Author Topic: Jet chart - The effect at different throttle openings  (Read 6433 times)

per_w_aberg

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Jet chart - The effect at different throttle openings
« on: July 14, 2014, 03:19:19 PM »
Sorry for starting yet another thread on jets and carbs but since it is a dear subject here I figure why not?

I'm so far of a stock Vision with pod filters an a free breathing exhaust that common knowledge hardly apply and the prospect of success seems slim but I'm willing to give it a shot!  That's why I would like to get my own basic understanding and build from there. Looking at common jet charts 2+3  there is a thorough understanding of mostly sliding throttle carbs. I started a chart of my own for the Vision, "jet chart 1", and here's where I would appreciate input and concerns. The chart aims to show where the jets come into play at different throttle openings. My chart is an estimate of what i think is going on. Could we get some talk going on this?

For starters, where does the pilot air jet come into play in pic 3?

Whats up with the main and pilot air jet being the same physical size and the same #number? Are they not interchangeable!? If not, how can you tell the difference?


QBS

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Re: Jet chart - The effect at different throttle openings
« Reply #1 on: July 14, 2014, 04:10:05 PM »
Regarding the last set of questions, air is less dense than gasoline.  Why the numbers are same, I do not know.

b_racuda

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Re: Jet chart - The effect at different throttle openings
« Reply #2 on: July 14, 2014, 04:12:33 PM »
This is close enough to this subject. I have now finished converting my 50hp Vision to 64hp version. So carbs, cams and fittings under carbs are changed. I have also repaired YICS, put new spark plugs and spark plug wires and new stator and now the bike is running quite nicely. But I wonder if the settings and jetting is just the right. So I asked my local shop if they can adjust my bike. They are doing it with the help of dynamometer. So first they are putting the bike to dynamometer with the stock settings and then taking the results. They told that with the result they can see what the jetting should be. I am planning to take my bike to there in the end of this week. I ask to give results from the dynamometer to me and I can then post them here.

Prophet Of Doom

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Re: Jet chart - The effect at different throttle openings
« Reply #3 on: July 15, 2014, 08:33:59 AM »
Just before I start, I'm no carb expert so please don't take this as authoritative, I may be partially or entirely wrong - just contributing to the discussion best I can.

Have a look at the carb cutaway from the (11U) Service guide Drawings EN

When the Throttle is closed you get fuel/air in two ways.  One is through the enricher (Choke) circuit (controlled by the 2 starter jets and the lever), the other through the pilot circuit.  The pilot circuit restricts incoming fuel with the pilot jet, and incoming air with the pilot air jet and then further restricts the volume of fuel/air with the pilot screw (which people call a mixture screw, but really it isn't, it's just a volume knob).  While these circuits remain active right through to wide open throttle (WOT) the effectiveness of the fuel coming out of the pilot outlet as a percentage will decrease - it's only a small hole.

As you start to crack the throttle open, you start to uncover the three (or is it four?) bypass holes.  These are fed by the same fuel/air mixture from the pilot circuit, but in stages till they are all uncovered. at perhaps 20% open (I haven't measured).   Again, these will remain active over the remainder of throttle, but as a rapidly lowering contribution as the main increases

The main circuit restricts incoming fuel with the main jet, and incoming air with the main air jet and then further restricts the mixture through the main breed pipe (emulsifying tube) the volume of fuel/air depends on air velocity past the venturi.  This will start as soon as the air velocity is enough to pull the fuel out - I don't know where this is. 

About this time, the acceleration pump will activate, providing raw fuel to aid transition between the pilot circuit, and the main circuit and compensating for the velocity lag.  Because  you can accellerate smoothly when doing it slowly, I think the transition point is probably about right, that the flat spot is caused by a lag in velocity not being compensated by the accellerator pump if the throttle is opened quickly.

I also think that the 130-135 pilot air jets in US models are too big.  This is compensated at idle by turning up the volume (pilot screw) but causing an over lean condition coming off idle.  I've tried both and had much better results with 120 pilot air

Mains are about right I think, I've not had any plug colour issues on long trips - not that there have been any for a while.

That's about it as far as I can tell.  So to answer your questions:
 
where does the pilot air jet come into play - partially from fully closed, increasingly effective from just open to 20-25% open, and then tailing off sharply - insignificant at 30%

Whats up with the main and pilot air jet being the same physical size and the same #number? There are only a few physical sizes of jets, so why would they make them different?  Why do you think they are the same number? Specs for your model are Main Jet 122.5F/127.5R, Pilot Air Jet. 120F/120R

Are they not interchangeable? Physically yes, but they are different sizes.  the sizes are written on them if you source them from Mikuni

QBS

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Re: Jet chart - The effect at different throttle openings
« Reply #4 on: July 15, 2014, 12:08:56 PM »
POD, your comments about the US models 130-135 air jets causing a lean condition off idle is enlightening and simple to check out.  Off idle flat spot has been one of my chief V complaints for many years.  I look forward to seeing what 120 air jets can do for me.  Thank you.

per_w_aberg

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Re: Jet chart - The effect at different throttle openings
« Reply #5 on: July 15, 2014, 05:54:18 PM »
Quote
where does the pilot air jet come into play? partially from fully closed, increasingly effective from just open to 20-25% open, and then tailing off sharply - insignificant at 30%

Whats up with the main and pilot air jet being the same physical size and the same #number? There are only a few physical sizes of jets, so why would they make them different?  Why do you think they are the same number? Specs for your model are Main Jet 122.5F/127.5R, Pilot Air Jet. 120F/120R

Are they not interchangeable? Physically yes, but they are different sizes.  the sizes are written on them if you source them from Mikuni

That's insightful writing POD and all is worth considering sorting this out. The thing is I read somewhere in ROV that air/main jets are not the same. But I think the are. See, I'm up to 130 mains now and ready to try it all for air and main jets in the range of 100-140. That's why I want to be able to swap mains and air jets regardless of what the are called.

I think its fair to put the air jet operational range at partly open throttle considering the pilot screws ability to compensate for its effect at idle.


Prophet Of Doom

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Re: Jet chart - The effect at different throttle openings
« Reply #6 on: July 16, 2014, 06:52:56 AM »
OK, except that the pilot screw throttles the pilot mixture, not just the air, so if you are going to start the pilot air from just open, then you should do the same for the pilot jet.

The mains and pilot jets are both Mukuni type N102 (I think from memory) this is a standard fitment and 100% interchangeable except that the jetting (hole size) is different.  I can understand larger mains as you are flowing more air, but why are you leaning the pilot circuit?  I'm not criticising, just interested in the rationale.

Rikugun

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Re: Jet chart - The effect at different throttle openings
« Reply #7 on: July 16, 2014, 07:01:04 AM »
This is a great discussion but I wish per started with a well sorted set of carbs. Then you would know what changes did what rather than wondering what part cleanliness of jets/passages, sync, etc. was playing.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

per_w_aberg

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Re: Jet chart - The effect at different throttle openings
« Reply #8 on: July 16, 2014, 07:16:25 AM »
POD: Good news on interchangable jets and god point on pilot jet register. I'm not leaning the air jets, they are stock 120's. I am going to reduce them and now I know what jet size to get. You mentioned air main jets, where about are they located? I guess the must be "hard coded" in the internal windings of the carb?

Rikugun: my carbs are ultra sonic cleaned, compressed air blown and kited whith repair kit parts as best I could. There is certainly room for errors and gremlins but I have a good foundation. i bet a couple of wrenching rounds in the carbs is called for. That and a good carb sync.

Prophet Of Doom

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Re: Jet chart - The effect at different throttle openings
« Reply #9 on: July 17, 2014, 04:31:58 AM »
You mentioned air main jets, where about are they located? I guess the must be "hard coded" in the internal windings of the carb?
It's a brass 1.8
« Last Edit: July 17, 2014, 04:38:41 AM by ProphetOfDoom »

Rikugun

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Re: Jet chart - The effect at different throttle openings
« Reply #10 on: July 17, 2014, 07:04:11 AM »
Rikugun: my carbs are ultra sonic cleaned, compressed air blown and kited whith repair kit parts as best I could. There is certainly room for errors and gremlins but I have a good foundation. i bet a couple of wrenching rounds in the carbs is called for. That and a good carb sync.
Were the cleaned and kited carbs proven with the stock air box? I'm glad to hear you got the rebuild kits. I'd think a new  accell pump diaphragm will be crucial to success with pods.
It is far better to grasp the universe as it really is then to persist in delusion, however satisfying or reassuring.  Carl Sagan

vl5150

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Re: Jet chart - The effect at different throttle openings
« Reply #11 on: July 30, 2014, 11:12:19 AM »
There's lots of information on tuning webers.  Here's a good example and it tells what jets do what at a particular RPM. http://www.scuderiatopolino.com/TuningofWebercarburetorsrev2.pdf

It's not 100% compatible with the Mikuni, but the principle is the same.

I have ALL the jets from 115 to 140 and tried them all.  I found that the flap is a response to having the US pilot jet adjustments blocked.  Weber carbs never had to use them.  So I think pods can work fine if the temps are stable and it's tuned for it.

From what I can tell the stumble is caused from the abrupt change in vacuum  when the butterfly valve opens and this affects the fuel delivery.  Normal slide/CV carbs have the needle valve to control this, but we go from pilot right to mains with the accl pump thrown in to complicate things.  Still, I'll take the Vision DD's over a bank of 4 Mikuni CV's any day.  The CV's diaphrams can get ripped and there's 16 screws just to get the bottoms off to clean the jets.  I can blow and change the vision mains in 30 seconds.
« Last Edit: July 30, 2014, 11:14:05 AM by vl5150 »

per_w_aberg

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Re: Jet chart - The effect at different throttle openings
« Reply #12 on: July 30, 2014, 05:30:13 PM »
Thanks for that link. I'm not sure what to make of my jetting. It stumbles, or rather runs weak at ~10-40% throttle at low rpm, better at higher with the same throttle. The air screws makes very little difference. It seems to run lean but there is soot in the exhaust. I responds as expected to choke indicating the mixture is not rich. The acc pump is squirting as expected. I would expect it to run lean considering the free flow exhaust and the pods but the soot tells another story. I'm really in the cold with this one. Thinking lean I found out that the 60 pilot jets are as large as the come and what I can tell it makes no difference if you up the pilot jets or down the air pilot jets, which of course is easier to change anyway. Talking to a really renowned Mikuni expert (30 years in business) made me puzzled, he says that the pilot screws are air screws and opening them make for a leaner mixture. That's the opposite of what I think. Anyway, there should be light once I start seriously tinkering with them.

What I wouldn't give for a CO2 meter! Anyone tried that? Are the around in peoples garages or shops? Had I just welded in nuts for lambda sensors I would be much better of I think.

supervision

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Re: Jet chart - The effect at different throttle openings
« Reply #13 on: July 30, 2014, 06:13:16 PM »
You are correct, on these carbs, lean is closed.
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Prophet Of Doom

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Re: Jet chart - The effect at different throttle openings
« Reply #14 on: July 31, 2014, 07:59:11 AM »
You Mikuni guy is right generally, but not for the BD34 - just look at the diagrams.

I suspect you will get soot with both a lean and a rich misfire - either way will result in unburned fuel out your tailpipe will it not?   I think just an o2 sensor won't tell between lean and a rich misfire as they both leave unburned O2.  Only a multi-gas analyser will help there.  For both methods you will need to drill your pipes as a sniffer won't work on a single muffler.

My bike used to be very stock, but now, like you I have all sorts of untuned mods including new carbs K&N filter and predator pipes, possibly cams.  Your post has got me thinking about how I'll tune my bike also - I thought I'd try something like this as I don't want to drill my stainless pipes for the EGA

Forget about idle, except to get the bike started and running 2.5 - 3 turns should do it.
Take off the airbox, and mark the throttle where the pilot bleed holes are just uncovered. Put it back together and get it out on the road. 
Take it up to the mark and note the speed.  The next run should be faster.  Then crack the enricher (Choke) just a small amount beyond your cable slack.  If the bike accelerates then it's lean -  go to the next lower number pilot air jet, if it slows or bogs then its rich go to the next higher number pilot air jet.
Repeat till the next run is slower, go back to the prior pilot air.

Now tune your idle with the pilot screw using vacuum gauges and shoot for highest vacuum and do sync at the same time

Now tune the accelerator pump.  Screw it tight a few turns so it has to be too much, then back it off a bit at a time till a smooth transition off idle is achieved.

Now tune mains.  Run up over 50% throttle and note the speed  Then crack the enricher (Choke) just a small amount beyond your cable slack.  If the bike accelerates then it's lean -  go to the next higher number main jet, if it slows or bogs then its rich go to the next lower number pilot air jet.


Does that sound like a good plan do you think?  Is that pretty much what you did with your choke?


per_w_aberg

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Re: Jet chart - The effect at different throttle openings
« Reply #15 on: July 31, 2014, 08:32:04 AM »
All is worth considering. The choke trick sounds like a good idea. Mind you I discarded the wire an operate the choke directly on the carbs for the sake of simplicity. Not so simple to operate though :) With only 2 screws holding the saddle and tank and a quick lock for the fuel line I will be able to get to the carbs in a minute so there is no excuse. I'm not so sure lean mixture gives high co2, I'll look in to that. Thanks for your support all!

QBS

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Re: Jet chart - The effect at different throttle openings
« Reply #16 on: July 31, 2014, 01:15:38 PM »
PoD, I love your plan for carb tuning.  It is very doable.  I especially like the way it uses carb enrichment to diagnose changes .  Well done.

Prophet Of Doom

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Re: Jet chart - The effect at different throttle openings
« Reply #17 on: July 31, 2014, 02:57:00 PM »
I was talking about O2 rather than CO2.  O2 sensors can be had for $100each and read with a multimeter, whereas my EGA is a $14,000 device (although I got it for $200 at a garage sale) :-)

CO2 will only increase with additional combustion, but 02 can be high, even increasing once you start misfiring.  The pictures you get in real life are nothing like the ones in the book because they typically omit those misfiring scenarios.

Can you wire up the choke just for tuning?  My bike needs 1/4 choke to start every time.  I wouldn't want to be without it.

per_w_aberg

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Re: Jet chart - The effect at different throttle openings
« Reply #18 on: July 31, 2014, 04:26:03 PM »
There will be some choke friction device for sure, now I need three hands to start the bike. Big news now is jet change #1. My shop only had the big Mikuni jets, same hole size for the jet number It seems and threads are the same so why not try file the down? That worked and now 100 pilot air jets makes the whole difference! I'm definitely  on the right track. It almost feels like a proper bike, no bog and a wee bit of hesitation until its up to temp. It's awesome!

Prophet Of Doom

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Re: Jet chart - The effect at different throttle openings
« Reply #19 on: August 01, 2014, 02:03:04 AM »
100 is a huge change.  That's 6 sizes richer that stock 12 sizes richer than stock US.
Not that I'm arguing against it, but how did you come up with that number?