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Questions about AF ratios

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Hi there guys. I am somewhat new to motorcycles but learning fast. I really enjoy all the discussions of carbs and mods and exhaust systems and jetting etc.

I have very much enjoyed reading about and looking at all sorts of dyno graphs depicting exhaust systems and mods that I am interested in.

I have been hearin and learning more about AF ratios and would like to know all that I can about it.

So here are a few questions:

What is a good AF ratio number(I read a post recently with someone talking about how his AF was 12.1 and he tuned to 12.7)?

Does the right AF ratio # depend on displacement, exhaust and mods of the engine?

Does a higher # mean a richer ratio? And a lower # number leaner?

On my drz400sm I intend on doing 3x3, JD jet kit, fcrmx 39 carb(is that a flat slide carb? Or are there slightly different versions of the fcr 39) and debating wheather or not i go with the mrd/ssw or yosh rs2 exhaust system. I will be riding street almost exclusively and will need to jet for sea level. Sorry for the long winded post :/

Edited by SupermotoAl

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A good af ratio would be 12.7 to 13/1 you can make more power at 14/1 but that is on the lean side.

Different mods change the af ratio.a pipe needs a richer mixture with a 3x3 mod.big bores like to be leaner. Stroker likes rich. Cams can be both. The only way to get it close to what you want is with a dyno with a afr readout

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without a bunch of equipment to actually read the ratio its a mute point .. the good thing is that these days many combinations of parts have been tested, we have the internet, and its a simple matter of copying someones setup to get very close ... in the old days you felt how it was running in the seat of your pants and read plugs to jet, you just have to have a basic understanding of which part of the throttle you are tuning and how it relates to the different circuits in the carb ie, low speed, mid, and high, coupled with response ... very generally for each of the 'sections' you tune over and under the peak power spot to locate it, then set it to slightly rich so you dont fry the motor .. alittle lean generally isnt safe especially in a motor that gets pushed ..

Edited by cowpie

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the good thing is that these days many combinations of parts have been tested, we have the internet, and its a simple matter of copying someones setup to get very close ...

 

I'm not sure that's a good thing. Now people don't want to take the time to verify, understand, or tinker with things until it's right. I found the settings recommended everywhere on the internet to be really far off for my DRZ. Sure my bike ran with the recommended settings, but it runs better, and makes more power now that I have gone away from those settings. I think people forget that those recommendations should be a starting point.

 

Not for a DRZ, but I have several numbers you can look at scattered throughout this directory, http://www.cdmfabrication.com/bbpics/bike/dyno/. It's from a discussion I was having with a few guys on the R6 Forum. In particular this is the HP vs AFR graph for 4 different fuel maps. Every bike / setup responds differently, so you really have to just expierment some and see what your bike likes. I found on my R6 it makes more power all the way down to 12.5:1, however I like the throttle repsonse of a leaner mixture (13:1 in this case). This ended up being a good compromise making only slightly less power, but having much better throttle response. The nice thing about fuel injection is I can tune in very specific areas, in each gear, which means I can move the AFR up or down as necessary through the RPM band to make more power. Also means I can lean out the cruising area in 6th to help fuel economy (not that my R6 gets good fuel mileage anyways).

 

My primary point is you need to do some testing to figure out what works best for your bike.

 

Back to the DRZ, below are two images of my air box, which everyone claimed would make my bike run lean. However I found that it was still really rich with the recommended jetting.

 

http://www.cdmfabrication.com/bbpics/bike/fcr21.jpg

 

http://www.cdmfabrication.com/bbpics/bike/fcr22.jpg

 

I'm on a 150 MJ, 38 PJ, 1/2 turn on PAJ, 200 MAJ, EMM Clip 4. The bike actually made more power on a 148 MJ, but that was as low as I felt safe testing, so I went back to the 150 just to make sure I was a hair on the rich side. Who knows it may have still made more power yet on an even smaller jet. However it was making smaller gains as I went down once I passed 150, so I think it's in a good spot. Also note with the setup above I don't really have any deccel popping (this went away after I got the PAJ adjusted).

 

Recommendations are good, but they should be a starting point.

Edited by OhioYJ

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