Bonneville America Forums Forums Technical Forums General Tech Idle Mixture # of turns out.

When adjusting the air mixture screws, I seem to remember seeing quite a bit of talk about starting at 2 1/2 turns out and also, it seems that alot of people are at 3 to 3 1/2 turns out.
I was just reading the tech vaults link to carb tuning ( http://justkdx.dirtrider.net/carbtuning.html ).
It says "The normal operating range is between 1 and 1.5 turns out so if you find the ideal setting is less that 0.75 turns out consider installing the next richer pilot jet (larger number ). If you find the ideal setting is more than 2 turns out consider installing the next leaner pilot jet ( smaller number )."

Which is more factual?
I just want to clear up the confusion.

I was always under the impression that backing the Idle Mixture screws "out" will richen the mixture.
Our bikes are set up very lean from the factory and removal of the AI makes this richening necessary.
The general concensus is if you have to back them out more than 4-4 1/2 turns, go up to 45's and start over.

See... according to the carb tuning in the tech vault (this is a direct quote) "The air screw controls the flow of air into the circuit. Turning the air screw clockwise reduces the air flow and richens the circuit. Turning it counter clockwise increases the airflow and leans the circuit."
If that is the case then "out" will lean your mixture.
Where's the big gunns?
Am I just in the weeds right now?

Turning it out definatly leans it out. Just remove the screw and you can see it's a tapered needle, so it's obvious. Anything more than about 4 turns and you need to go to a bigger pilot. But the only question i have is why is it that most people, even those with freaks or max airbox mods find the stock pilots as rich as they need to go when the mains and needles always need to go higher with such mods? Seems odd, and even i tried but found the stockers were plently rich enough with my uni, snorkel removal and drilled bottom. Mains are at 130 and TBS needles with one shim, yet stock pilots are rich enough.........wierd.

Too lean causes popping on decel, backing screws out diminishes decel pop, right? Do you also get decel pop if too rich?

Lonzo. No

Seems backward. Turning out would bring the needle farther out and allow more fuel to flow. Least I think so.

Thats what I thought.
Turning out decreased the popping on mine.

Turning out decreased my popping too.

Are the stk pilots on our bikes 42s?

Yes, the stocks are 42.

seems backwards all these statements.
If Im running rich, do I go to 45s or down to 40s

Well this all seems strange. Always have heard that decel popping is called by a too lean condition. Recommened cure for this is to turn out on the pilot screw. Therefore according to this you would be leaning it out more? Dosen't make sense to me.

Doesn't make sense to me, either. Backed out screws til popping on decel almost dissappeared. When I had 45 pilots, only had screws turned out 1 1/2 or so. With 42s back in, screws are about 3 1/2 out. I'm already confused about other things, going to quit thinking about this.

Wheres Pat when you need him?! I always thought it was an air screw but at this point i dunno what to think. I'll tell you one thing......i never notice much or any difference between 2 turns a 4. I can't even use that method of turning one till i get the fastest idle then the other one because it doesn't change till i'm at almost no turns. Oh well, it's always run good no matter where i set it so WTF.

45's will make it richer.
As for the screw (in or out)....??? I'm confused by the tech vaults carb tuning thread...

The pilot circuit is essentially a venturi.

When you either shrink a venturi or pass more air through it, the flow increases at the stricture. Therefore, there is less pressure on the top of the pilot jet. By backing out the air mixture screws, you are injecting more air, more fuel gets sucked up the pilot jet, and it richens the circuit.

Thank you phil for clearing that up.

What Phil is saying makes the most sense. It is however the exact opposite of what the thread "carb tuning" in the tech vault is saying. There it clearly says, and I Quote, "The air screw controls the flow of air into the circuit. Turning the air screw clockwise reduces the air flow and richens the circuit. Turning it counter clockwise increases the airflow and leans the circuit."
The thread in the tech vault may be the culpret here and if so, it may need a swift kick out.

Well I guess it ain't cleared up then. I see what you're saying Reido. This would be a good time for lecture on carb theory pretaining to cvk carbs. If someone with ultimate knowledge would kindly step in please.

I hate when I am wrong. I wanted to know why that article would say what it did so I looked here and there for information on the CVK pilot circuit and then went back to that article. They are absolutely correct.

First, Id' like to give some info on the venturi effect for my own benefit as well as yours. Check out this graphic. I know you all recognize it (except for the yoots! ):



High pressure from the ram exits the tip at a lower pressure but with a higher velocity (the venturi effect which is an aspect of Bernoulli's principle) which sucks whatever fluid is in the can up and sprays it out. What if we introduced a metered air control into the can and opened it up (counter-clockwise)? Less fluid would be sprayed! Lean! Closing it (clockwise), would richen the spray!! Less air, more spray! Like I said, I hate to be wrong...

Anyhoo, I broke the animated graphic out into its two pics as I wanted to look at the idle side. Here's that graphic:



First, I was mistaken about where the venturi effect is taking place. It actually occurs in the throat of the carb itself and the outlet for the pilot jet's gas and air screw's air (right below the 'P' in Low Pressure).

Check out the green (air) and purple (gas) bubbles (sheesh, how corny! ). Air flows (red arrow) into the carb passing over that outlet. If you screw the air screw in (clockwise), it will richen the circuit because more gas will be sucked by the pressure differential between the tube and venturi instead of air. If you screw the air screw out (counter-clockwise), it will lean the circuit because more air will be sucked by the pressure differential between the tube and venturi instead of gas.

So how does this affect popping? My theory is that if you richen the circuit (screw the air screw in, clockwise), you're introducing more fuel which produces popping. If you lean the circuit (screw the air screw out, counter-clockwise), you have an opposite effect.

Have some coffee and discuss!

This is correct if the air mixture screws are on the air filter side which ours ARE NOT. Dang it!

I hate to raise debates when I'm not sure myself.
A+ Phil!!!!
I am now having "the moment of clarity". Ahhhhhhhhhhhhhh.

It wasn't untill I decided to do some carb homework that I ran across this, but it seems to be solved.

Quote:

---

So how does this affect popping?

---

I think Greybeard hit on this once (ok I'm sure more than once ) It had something to do with being lean, then instantly after letting off the throttle getting rich, then the excess fuel burns in the pipes. (isn't that what you just said Phil?) Hopefully we could get Greybeards take just 1 more time?

This brings up a question. I'm not suggesting anyone is right or wrong. But if screwing them IN makes it richer, why does thunderbikes tell you you don't need to rejet with thier pipes, only to turn the screws OUT to 4 turns? Wouldn't that be the opposite? I mean, they're saying to turn them out more to richen it instead of upping the jets. So if turning them out makes it lean then someone is wrong, and i'm certainly not suggesting Tbikes could'nt be wrong. But now i'm even more puzzled.

Do they have that statement about the mixture screws on their website?

I'm not sure, but it's something i recall a number of people mentioning. And i think it mentions it in the paper that came with them, tho it's been a year so i can't say for sure. But i know they've said this one way or another.

Not that I don't believe you but that's kinda vague at best. I'd like confirmation from other Tbike owners.

However, I'll still stand by my corrected version. Screwing them out will lean the idle circuit and not richen.

Post the question. There are plenty of owners here and at RAT. But i assure you that Tbikes says this. Or better yet email them. I'm not doubting your theory, i'm just saying it's odd and i wonder what the deal is. They may well be clueless. Wouldn't be the 1st time i saw so called experts say something thats totally wrong.

So, instead of our bikes being set up lean from the factory, its just the opposite? Our bikes come extremly rich from the factory there for we get decel popping with more open pipes? And to cure this we screw the pilot screw out to lean it more?

Where's Pat? He must be out riding. Or brewing up some good stuff.

Thinking more about this and I have a simple visual that proves out my latter description.

What would happen if you screw the air mixture screw all the way in? Yep, that's right boys and girls. The only thing going in now would be gas! In essence, by unscrewing the air mixture screw (from bottomed out), you are leaning the circuit. If you are sitting at 2 turns out and unscrew it more, you are further leaning the circuit!.

Think about it.....

So we've been led astray into the fields of battle, only to now find out that the weapons of mass destruction we thought we possessed were just a figment of our imaginations.

Great debate? You flatter us but naw, this is not great debate. Great debates are:

which color is best
which are the best tires for our bikes
AND of course
What is the best oil!!!



Personally, I welcome all comments and if I am wrong, I will be the first to admit it. That's called the mark of a mature man (and I don't mean old! )

Rob if it such a great color why is yours the only one?
Obviously the best color is a blue speedy covered in grime for that fresh from the road look.

Rob if it such a great color why is yours the only one?
Obviously the best color is a blue speedy covered in grime for that fresh from the road look.