Air fuel ratio?

So when I bought my xj750 with pods and full exhaust, it ran nicely. It slowly got worse and I rebuilt the carbs but never got it running perfectly before I put it into storage.

a month ago I borrowed my buddy's wideband O2 and found out it was way too lean (no wonder motor gets so hot).

Now the bike is running like a dream aside from it smelling a little rich at idle.

What air fuel ratio do you guys prefer? I love highway riding but I do occasionally get stuck at stop and go traffic for up to 30 minutes, hence it's tuned to 12.5 right now

 

dont take my word on it but at 12.5 i think it might be time for you to rejet your carbs instead of that many turns

 

yeh... I dont think they have that many turns out. surely its shorthand 12.5:1
sounds rich, but then I found this on a ducati forum:
"The air/fuel ratio that maximizes power has been shown to be about 12:1 or 12.5:1 and is about 20% richer than stoichiometric (14.7)."

curious on my ratio now

 

...for that engine design. XJ's make more power when running lean, and the best power when lean enough to hole a piston.

 

Isn't it odd that "stochiometric" just happens to be numerically the same as atmospheric pressure - 14.7 PSI at sea level.

Years ago, air-cooled Porsche Turbos ran 12.5:1 when on boost. Porsche knows best !!
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NEW MEMBERS - - Your bikes need brakes and tires before you ride.

 

Found this....

Bosch state that most spark ignition engines develop their maximum power at air/fuel ratios of 12.5:1 - 14:1, maximum fuel economy at 16.2:1 - 17.6:1, and good load transitions from about 11:1 - 12.5:1. However, in practical applications, engine air/fuel ratios at maximum power are often richer than the quoted 12.5:1, especially in forced induction engines where the excess fuel is used to cool combustion and so prevent detonation.

 

12.5 average or each cylinder, is that idle or under load or mid-range or red line.
What size jets are you using

 

As i understand it the a/f mix and jetting is specific to whatever pods and or exhaust is on the bike. And that it's difficult to dial in each carb because the intake air with pods is too great and erratic for cv carbs and why adding the cut down boots help choke down the air making it more constant which help with getting the correct mix in each chamber to produce max hp which help with syncing (to have all cylinders produce max equal hp for a specific model). Am i not understanding something?

Gary H.

 

Hi Gary, I've re-read your comment a few times and it's confusing, but sounds right, so I'll paraphrase the whole thing to make sure we're all (myself including) on the right page.

We all aim to get the perfect stochiometric ratio, in each carb, across all conditions. It's impossible but we get close.
intake mods and alternate exhausts can upset the factory-set ratio, so we need to re-jet to fix this. (more air means we need more petrol to get to the right ratio)
Pods supply an erratic disturbed airflow compared to the factory airbox, which can make jet adjustments more difficult.
Adding a cut down boot can help smooth down the airflow like a velocity stack is designed to. This makes jet selection easier.
Using a ratio of 12.5:1 looks rich (to me and maybe k-moe) but it works well for Rd337

 

14.7:1 is going to give WHITE plug insulators indicating too much heat for an XJ, but just right for a Toyota Corolla.

If you listen carefully to an idling car engine, you will hear the idle change every 4 seconds, as the computer rocks a little richer/leaner than 14.7:1 for the purpose of keeping the catalytic converter burning clean. And if stuff is burning in the converter - - it's not burning in the cylinders, and it's a waste of fuel all-the-time.

 

I apologize for the confusion. Chacal has a nice write-up about this in the Information Overload thread. It's a much better explanation than I can give. Hope this helps.

Gary H.