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Trying to resolve pilot jet effect

  • Thread starter Thread starter Billy Ricks
  • Start date Start date
B

Billy Ricks

Guest
There seems to be some confusion regarding pilot jet sizing and its effect on mixture of the pilot circuit. Some here seem to feel a smaller pilot jet richens the pilot circuit. This is incorrect, just the opposite is true. Here is a link and an excerpt to prove this.
http://www.siue.edu/~rsutton/SR/mikuni.htm
"The pilot jet meters fuel to the engine all the time the engine is running and has less effect on mixture as the throttle opening increases, it's an cumulative thing. Since the pilot jet effects the entire range it should be adjusted first. The air adjust screw is a fine tune for the pilot jet by adding air to the mixture to get it just right. The pilot jet is adjusted by replacing it with a different size, larger number gives richer mixture. They are standard Mikuni pilot jets and are available at your local motorcycle shop."
And another link to a pdf file that states the same thing.
www.boyesen.com/filemanager/download/2/reedinstall.pdf

Most of the CV type carbs don't even have pilot air jets. Only VM and a few CV carbs have pilot air jets and are normally a screw type adjustment. RS flatslides and HSR series carbs do indeed have pilot air jets of fixed value and I will concede that smaller values do indeed richen the mixture.

Most of the BS series carbs do have main air jets, but they effect the overall mixture, not just the pilot system.
 
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Just a small issue Billy.
All my BS series CV carbs have a pilot air jet as well as a pilot jet; the pilot air jet is in the air horn at the rear of the carb body.
This is true of all the bikes I have, GSX250, GSX750SZ, Gsx1100SZ,and even the mighty Tractor.
Dink
 
Dink said:
Just a small issue Billy.
All my BS series CV carbs have a pilot air jet as well as a pilot jet; the pilot air jet is in the air horn at the rear of the carb body.
This is true of all the bikes I have, GSX250, GSX750SZ, Gsx1100SZ,and even the mighty Tractor.
Dink
Click to expand...

Yes, and additionally the BS series CV's have a pilot air screw.

While the slow speed circuit is the most complicated action of the carb, it's not unlike any of the other circuits. Bigger fuel port means more fuel and that makes a richer mixture.
 
There was just a little confusion between another regular and I. I was stuck on pilot jet and he was referring to pilot air jet. Pilot jets get richer with larger values and pilot air jets get leaner with larger values. Part of my confusion was the fact that Suzuki doesn't publish pilot air jet values for later models. Probably due to emissions regs. As long as the stock pilot air jet is still intact if the low speed mixture can't be corrected with the mixture screws you change pilot jet size, that's if the carbs are clean and mixture screws are in good shape..
 
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Just browsing through his site (first link), I'd like to add a thought.
He says to adjust the pilot circuit first, before the needle and main circuits. I can see his reasoning. There IS some overlap effect between circuits. I won't say what the exact order should be. Most actually say you must jet the main circuit right first, then the needle, then the pilot circuit. I used to try getting the main right first, but then found that getting the needle perfect and then going with the largest main I could without causing any full throttle roll on problems was just easier and quicker in most cases. I always set the pilot circuit to a "ballpark" guess first but actually re-adjust/fine tune it last. But that's what I choose. It works for me. When the throttle is opened beyond about 1/4, the amount of fuel being drawn through the pilot circuit is so minimal it doesn't really effect the other circuits. Many believe the cut-away and jet needle take over much sooner than 1/4. Depends what charts/sites you believe.
His last photo of the carb "air screw" isn't correct. Any screw "after" the throttle slide/plates would be a mixture screw or pilot fuel screw. Any air screw located here would amount to an intake leak. I only mention this because his site is meant to clarify and simplify. His true air screw location was not labeled/identified. It's in the other photo, on the filter side, part of the "diagonal" chamber going to the unseen pilot air jet inside the carb throat.
 
KEITH KRAUSE said:
Just browsing through his site (first link), I'd like to add a thought.
He says to adjust the pilot circuit first, before the needle and main circuits. I can see his reasoning. There IS some overlap effect between circuits. I won't say what the exact order should be. Most actually say you must jet the main circuit right first, then the needle, then the pilot circuit. I used to try getting the main right first, but then found that getting the needle perfect and then going with the largest main I could without causing any full throttle roll on problems was just easier and quicker in most cases. I always set the pilot circuit to a "ballpark" guess first but actually re-adjust/fine tune it last. But that's what I choose. It works for me. When the throttle is opened beyond about 1/4, the amount of fuel being drawn through the pilot circuit is so minimal it doesn't really effect the other circuits. Many believe the cut-away and jet needle take over much sooner than 1/4. Depends what charts/sites you believe.
His last photo of the carb "air screw" isn't correct. Any screw "after" the throttle slide/plates would be a mixture screw or pilot fuel screw. Any air screw located here would amount to an intake leak. I only mention this because his site is meant to clarify and simplify. His true air screw location was not labeled/identified. It's in the other photo, on the filter side, part of the "diagonal" chamber going to the unseen pilot air jet inside the carb throat.
Click to expand...
I've always done the same, get the main right then go from there.
 
I agree with getting a main jet that is close, then getting some "drivability" with the pilot and needle circuits. Then fine tune each range of each circuit (pilot- idle, needle and emulsion tube- mid range, mains- WOT).
Keeping in mind that each circuit DOES have an effect on wide open throttle.
The needles circuit IS effected with pilot jetting (air screws, fuel screws, air jets, fuel jets- whatever your carbs have).
Its just that easy!!hahaha
I have a prostock dragracing snowmobile in which I make 218 horsepower with an 800 cc engine- the entire "tune" of the carbs has to be right or at the least it wont make 218hp and at the worst will melt expensive custom pistons and stuff.
Attention to every detail is key to a good tune= best performance.
When everything is right, Its a beautiful thing!

ECR
 
Due to a little tunnel vision on my part I wasn't getting through to another forum regular. If you have the right carbs on your bike and there are issues with the pilot circuit you would be best served by changing pilot jets, not pilot air jets. By changing the air jets you would just be opening another can of worms.
 
Deesel said:
Keeping in mind that each circuit DOES have an effect on wide open throttle.
Click to expand...
I do agree that there's some overlap between circuits at specific throttle positions but at wide open throttle, the jet needles positioning will not have any effect on how much fuel flows through the main jet. Maybe at the moment of roll on from, say, 1/3 or 1/2 to wide open, it will take a moment to burn off any overlap effect, but beyond that, the flow will be completely regulated by the size of the main jet for as long as the throttle is held wide open.
The smallest opening to fuel flow will always be the regulator.
 
I used to agree with you Keith Krause, but I have seen the size of the emulsion tube effect the flow characteristics of the main jet. This was on some TMX flat slide Mikunis.
Maybe needle jet and jet needle dont effect WOT on Suzuki GS carbs.
But I wouldnt take it for granted that it doesnt.
ECR
 
A site that I've always found useful in slogging my way through jetting issues, can be found at http://factorypro.com/tech/carbtun.html

Another site for basic info. and tuning tips (you just have to ignore the Harley-oriented slant to things :-s ), is http://www.mikuni.com/fs-tuning_guide.html

There's also a useful diagram showing which jets are primarily supplying the fuel as the RPM rise, at http://www.mikunioz.com/jets.htm -- scroll towards the end of the page. OK, the info. is for some pretty amazing Mikuni flatslides, etc. but the principles are the same.

Mike.
 
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Deesel said:
I used to agree with you Keith Krause, but I have seen the size of the emulsion tube effect the flow characteristics of the main jet. This was on some TMX flat slide Mikunis.
Maybe needle jet and jet needle dont effect WOT on Suzuki GS carbs.
But I wouldnt take it for granted that it doesnt.
ECR
Click to expand...
OK. I'm not arguing either.
But it's still the smallest hole that will regulate flow. The jet needle, on any carb I've seen, only regulates to approx' 3/4 throttle. After that, the space between the tapered jet needle and the needle jet is greater/a larger area and at that point (except for small overlap) the smaller main jet hole will regulate completely. Sustained full throttle is completely regulated by the main.
It's the only way a correctly jetted would work well.
 
Thanks for the thread Billy Ricks. We can presume that the issue is resolved, can we not?
 
dtkid said:
Thanks for the thread Billy Ricks. We can presume that the issue is resolved, can we not?
Click to expand...
Yup, I had a little senior moment there didn't I? The reason I deleted the post in the thread that got this started just after I wrote it is because I knew I was wrong then. Then when you PM'd me about it I got quite fixated on pilot jets when you were asking about the pilot air jets.

The guy was trying to go leaner if I remember right. What would have probably been the first thing for him to consider is new mixture screws but that's all hindsight now.

In short, you were right and I had tunnel vision.
 
Your a good man Billy Ricks. Now I have to go home and push my 1100 back UP the freaken' mountain.

I thought I would sneak off to work this morning without waking the kids so I rolled out of the drive-way down the mountain to "pop" clutch. But to my dismay.........no fire!!!!

I had just replaced the igniter with a used one and now I fear that the used igniter is a "used-up" igniter. Fortunately, I keep my feathers numbered for just such an emergency. Got another used one somewhere.

Have a good day.
 
Keith, There are VERY VERY few bikes that the jet needle loses some effect at wide open throttle. MOST bikes are adjusted (part of the tuning process) to NOT ALLOW the jet needle to completely come out of the needle jet. This is a safety issue. If you have ever seen the effect of when a jet needle comes out of the needle jet, it can then move laterally a tad and not allow the slides to close. That is why there is a screw stop on Mikuni VM series carbs for limiting the amount the slide can raise to allow the jet needle to stay in the needle jet just a fraction... the way it is set is to open the slide manually and adjust the stop bolt so that the slide stops even with the top of the venturi. It is an important adjustment for safety sake.

Thus, the jet needle is still having a slight effect at wide open throttle. In addition, the needle jet on most bikes will be larger (the orifice) than the main jet. The MAIN JET is the limiter as far as gas flow at wide open throttle.

Most carbs have three circuits... pilot, mid-range (jet needle/needle jet) and main jet. ALL flow air/fuel all the time... the main jet and jet needle share an air jet located on the intake venturi edge. The pilot jet does as well. In addition, the slide cut out or gap between the slide and bottom of the venturi allows air through to add to the air fed by the pilot air jet, also on the venturi edge adjacent to the main air jet. Some carbs (CV type) put air jets in the slide diaphram top as well...

I generally agree that you should probably tune the main jet FIRST if you are tuning but you have to get the bike idling and away first so that usually necessitates a bit of tinkering with the pilot system PRIOR to main jet tuning. The pilot jet is a critical tuning component and should be based not only on the fixed diameter pilot air jet but the slide cut out... bigger air jet and/or bigger slide cut out mean you really need a bigger pilot jet. The earliest GS750 VM26s were a dandy example! Look at the pilot air jet and slide cut out and you will soon figure out why those carb assemblies came from the factory with #25 pilot jets. Those buggers are nasty to tune for idle. later, Suzi spec'd smaller air jets and cut outs and subsequently over a few years ended up with #15 pilot jets which are far easier to tune. Try sticking #15s in the early carbs and it is so lean it won't start, much less idle.

The pilot circuit affects not only idle, but off idle transition. If the bike is too lean in the pilot circuit, you might be able to rev the engine with the bike on its centerstand but when you try and take off, the bike will just fall on its face... Tune the pilot circuit a bit first... get the bike going and get the right main jets and jet needle clip position, then bring it back in and sort out the idle would be the ideal, I think... Anyway, this is one of the more interesting discussions...
 
With all respect George, I know how the carbs work.:)
But I don't understand how anyone can confuse such a simple thing as the fuel flow going through a main jet and then through a jet needle/needle jet space.
The smallest hole in the main circuit will COMPLETELY regulate fuel flow at full throttle. Once the jet needle raises to approx' 3/4 throttle position, the space between the needle and needle jet simply becomes larger than the hole of the main jet. The main will now control flow regardless of how much higher the needle raises. I don't see where that's hard to understand.
I DO agree that the pilot circuit will add a little mixture to what the main circuit is supplying to the cylinder. But it's feeding the carb through a separate passage. At full throttle, the vacuum isn't acting on the pilot circuit as it would with closed/nearly closed throttle plates, but I'm sure there's still fuel some entering the carb throat. There's SOME vacuum present at the pilot circuit.
The jet needle and main jet are "in-line". And as I said, at some point, the jet needle has to give way to the main jet. Completely. That's the whole purpose of the design. That's why there are needles and mains, and not just needles. A tapered needle rising out of a space will create a larger and larger hole until that hole becomes larger than the main jet. Smallest hole regulates.
If anyone can show me how a jet needle will effect the main as in the above condition, let me/us know.
But if this is some silly miniscule amount and is just for the sake of some scientific argument, please don't make jetting any harder than it can be. In the real world, it wouldn't matter.
 
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