# Air/Fuel Ratio

Hey guys,

I have an Innovate LM-1 air/fuel ratio meter that I use for tuning my boat and thought I might give it a try on my YZ450F. I got a piece of 1 3/4 exhaust pipe and welded in a bung for the O2 sensor then slipped it on the tail pipe and fired her up. My question is.... What A/F ratio should these engines be? I'm not sure that I have everything with this rig all lined out but it is reading about 17.8:1 at idle. That seems awfully lean to me. I run my boat motor (supercharged 383) at aprox. 12.5:1 which is a little on the rich (safe) side. Has anyone here take actual A/F reading on their bikes?

Sounds like dilution to me. On a single cylinder engine, I'd suggest the pick up point be at least 12" into the exhaust.

seems like its 14:7 to 1 . if i remember correctly . i think gray is right the o2 needs to be moved in closer.

Sounds like dilution to me. On a single cylinder engine, I'd suggest the pick up point be at least 12" into the exhaust.

At least...the farther the better. Tdub

I think you may be right. I am going to add some pipe past the sensor and see if that helps. I don't want to weld a bung into the head pipe if I can avoid it.

seems like its 14:7 to 1 . if i remember correctly . i think gray is right the o2 needs to be moved in closer.
14.7 is held to be the stoichiometric ratio, that at which the fuel being burned is completely consumed in the reaction, and maximum efficiency is achieved. Maximum power actually is developed at ratios of more like 13.5-14.0:1.
14.7 is held to be the stoichiometric ratio, that at which the fuel being burned is completely consumed in the reaction, and maximum efficiency is achieved. Maximum power actually is developed at ratios of more like 13.5-14.0:1.

I concur(not that I needed to)Tdub

We had a probe that went a foot into the mufler on a 610 husky and it showed a major lean spot showing at 4500rpm but the bike did not respond to richer jetting. When a longer, flexible copper probe was inserted that went in past the collector it was determined that the bike was actually rich in this zone. The o2 sensor gave accurate readings everwhere but in the area we were having problems. These big singles have big pulses that go in both directions drawing fresh air into the exhaust in certain rpm ranges. You need to get the o2 sensor past the collector if you want an accurate reading at all rpms. In my experience the bikes run best around 13.0. Engines reading 13.5 or slightly leaner may make slightly more power on a dyno but they seem to run best around 13.0 While14.7 provides the cleanest burn with the least amount of polutants in the exhaust but in my experience carburated bikes run terrible when set up that lean. Steve

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I concur(not that I needed to)Tdub
But I always welcome your affirmation, and others should, too. And please speak up whenever you think I'm wrong (even if I dispute it).

14.7 is held to be the stoichiometric ratio, that at which the fuel being burned is completely consumed in the reaction, and maximum efficiency is achieved. Maximum power actually is developed at ratios of more like 13.5-14.0:1.

I'm not sure what the unit of measurement for these ratios are. When I was in school, we talked mass ratios of fuel:air and 16:1 was the most power you could achieve, with 13:1 being the most economical. Anything in between is a compromise, and anything outside of that range is a poorly running motor....

I'm not sure what the unit of measurement for these ratios are. When I was in school, we talked mass ratios of fuel:air and 16:1 was the most power you could achieve, with 13:1 being the most economical. Anything in between is a compromise, and anything outside of that range is a poorly running motor....
Think about what you have said here: 13:1 is more economical than 16:1

That says that an air/fuel charge containing 13 parts air to each part fuel produces better economy than a leaner one with 16 parts air to each part fuel. It's backwards.

14.7 is ideal for efficiency, and max power occurs somewhere in the mid 13:1 area. Max efficiency ostensibly should produce maximum economy, but in practice, engines can be run leaner than stoichiometric for the sake of economy and still operate well. In fact, modern port fuel injection and engine control systems can run a car engine at ratios near 17:1 at a cruise and produce ridiculous steady state fuel consumption numbers, like 27 mpg at a steady 65 mph on flat ground in a 5.7L Corvette.

I added some additional pipe and curved it down. It seemed to help when running up in the rpm range, idle still reads lean unless you add some back pressure to the end of the pipe. It reads 12.5:1 at high rpm with no load, how much different would that be on a dyno (loaded)? I'm just goofing off right now, I don't entend to make any jet changes based on these readings unless I'm sure they are accurate.

It reads 12.5:1 at high rpm with no load, how much different would that be on a dyno (loaded)?

Completely different. The only reading you'll get that means anything as far as the main jet is full throttle, under a load. If your meter is portable, take it for a ride.

I need a 12vdc source to take it for a ride.

Think about what you have said here: 13:1 is more economical than 16:1

That says that an air/fuel charge containing 13 parts air to each part fuel produces better economy than a leaner one with 16 parts air to each part fuel. It's backwards.

14.7 is ideal for efficiency, and max power occurs somewhere in the mid 13:1 area. Max efficiency ostensibly should produce maximum economy, but in practice, engines can be run leaner than stoichiometric for the sake of economy and still operate well. In fact, modern port fuel injection and engine control systems can run a car engine at ratios near 17:1 at a cruise and produce ridiculous steady state fuel consumption numbers, like 27 mpg at a steady 65 mph on flat ground in a 5.7L Corvette.

Sorry, I should have put in bold (for the old guys) that I was referring to FUEL:Air, not air:FUEL......makes more sense now, doesn't it?

Are these readings actually going to have any meaning considering how the bike is used on a track ?

Sorry, I should have put in bold (for the old guys) that I was referring to FUEL:Air, not air:FUEL......makes more sense now, doesn't it?

No, because then it would be 16 parts fuel to one of air...kind of like carbonated gasoline.

And watch the "old guy" stuff.

No, because then it would be 16 parts fuel to one of air...kind of like carbonated gasoline.

And watch the "old guy" stuff.

Carbonated gasoline, that's funny old gray guy...

No, because then it would be 16 parts fuel to one of air...kind of like carbonated gasoline.

And watch the "old guy" stuff.

Not when you're talking mass flowrates......trust me on this one, I won't steer you astray.

Edited: Now you've got me really thinking and questioning my own thoughts......I'll have to do a little research on this to see if I've twisted my brain somewhere in the years since college....