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Air/fuel ratio

Old 08-08-2016, 05:51 PM
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Air/fuel ratio

Can someone explain what my air/fuel ratio should be across the length of the power band? (beyond 14.7 @ idle)


Am I correct in that my fuel screws are for idle/low range, pilot jets are for mid range and main jets for high range?


Thanks for your input in advance guys!
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Old 08-08-2016, 06:35 PM
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Here's a link to the Honda Common Service Manual. Lots of good background info on the function of the carbs in here.

https://www.dropbox.com/s/xz39vanvr6...anual.pdf?dl=0
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Old 08-09-2016, 10:06 AM
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Originally Posted by Cadbury64 View Post
Here's a link to the Honda Common Service Manual. Lots of good background info on the function of the carbs in here.

https://www.dropbox.com/s/xz39vanvr6...anual.pdf?dl=0
A lot of really good information there. Appreciate that one Cadbury. It explains well about the needle and the main jets role in detail. A good read.
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Old 08-10-2016, 03:27 PM
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So, after a bit of research, I still can't find exactly what I need.


I have a wideband meter for my air/fuel ratio. So, at idle I sit at 14.7(ish), but I want to know if my mains are correct. My research indicates the switch from pilots to mains occurs at 3000-3500 rpms, so what should my a/f ratio be at say, 4000? I have a flo commander I JUST installed, but I want to be able to get it spot on.


If my pilots were fine, but my mains were out, would it still be brown on the plugs?
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Old 08-10-2016, 07:31 PM
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My two cents. Roughly said, to 4k is pilot jet area, 4-7 needle, 7k above main jet.

BUT, everything is affecting each other various ways. Few examples,

- if you change pilot jet setting, it has effect at bottom, and slight effect above
- if you change needle position, whole range is affected, but mainly middle
- if you change mains, everything change

The goal is to balance all the changes to get as linear AFR as possible from 3k to redline. If you achieve to get the same AFR through the rev range, you will get the best power and torque characteristic.

AFR value is between 12.5-13.5, depends of what you manage to keep stable trough rev range, every change in afr results in torque drop.

14.7 at idle is ok.

Last edited by Stephan; 08-10-2016 at 07:34 PM.
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Old 08-11-2016, 02:10 AM
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I can't help but wonder what Mikes AFR is with the radical mods to his carbs ?
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Old 08-11-2016, 09:13 AM
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Originally Posted by Stephan View Post
My two cents. Roughly said, to 4k is pilot jet area, 4-7 needle, 7k above main jet.

BUT, everything is affecting each other various ways. Few examples,

- if you change pilot jet setting, it has effect at bottom, and slight effect above
- if you change needle position, whole range is affected, but mainly middle
- if you change mains, everything change

The goal is to balance all the changes to get as linear AFR as possible from 3k to redline. If you achieve to get the same AFR through the rev range, you will get the best power and torque characteristic.

AFR value is between 12.5-13.5, depends of what you manage to keep stable trough rev range, every change in afr results in torque drop.

14.7 at idle is ok.

I agree, a friend of mine said to shoot for 12.5-13 at full throttle....I'm at approx 10

I have a two brothers exhaust and a K\N filter.
I have 48 pilots and 180 main jets (guess I have to go up to my 190s)
and I think I need to go down on the needle because the fuel screw on the front is all the way in and still a little lean on idle.

I also have velocity stacks coming. I hear they will enrich the mixture?

Guess I'm going to be doing some tinkering this weekend....

Last edited by drgreenthumb04; 08-11-2016 at 09:15 AM.
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Old 08-11-2016, 09:45 AM
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If you're reading 10 parts air to 1 part fuel at full throttle you are too rich, and your main jets should be slightly smaller. Where is your sensor for the meter mounted? Right pipe or left? The left pipe tends to see more of the exhaust.


I have a little bit of a hard time believing that, as stock main jets are 175 front 178 rear and you are at 180's. If they are dyno jet 180's, some folks say those flow slightly more fuel than OEM jets of the same size.


13:1 air to fuel is supposed to be best ratio for maximum power.


With an A/F meter and knowledge of the different circuits, you are pretty well armed.


Toss the K&N, use a stock filter, and you will find stock jetting is pretty darn good. Save yourself 1,000 years and steer clear of all the jetting threads on here - they will lead you down many stray paths with very little benefit.
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Old 08-11-2016, 09:57 AM
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Originally Posted by AndyMX47 View Post
If you're reading 10 parts air to 1 part fuel at full throttle you are too rich, and your main jets should be slightly smaller. Where is your sensor for the meter mounted? Right pipe or left? The left pipe tends to see more of the exhaust.


I have a little bit of a hard time believing that, as stock main jets are 175 front 178 rear and you are at 180's. If they are dyno jet 180's, some folks say those flow slightly more fuel than OEM jets of the same size.


13:1 air to fuel is supposed to be best ratio for maximum power.


With an A/F meter and knowledge of the different circuits, you are pretty well armed.


Toss the K&N, use a stock filter, and you will find stock jetting is pretty darn good. Save yourself 1,000 years and steer clear of all the jetting threads on here - they will lead you down many stray paths with very little benefit.

My jet kit is a 6sigma. I told them what I was running and they sent the kit with 48 pilots and 180 mains as well as 190s if too lean still. As for my wide band meter, its an LM-2 digital meter, so I mounted it to the front carb for this run, but I set it up to be removable and interchangeable between the two pipes so I can read either carbs afr.

I probably never should have mentioned the K/N, ( I know its a sensitive subject) but for the intended purposes, it was necessary. I have a hi-flo filtro as well, but I prefer the K/N. Is 13:1 not a little lean? when does damage start to occur?

Last edited by drgreenthumb04; 08-11-2016 at 10:00 AM.
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Old 08-11-2016, 01:57 PM
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Okay, I missed this amazing write up and just found it....I get it now


www.vtr1000.org ? View topic - Carb Tuning guide (setting up a Dynojet or Factory Pro kit)
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Old 08-12-2016, 10:51 AM
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less than 14 to 1 is RICH (more fuel than needed) - i.e. 13:1, 12:1, 11:1, 10:1. Less air to 1 part Fuel. More than 14 to 1 is LEAN - i.e. 15 parts air to 1 part fuel. You have it backwards. Unless you know what brand/size of jets 6sigma is using - you could just be making guesses. 6sigma is one of those Chinese ebay groups, right?


Buy yourself some OEM jets from a reputable vendor - sudco, or even rockymountainatv will have them.
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Old 08-12-2016, 07:00 PM
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Originally Posted by AndyMX47 View Post
less than 14 to 1 is RICH (more fuel than needed) - i.e. 13:1, 12:1, 11:1, 10:1. Less air to 1 part Fuel. More than 14 to 1 is LEAN - i.e. 15 parts air to 1 part fuel. You have it backwards. Unless you know what brand/size of jets 6sigma is using - you could just be making guesses. 6sigma is one of those Chinese ebay groups, right?


Buy yourself some OEM jets from a reputable vendor - sudco, or even rockymountainatv will have them.

Your absolutely right, I am running rich (better than lean I suppose) and I did have it backwards

I don't think that 6sigma is a Chinese group? I could be wrong though. The kit seems fine. As I stated before it's 180 mains, 48 pilots. I've never done dyno runs or the sort, so reading running afr is new to me.
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Old 08-13-2016, 08:32 AM
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Just for reference my VTR with a K&N and dynojet kit runs 185 mains.

www.vtr1000.org ? View topic - K&N, Dynojet Kit, Micron (NFRU) Cans

Performance is fine but fuel consumption is awful.
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Old 08-13-2016, 09:58 AM
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Originally Posted by Wicky View Post
Just for reference my VTR with a K&N and dynojet kit runs 185 mains.

www.vtr1000.org ? View topic - K&N, Dynojet Kit, Micron (NFRU) Cans

Performance is fine but fuel consumption is awful.
I'm running rich with 180's. Hows your 185's? Whats your pilots?
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Old 08-14-2016, 07:16 AM
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Have been following this thread in the background.
I thought there is a difference in orifice size between (some) brands even though they have the same number.
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Old 08-15-2016, 08:45 AM
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The local dyno guys here in Charlotte say the Dyno Jet jets flow slightly more than OEM jets. I'm not sure exactly how they know that, or if they've measured it.
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Old 08-15-2016, 08:47 AM
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Where is your O2 sensor mounted? How far from the cylinder is the bung on the exhaust pipe? Can you just drop the O2 sensor into the left muffler and see what it reads?
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Old 08-15-2016, 10:50 AM
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Originally Posted by AndyMX47 View Post
Where is your O2 sensor mounted? How far from the cylinder is the bung on the exhaust pipe? Can you just drop the O2 sensor into the left muffler and see what it reads?
yes you can, but you will be reading a mix of cylinder gases. It works and i've done dozens of Dyno runs that way on the SH, and hundreds of dyno runs on other bikes that way.
Newer bikes we no longer use a stand alone sensor, but use the onboard ones.
I have a set of SH headers that have a bung in both pipes, 8" exactly away from the header end (head side) Allows me to check both cylinders with as fer variables as possible.
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Old 08-15-2016, 02:53 PM
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Originally Posted by E.Marquez View Post
yes you can, but you will be reading a mix of cylinder gases. It works and i've done dozens of Dyno runs that way on the SH, and hundreds of dyno runs on other bikes that way.
Newer bikes we no longer use a stand alone sensor, but use the onboard ones.
I have a set of SH headers that have a bung in both pipes, 8" exactly away from the header end (head side) Allows me to check both cylinders with as fer variables as possible.
That is how I also have mine set up, but I don't have any dyno experience. Just using the afr gauge makes the data hard to interpret what my afr should be at certain rpms at different throttle positions. There is computer software and data logging abilities with it. I will have to dig a little deeper and map out my findings for each carb.
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Old 08-15-2016, 02:55 PM
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Originally Posted by AndyMX47 View Post
Where is your O2 sensor mounted? How far from the cylinder is the bung on the exhaust pipe? Can you just drop the O2 sensor into the left muffler and see what it reads?
Just using the afr gauge makes the data hard to interpret what my afr should be at certain rpms at different throttle positions. There is computer software and data logging abilities with it. I will have to dig a little deeper and map out my findings for each carb.
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