuel is getting in from anywhere else, you essentially lose control of it with the idle jet. It sounds like this is what is happening in your case. So where else can fuel come from?

1. The transfer ports, if the throttle plate is open too far and is exposing them to manifold vacuum.
2. The needle jet port, if the fuel is too high in the bowl, i.e. float level too high.
3. The enrichener circuit, if that valve on the end of the cable is not seating against it's seat all the way, or if the rubber gasket on it is worn out. This will also let air bypass the throttle plate, and will thus circumvent the idle jet control.

I'd suggest to pull the carb, take it all apart and give a good cleaning and inspection, then put back together. Pretty much all explained in TFM.

-JW

[cheech]

Can you please give me more detail about the air bleed hole?

JW is steering you right where you need to be.

The air bleed hole usually drilled in the butterfly is usually only needed on heavy modified engines.
There would be a hole drilled in the throttle plate.

Below shows the transfer ports.
This was off a maybe 500 mile bike so I'm sure the idle is spot on for it. As a generalization to show where the butterfly sets at idle.
Left pic: See the 2 black holes. One on left is idle mixture screw. Center is 1 of 4 transfer port.
Right Pic: Butterfly open to show the 4 total transfer ports.
[attach=0,msg1344587]    [attach=1,msg1344587]

[cheech]

Modded carb butterfly with bleed hole image from the WWW

[PJultra]

Thanx everyone for all of your knowledge, This is very educational.