Author Topic: Pilot jets  (Read 912 times)

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Offline jelbon

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Pilot jets
« on: November 10, 2019, 11:48:41 pm »
In the process of resurrecting a crashed 2002 c10.
Stripped the carbs down to clean and check them over. One slider was slightly sticky.
I had to drill out the pilot plugs to get at the pilot jets, which I thought was only a California or European thing (Canadian bike).
But what I found interesting was the amount of turns to zero the jets. 1/8, 3/4, 1/8 and 1 1/4.
Why the vast difference? 
Had lots of carbs apart previously and found that they were usually close.

Offline GeorgeRYoung

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Re: Pilot jets
« Reply #1 on: November 11, 2019, 01:17:52 am »
Mine were all over the map, too.
Factory settings 0, 7/8, 1 1/3, and 1/4 turns CCW from lightly seated.

I set them all to 1.5 turns out, worked fine.

Offline connie_rider

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Re: Pilot jets
« Reply #2 on: November 11, 2019, 01:51:07 am »
For clarity; You are  talking about the pilot screw, not the pilot jett.
When drilling thru the cap to reach it, many touch the screw with the drill bit, and turn it in...
Is there any chance you touched the pilot screw with the drill bit?

But I agree, stock; the adjustment's you find are all over the place.

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Offline Lee

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Re: Pilot jets
« Reply #3 on: November 11, 2019, 03:45:54 am »
Anyone know the factory criteria or procedure for setting on assembly line?

Offline GeorgeRYoung

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Re: Pilot jets
« Reply #4 on: November 11, 2019, 01:21:39 pm »
Anyone know the factory criteria or procedure for setting on assembly line?
Turn them in until the engine pops on deceleration. ;)

Offline cra-z1000

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Re: Pilot jets
« Reply #5 on: November 11, 2019, 02:02:57 pm »
I believe they are set up on a flow bench .
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Offline jelbon

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Re: Pilot jets
« Reply #6 on: November 11, 2019, 03:58:56 pm »
"Is there any chance you touched the pilot screw with the drill bit?"
No, I was very aware of that while drilling.
Interesting to see that others had the same experience.
Currently I have reset them to the same settings, left the plugs off and may adjust them if needed later.

Offline cra-z1000

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Re: Pilot jets
« Reply #7 on: November 11, 2019, 04:17:02 pm »
I recently went through mine and set them all to 1-3/4 out . Running better than ever .
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Offline Lee

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Re: Pilot jets
« Reply #8 on: November 12, 2019, 02:21:51 am »
If the factory used a flow bench would there really be such a wide range of turns between carbs? Curious.

Offline kkja13

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Re: Pilot jets
« Reply #9 on: November 12, 2019, 05:18:03 pm »
I've never had my carbs off.  Where is the plug?  Does anyone have a picture?
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Offline jelbon

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Re: Pilot jets
« Reply #10 on: November 13, 2019, 12:52:24 am »
They are under the engine side of the carbs, right in front of the float bowls.

Offline GeorgeRYoung

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Re: Pilot jets
« Reply #11 on: November 13, 2019, 01:38:18 am »

Offline cra-z1000

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Re: Pilot jets
« Reply #12 on: November 13, 2019, 01:51:46 am »
If the factory used a flow bench would there really be such a wide range of turns between carbs? Curious.


It does seem strange . I never knew how mine were set from factory . Steve did my first service . I have heard reports of them being set all over the place from folks here .
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Offline Lee

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Re: Pilot jets
« Reply #13 on: November 13, 2019, 10:52:56 pm »
Strange indeed!

Offline MAN OF BLUES

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Re: Pilot jets
« Reply #14 on: November 14, 2019, 01:21:24 am »
really not strange at all..... I don't think any of you have your finger on the pulse of the actual components,how they really work,  and how the bike was manufactured for that long of a time.

First off, the Pilot Screw, is not a FUEL screw needle, it controls airflow, which comes from the intake side of each carb,(where by the way, it is drawn thru a precise brass orifice, pressed into place, and upon inspection I have seen various hole diameters.. which may explain my last sentence in the diatribe I'm typing) to forward of the butterfly plate; this in turn, controls just how much fuel can be supplied, during starting, and low speed idle/up to 3k rpm, where the slide lifts.. and the secondary and main jets supply fuel thru very precise "drilled jets". Liquids, specifically fuel, are controlled pretty consistently by "drilled jets", as liquids behave differently than "air"..
This air "circuit" is prt of the casting of the carb bodies, and each carb has a different body by nature of how they fit "the rack".. now, withing each aluminum body, a series of torturous pathways are "cast" and some machined, carrying the air up to the pilot air screw "needle". Some have been drilled, and have pressed in BB plugs, sealing the opening where a dril was used to "connect" passages, to complete the circuit pathway... without having to say it, there are many areas that actually inhibit/change the "flow" of air, when vacuum is pulled from the rear to the front. Then add in they are all produced and tossed into a "bin" for assembly, be it finishing the machining, of final assembly. There is so much "deviation" that can occur in this vacuum/air path, that necessitates the need for that "pilot air needle screw", to pull fuel up, and thru the 4 small holes ahead of the throttle plate, consistently.
During the "actual carb buildup", each one is calibrated by drawing a specific vacuum, and that needle is adjusted, and capped off... then, each one is tossed, once again, into a bin, and assembled into a "rack" prior to the install.. each and very carb is different, thus the needs of the adjustment of each pilot screw will be different.
Now, couple this, with the fact that each cylinder, based on it's intake valves, pulls differently, as a result of flow path and adjustment, it becomes even more clear "why" you see the variations..

Mind you, when people like SISF and I do carbs, we try to "optimize" as well as we can, as we don't have flow benches setup to measure each carbs "flow"; but, as the carbs were built, and the needle was seated, when we re-set them, it is pretty consistant as any deformation on that needle's seat, during it's usage, and replacement, makes it the most viable means "we have" to reach some equilibrium with the system.

That's the short version that explains "why they differ" from the factory.. (but, it does not explain why you would find one, that came from factory, with the seal plug.. that had the pilot "bottomed out"..... that to me was an erroneous assembly, and there was another problem in the fuel pathways that allowed that bike to "run")...

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Offline Lee

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Re: Pilot jets
« Reply #15 on: November 16, 2019, 12:34:31 am »

MOB,
Your explanation of needle controlling air to pull fuel through the holes post the butterfly is good as gets perhaps. That on why Kawasaki with the ability to measure such might compensate for parts stack variations on assembly line. This wide range of variance in turns from factory is about emissions and not performance as well would seem unsaid.

That is not the strange part. The fact we do not set same parts { same needles in same carb body } as they were from factory is the strange fact. Most have a prefered number of turns for all carbs in a bank on a clean and rebuild. Some might use color tune or EGT to set spec to be more accurate.

The stranger still part is all methods work in some fashion.  My 87 C10 had all sorts of turn variance when I pried them from under the bungs. I did not drill all the way. Used an awl to finish peirce and pry.

The bike was not running when first had so can not speak of it's smoothness then. But on rebuild of carbs took great pains to set all the same in every measure. I was rewarded by extremely smooth idle down to 750 rpms though did not leave it set there of course. Perhaps just luck of the draw on a low miles bike that sat for 12 years with gas in tank and dried fuel in carbs.

Would luv to know if factory turns of pilots as spec would have been better. Next carb bank with bungs, I will rebuild with turns as found just to see. If that day ever comes to be.

Offline GeorgeRYoung

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Re: Pilot jets
« Reply #16 on: November 16, 2019, 02:14:09 pm »
. . . First off, the Pilot Screw, is not a FUEL screw needle, it controls airflow, . . .
I beg to differ. The pilot screw for the Concours is on the engine side of the butterfly valve. A mixture of air (from the air valve) and fuel (from the pilot jet) is delivered to the pilot screw, and thence to the engine with the quantity depending on the pilot screw setting.

There are carbs where the pilot screw control air, but not on the Concours.

Two cents worth from a guy who now has a fuel injected bike.

Offline jelbon

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Re: Pilot jets
« Reply #17 on: November 16, 2019, 07:01:42 pm »
Yup, It definitely adjusts fuel not air.

Offline Lee

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Re: Pilot jets
« Reply #18 on: November 16, 2019, 08:10:32 pm »
Conventional wisdom says engine side of throttle plate pulls fuel as I have always read. MOB if understood correctly offers a different slant on such for C10 in his words above. Myself do not have an opine past what is already said. It meters something relative to emissions that is why it is plugged with a bung. But would definitely surely like to know the real deal for the truth in understanding of it on a C10 if different than conventional thought on this subject.

Offline MAN OF BLUES

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Re: Pilot jets
« Reply #19 on: November 16, 2019, 09:45:53 pm »
Well, it seems that people are not getting what I described;
That actual needle, does in fact control the amount of air, which is drawn (in a passageway down inside the body of the carb, ) to "supply" fuel there, to the other 4 tiny drilled low speed "drilled" fuel orifice's in the forward throat.... that is why it is called the "Pilot Air Jet".... it's prime function is to provide a vacuum to pull fuel, to be expelled thru those 4 holes... the throttle "plate" when the bike is at idle, is actually closed to the point only 1/2 of the forward most orifice is exposed/open, and as the plate is further moved, by the throttle being turned, the other 3 holes become exposed one at a time, so yes, fuel is allowed to be "ejected" into the tract, as each is uncovered, but only as the edge of the plate exposes each individual hole.. (ie, fuel is not sucked from them, until each is exposed)  so that Pilot Air Screw Needle, is in fact just drawing fuel to the circuit, to be released as the plate exposes them....
and yes, I currently own a C14 fuel injected bike, but have done service on so many C10 model CV carbs, I think I know of what I speak. It has little to do with "emissions", but when mal adjusted does tend to over supply fuel...
What I mean by that is; the needle does not supply any real amount of
fuel, Idealy it doesn't need to, and if opened by say 4 turns, will in fact allow fuel to be sucked along that pointy needle, forward of the throttle plate... not what was intended, and it also make more fuel drawn up, and behind the throttle plate, where it can't be utilized correctly, as the plate is limiting the "dump". The opposite of that, is when the air screw is shut down too much, and cannot "pull" enough fuel into the same 4 hole circuit, to be atomized during the exposure to the opening throttle plate and subsequent increasing airflow.

Might want to ask SISF, as everyone tends to believe him, but I'm positive he will say the exact same "mode" of the function of that needle.

Also note, that most of us, SISF included, will actually replace that needle, the o-ring, washer, and spring, installed on it, and that in turn makes "settings" different from what were on the OEM installed one... That said, we both use the 2 turns out from seated, as a start point, and "if" needed we adjust in very small increments (hardly 1/4 turn either way), to suit... but 9 out of 10 times, that 2 turn adjustment seems fine.... when using the K&N needle replacements, I actually start at 2-1/2 turns out, as thee taper on those needles are different from the OEM ones...
« Last Edit: November 16, 2019, 10:01:54 pm by MAN OF BLUES »

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Offline cra-z1000

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Re: Pilot jets
« Reply #20 on: November 16, 2019, 10:54:33 pm »
Another reason I don't mind paying my dues here . Every time I open this forum I gain more knowledge  :beerchug:
1987 C 10
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73 Suzuki 550 triple
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