Air/Fuel ratio

[truck]

I don't want to clutter up the A/F board with an simple question.
The EPA says 14.7:1 but the tuners try for 13:1. Why is that?

[fxstdavew]

14.7 is optimum for emissions and fuel economy and 13:1 -13.5:1 is ideal for max power. This is the simple answer someone else may want to go in to a whole essay but this is the quick and short of it.

[wolf_59]

Wide open throttle 13:1 -13.5:1 is ideal for max power.
Part throttle/cruise 14.7 is optimum for emissions and fuel economy
from what I have learned on this board 14.2 part throttle and 13.5 WOT

[truck]

Here's what I've got, slightly rich, but that's OK as far as not harming the motor, right?

[attachment removed after 60 days by system]

[PoorUB]

Here's what I've got, slightly rich, but that's OK as far as not harming the motor, right?

I hope you did not pay for that tune! It looks like a "before" pull!
You really need to get it tuned. 12-1 AFR is very rich. I doubt the engine is even burning all the fuel. Any excess could be washing down the cylinder walls and diluting the oil.
Personally I would shoot for 14.5 at cruise, and 13.5 about ~3500 RPM and over 30 % throttle position. You will get decent cruise MPG, and still have power when you need it.

[rking1550]

 

all though I'm no expert, I do think thats too rich especially if it's that rich at cruise, or partial throttle positions.
mine was lean at a WOT run , but still rich at a cruise (part throttle )
so what did the tuner say about your cruise  A/F ratio ?
it is possible to be ok at cruise and still be rich on a wot run. which IMO would be ok, because your not wide open all the time. but WTFDIK.  good luck

[attachment removed after 60 days by system]

[truck]

I normally run at ~2,200 to 3,000 rpm which is about 65mph. When I get into the western states with higher speed limits I could cruise @  35-3,700 rpm, but that's not common.
It was explained to me that the richness was from the accelerator pump, but at cruise the mixture is OK and should give me good mileage.
Then I got to thinking, I don't cruise at 4,000 rpm or more.
I don't know the ins and outs of tuning and what is good and what isn't, so any advice is appreciated.
So far, I don't like the idea of the gas washing down the cylinder wall.

[surf]

Truck, Did the tuner keep the sportster needle in there?

[truck]

He didn't touch the needle as far as I know. Still has the Sporster needle.

[Sonny S.]

N65C needle is not known for good MPG.....but they sure are popular   

[mayor]

I hope you did not pay for that tune! It looks like a "before" pull!
You really need to get it tuned. 12-1 AFR is very rich. I doubt the engine is even burning all the fuel.

easy there...he's running a carb , not an efi.    For a CV carb, that afr looks fine.  No worry's. Most will target 13.5 for wide open throttle and it looks like your around 13 (after 4k).  You can probably pull the 185 main out for a 180 and still be fine....or you can just leave it go and not worry.  It's fine..   keep in mind though, that's only wide open throttle afr....

[PoorUB]

I hope you did not pay for that tune! It looks like a "before" pull!
You really need to get it tuned. 12-1 AFR is very rich. I doubt the engine is even burning all the fuel.

easy there...he's running a carb , not an efi.    For a CV carb, that afr looks fine.  No worry's. Most will target 13.5 for wide open throttle and it looks like your around 13 (after 4k).  You can probably pull the 185 main out for a 180 and still be fine....or you can just leave it go and not worry.  It's fine..   keep in mind though, that's only wide open throttle afr....

I will give on this one, did not know it was a carb. It could be the accelerator pump shot showing up too. It would be nice to know what part throttle AFR is at cruise.

[Jeffd]

I would go one size smaller on the main jet.  It will let you know if it is too lean in cruise mode. 

[truck]

I would go one size smaller on the main jet.  It will let you know if it is too lean in cruise mode.

How will it let me know, and what is one sizes smaller than 185?

[mayor]

I would go one size smaller on the main jet.  It will let you know if it is too lean in cruise mode.

How will it let me know, and what is one sizes smaller than 185?

one size smaller is a 180 main.  The bike will pop (burp...fart...what ever you want to call it) at light throttle indicating that it is lean or it will feel like it's surging.  The wide open throttle is controlled my the main, but the cruise is controlled by the needle (..and Sonny and I would argue the main as well).  If you find the bike is indicating that the cruise is lean after switching to a 180 main (27114-88), you can switch the needle out (to a NOKK needle) or you can use a #4 brass washer to shim under the needle to richen the cruise up a pinch....

[truck]

Thanks for the education.

[hrdtail78]

I wouldn't blame that richness on accel pump.  If you are starting the run at 2000 by 2500 accel fuel is way past being burnt up.  Can always set the brake for 2000, hold WOT, and start recording which will release the brake.

Type of needle will do this, pipes will do this, needle placement.  On mild biulds, the slide isn't open all the way untill around 3500-4000.  So I read the graph as the MJ isn't metering fuel during your rich condition.  The needle is.  I would drop the needle and check cruise A/F.

A smaller metering device will pull more fuel than a larger one if the signal is stronger.  CV carbs are made to keep the velocity higher at partial slide openings.  This is where I agree w/ Sonny's comment about the sportster needle.  They usually dump to much fuel in the lower rpm ranges for WOT runs.

[dynaglide]

back to the original question:
gas and air molecules combine during combustion, but it's not a 1-for-1 exact match.  14.7 AFR is the ratio at which every gas molecule hooks up with air, but some air will be left over.  A richer AFR provides more gas, which allows every air molecule to combine with gas, but will leave some gas left over.
For EPA purposes, no left-over gas is good, left-over air is OK.
For max power purposes, you want every bit of air you can get in the cylinder to work for you during combustion, and we don't care as much about left-over gas in the exhaust.

[mokeypoole]

one size smaller is a 180 main.  The bike will pop (burp...fart...what ever you want to call it) at light throttle indicating that it is lean or it will feel like it's surging.  The wide open throttle is controlled my the main, but the cruise is controlled by the needle (..and Sonny and I would argue the main as well).  If you find the bike is indicating that the cruise is lean after switching to a 180 main (27114-88), you can switch the needle out (to a NOKK needle) or you can use a #4 brass washer to shim under the needle to richen the cruise up a pinch....


So you guys are saying that the main jet (larger jet) has an effect on the mixture at idle or low throttle?
I thought the mixture screw controlled the idle mixture and the low speed jet controlled the mixture at low rpms.  I also thought the high speed jet only controlled the mixture at close to WOT. 

If someone can set me straight on this, it may explain why my bike runs as it is.  when rejetting the carb for an air cleaner, I left the stock high speed jet in it.  I get that cold nature effect described here. 

Should I go up one size on the main jet too?  I had always heard it was usually ok to leave as is. 

[mayor]

So you guys are saying that the main jet (larger jet) has an effect on the mixture at idle or low throttle?
I thought the mixture screw controlled the idle mixture and the low speed jet controlled the mixture at low rpms.  I also thought the high speed jet only controlled the mixture at close to WOT. 

main jet at idle- no
main jet at low throttle- no
main jet at cruise- I say yes....some say no

needle at idle- no
needle at low throttle - yes (needle size, shape, and height affects afr)

pilot at idle- yes

just to be clear, this is based on a CV carb....Mik's are different

most popping that I've experienced occurs at the transition from pilot to needle.  This is usually under light cruise (say 2-2.2k).  Needle length and shape will have a big affect on this.  Assuming that the pilot is right and the main is right....  Try shimming the needle one #4 brass washer to see if that helps...are you sure you have the right size pilot jet?....have you adjusted the pilot to the bike (meaning adjusted the idle a/f screw based on lean/rich...split the difference...I generally go towards the richer side of that difference)?   

...I had the plugs on one of my bikes  black from being rich and still couldn't tune a pop out, a needle switch on that bike was the only thing that worked.

[Jeffd]

most all circuts overlap to some degree.  I don't know squat in this arena but I have experimented over 1000's of miles and I have done the steady cruise at 50mph for miles on end with different main jets and I have found in my experience that main jet does effect low throttle position ie 1/8 or so.  i have my throttle marked in 1/8 increments.  I think air fuel adjustment has more influence than given credit.