Spanky's jetting tip

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midlifemoto
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Spanky's jetting tip

Post by midlifemoto »

"If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet".

I thought it was the opposite? If the air screw is in too far it's too rich and out too far too lean....too lean means a bigger pilot, right?
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skipro3
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Post by skipro3 »

Let's think about this;
Turning the air screw out adds more air. The right spot of the air screw is to have the correct balance of fuel to air. Less than 1 turn out means that there is just a little air to balance what must be just a little fuel. More than 2.5 turns out means that there is a lot of air to balance a lot of fuel. So if you want your air screw at some fixed setting, say, 1.5 to 2 turns out, then you would need to add more fuel to the setting where the airscrew is less than 1 turn out and less fuel to the setting where the airscrew is more than 2.5 turns out. You add or take away fuel with the pilot jet. So, it looks like your statement makes sense to me. Of course, if I'm wrong, I'll find a way to blame someone else. ha!!!
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Post by bmcd308 »

The air screw is a big deal at idle primarily. While it does have an effect all the time, the only time I have ever really been able to tell a difference is at way low throttle settings. There are those who will disagree and claim the idle screw has a noticeable effect at 1/2 or event WOT, but I really think that what they are feeling is the bike's response from idle to 1/2 or idle to WOT, not really the effect of the air screw at 1/2 or WOT.

Anyway, to think about this we have to think about what is going on in the carb.

When you diddle with the air screw, you are looking for the place with the best mix of air and gas. Conventional wisdom is that you should find it at somewhere between 1.0 and 2.0 turns out. The air screw controls the amount of air, and the PJ controls the amount of gas.

If your air screw is less than 1 turn from closed when the mix is correct, it means you are letting in almost no air when the mix is right. So you must be getting almost no fuel. So you could add both air and fuel. So you need a bigger PJ, because when the bike is on the pilot it is starving for fuel.

If your air screw is way open, it means you are having to let in lots of air. So you must have lots of fuel. So you need a smaller PJ, since having to back out the air screw too much at idle will mean that it will run rich as your throttle position moves up from 0 and the air screw can't let in additional air.
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Post by fuzzy »

Sorry, but that's wrong.
I thought it was the opposite? If the air screw is in too far it's too rich and out too far too lean....too lean means a bigger pilot, right?
This is correct.

The screw only has so munch range....If you are overcompensating to the point of having the screw fall out or be shut, then you have to go in THAT direction to get it back into an adjustable range. Basically to elaborate on where I think mbc was thinking...The PJ flows X amount of gas. X is constant. You use the air screw to fine tune the amount of air (Y) to mix with X to provide the perfect Air/Fuel ratio (for the given pilot jet).

The equivelant on the main jet is the needle clip (fine tune). There is a little more tolerance here as generally the bike is already moving and less sensitive to minute A/F errors assuming the main jet is close or even rich. For off-idle, high load, the A/F needs to be dang near perfect for the throttle response we all generally look for. Once you have a motor that runs super-crisp off idle, you won't want to ride without having it.

MX guys could generally care less as long as it pulls through clean....Even then they can just use the clutch to slip through the bad pilot tune.
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midlifemoto
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Post by midlifemoto »

I have been conflicting opinions even from bike service shops....basically trying to get my pilot right for my first Vintage race next week...it's a 1979 Honda 125 elsinore...the main is spot on...OEM pj is 55, it has a 60 in it...the air screw at 2 1/2 turns out gives the crispest throttle response...So I was told that if I wanted to get the same results within the 1-2 turns out range I would need to get a 65 pj, which I had to order...now I'm confused if I shouldn't have gone the other way and put in a 55.....
I spent most of my money on women and beer, the rest I just wasted

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Post by scheckaet »

:rolleyes:
my head hurts, trying to figure that one out.
What about this "general" statement:
if you are spot on at say 50F and you ride at 30F (all other conditions beeing the same like altitude and humidity...), you'd need to turn the air screw in? the way I understand it is since it's colder the air is denser which mean you need less in your mix.
Am I right?
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Post by scheckaet »

"If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet".
this makes sense to me.
air screw turned in means less air in the mix, so concidering the fact that the ideal spot is between 1 and 2.5 turns, you need to increase your jet size
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Post by scheckaet »

>|<>QBB<
midlifemoto wrote:I have been conflicting opinions even from bike service shops....basically trying to get my pilot right for my first Vintage race next week...it's a 1979 Honda 125 elsinore...the main is spot on...OEM pj is 55, it has a 60 in it...the air screw at 2 1/2 turns out gives the crispest throttle response...So I was told that if I wanted to get the same results within the 1-2 turns out range I would need to get a 65 pj, which I had to order...now I'm confused if I shouldn't have gone the other way and put in a 55.....
I would try that 55 and see how it goes.
If you put a 65 you will need more air in the mix and if I understand the phyisics of the air screw, you'd need to turn it OUT.
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Post by midlifemoto »

My head hurts too...I called the guys at Sudco, they said go a size bigger....I'm guessing there is no flow chart..."if this, do this" to carb tuning...all the atmospheric, humidty elevation, ambient temperatures, etc. probably dictate what works and what doesn't..I'm just going to experiment and learn by trial and error...do appreciate the responses tho, thanks
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Post by Indawoods »

" If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet."
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Post by fuzzy »

:supz:
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scheckaet
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Post by scheckaet »

BTW, who the hell is SPANKY???
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Post by Jeb »

Spanky is this dud that as a kid in the 40's built squirrel-powered wooden go carts . . . I think.

One point of confusion on all of this may be the difference between a two stroke carb and a 4-stroke carb, i.e. advice used in the wrong context. Sizing the pilot with the "2 and 1/2 turns and your out" would be as Fuzzy and Indawoods describe. On a four stroke it's the opposite; that adjustment screw functions as a fuel screw and thus the logic of PJ changes is the opposite.

I don't understand carb design enough to know if this is coincidental or not, but my experiences are that an airscrew on a smoker is located closer to the carb inlet and the fuelscrew on a thumper is located closer to the carb outlet.

Maybe someone can explain this in better detail.

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Post by Mr. Wibbens »

Nice to be RB'd

If I'm 2.5 turns out I'm pretty close to where I want to be
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Post by midlifemoto »

well, I tried the 65, simply because I ordered it and didn't have a 55 on hand....it works better than the 60 and I'm around 2 turns out and it has a very crisp throttle response....the plug is a light chocolate color and all is well...does this validate that over 2.5 turns out means a bigger pj is recommended? I don't claim to know, but in my case it appears to have been the right way to go rather than the smaller pj.

sumpin' to be said about good ol' trial and error...

thanks for all the info...
I spent most of my money on women and beer, the rest I just wasted

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2007 Harley Softail Custom
2004 Harley Heritage Special (was owned by Evel Knievel)
2006 CRF100 (sons)
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Post by canyncarvr »

Is this all screwed (ha ha ha) up to anyone but me?
"If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet".
That is correct.

If the air screw is in too far it's too rich and out too far too lean..
That is correct, too...and the operative word IS 'TOO'.
..it has a 60 in it... the air screw at 2 1/2 turns out gives the crispest throttle response...So I was told that if I wanted to get the same results within the 1-2 turns out range I would need to get a 65 pj...
That is NOT correct!

It is A Question of Balance (good name for an album), as skipro said.


Now...are you sure the screw IS an air screw? Maybe it is a FUEL screw!!??

Where is it on the carbonator? Upstream or down? To the cylinder side or the intake side of the carb.

Generally air screws are UPstream (intake), fuel screws are DOWNstream (cylinder side).

**edit**

OK...so I just now read ALL of Jeb's post....sorry!! And I can't even claim to have been snookered by the page two thing this time!!

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Post by midlifemoto »

the screw is located right side nearest the airbox...i was told fuel screws are located toward reed cage
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2007 Harley Softail Custom
2004 Harley Heritage Special (was owned by Evel Knievel)
2006 CRF100 (sons)
2006 CRF 150 (girlfriends)
1978 RM250
1979 CR 125 Elsinore
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Post by midlifemoto »

From the Vintage Suzuki Carb Jetting 101 tutorial..the last sentence is what confuses me...how ya' supposed to know which way to go?

The pilot circuit has two adjustable parts, fig 2.
The pilot air screw and pilot jet. The air screw can be
located either near the back side of the carburetor or
near the front of the carburetor. If the screw is located
near the back, it regulates how much air enters the
circuit. If the screw is turned in, it reduces the amount
of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the
circuit which results in a lean mixture. If the screw is
located near the front, it regulated fuel. The mixture
will be leaner if it is screwed in and richer if screwed
out. If the air screw has to be turned more than 2 turns
out for best idling and performance, the next smaller or bigger size pilot jet will be needed.
I spent most of my money on women and beer, the rest I just wasted

2006 KDX 200
2007 Harley Softail Custom
2004 Harley Heritage Special (was owned by Evel Knievel)
2006 CRF100 (sons)
2006 CRF 150 (girlfriends)
1978 RM250
1979 CR 125 Elsinore
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Post by canyncarvr »

..the next smaller or bigger size pilot jet will be needed.
Now we get to the heart of the matter!

It's simple, really. You have to go either in or out, bigger or smaller, more or less, left or right, up or down!

It appears to be an non-judgmental, equal opportunity carburetor...it doesn't care what you do to it!!

OK...so much for hilarity....


1. That makes no sense at all and is plainly an error in the write-up.

2. While this is NOT what it says, the intent may have been this: Having referred to both air and fuel screws maybe they meant to say, 'If the screw has to be turned out more than two...the next smaller or bigger size pilot...'

Saying JUST 'screw' follows the earlier writing about BOTH air and fuel screws.

So...if air? The next smaller.

If fuel? The next larger.

Capiche?


Forget trying to understand what is likely a goober. You know how each (air/fuel) adjustment works.

If your situation improved with a larger pilot (screw being 2+ out) AND a then less than 2+ out adjustment, that indicates a fuel screw is what you've got. The aforementioned up/down stream positions are not always true.

All good?

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Post by midlifemoto »

clear as mud...I just tonight...had a similar bog on my 78 rm250 and went with the next size up on the pilot and had the same results as on my 79 elsi, much better throttle response and the pulg read is great...the pacific nw must have something to do with it, right? i'm in the columbia river border area of portland/vancouver area...all i know is both these VDR bikes run very crisp and clean after my trial and error method..i don't know the techno intracasies of all this....but...i feel feel i have a good chance of a holeshot this weekend at Woodland with the settings I have :-)
I spent most of my money on women and beer, the rest I just wasted

2006 KDX 200
2007 Harley Softail Custom
2004 Harley Heritage Special (was owned by Evel Knievel)
2006 CRF100 (sons)
2006 CRF 150 (girlfriends)
1978 RM250
1979 CR 125 Elsinore
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