Carb Tune w/AFR Meter_Which O2 Sensor?

[djl]

FM head pipe with merge collector; front cylinder sensor located just in front of the collector and rear cylinder sensor located up near exhaust port.  I assume that if I was to check AFR with meter, I would work with the front cylinder sensor??

[speedzter]

No guarantee if the front will be richer or leaner than rear, but likely it will be different.
The correct way is to check both, and try to jet somewhere in the middle.
I wish my Innovate LM2 meter had the dual O2 sensor option fitted, but $$ !

[djl]

No guarantee if the front will be richer or leaner than rear, but likely it will be different. The correct way is to check both, and try to jet somewhere in the middle. I wish my Innovate LM2 meter had the dual O2 sensor option fitted, but $$ !

I figured that was the answer.  Not sure how to jet somewhere in the middle.  Seems like maybe jet for the rear since it typically runs leaner? Both should be sampled and decide which one to jet for based on the sampling.

[speedzter]

It's a compromise, and I've found it more pronounced when using the main jet circuit.
Jet the rear so it's just on the verge of being slightly lean (13.8 ), then check the front.

[HD/Wrench]

No guarantee if the front will be richer or leaner than rear, but likely it will be different.
The correct way is to check both, and try to jet somewhere in the middle.
I wish my Innovate LM2 meter had the dual O2 sensor option fitted, but $$ !

I run that same meter with the dual sensors on my race truck .. I have to say its a very nice item, I have the tach pickup up and EGT as well

[Y2KRKNG]

I'm running the AEM AFR meter and a single sensor in front pipe mounted down by my footboard on my TH. It's a blessing AND a curse! The readings seam to change some based on ambient and engine temp, or clean vs dirty AF, ect.
As far as frt vs rear, my rear S. plug always showed a bit more rich than the frt so id like to believe im good with going with that

Side Q, as far as tuning; could you share the quick, cheap, and dirty, on your realtime afr reading going down the road.
Mine;

Idle 13.8
5th gear cruising 13.8-14
WOT 11.7-12

I get real rich at certain rpms with a lite load, like 3-4th at 3-4k rpm lite to moderate load will go to 10.0 afr. I'm thinking that is from reversion causing a double charge of fuel

[speedzter]

Y2K , what carby are you using ?
I would pull back on your main jet at least one size.

[Y2KRKNG]

Mik45, 98 needle second from top, 175main, 50Acc pump.

I thought 12 was good on WOT for best power and heat

[speedzter]

Personally , I would usually try to target around 13.2  WOT.
The AFR readings do move around a lot when riding.
I've found wind direction will change things a lot, say between a head wind and a tail wind.
This is with my version of a "heavy breather" style air cleaner.

I've also experienced some rich "spikes" when using a Mik 45, usually around the transition
between the idle circuit and the needle (straight section)
Mostly cured with fine tuning the idle circuit, leaner needle, and needle position.
Just FYI, for me, the 45 seems to take more effort to tune correctly than the 42 or 48.

I also think your sensor placement may not be ideal.

[dynaglide]

  sort of...

I also see the "rich spike" that the Mik 45 tends to have at the needle transition, especially from the straight to the tapered part of the needle - drove me nuts trying to flatten it out.
I've found that if the needle size is correct (right size for cruise AFR on the straight part of the needle), the stock needle jet tends to makes the carb too rich when on the tapered part of the needle.
Also, the moving the clip up or down with the stock plastic washer results in AFR jumps of a point or more - not good for fine-tuning cruising AFR.  Using a combination of small metric metal washers (about .020" thickness) and/or the plastic washer (.030" +/-) allows you to tune to a more precise and flatter AFR. (Moving the needle up or down a bit DOES affect cruise AFR, even though the Mikuni tuning manual says it doesn't...)
I've found 12.8 or so to give me the best MPH at WOT at the track.

On my 95" w/Mik 45, I use a Y-4 needle jet (one leaner than the Y-6 it came with), 1x metal and the plastic washer under the clip in the center notch (.050" total), and a -97 needle.  This combination gives me the flattest AFR across the entire throttle range, as measure by my MTX-L wideband AFR gauge.

On my bike (YMMV), the front runs leaner than the rear.  Rather than trying to average AFRs between the two, I tune so that the front is "ideal", and the rear is whatever it is.  In my opinion, it's more important that the leaner cylinder doesn't run lean, so I don't worry about the richer cylinder.

[Y2KRKNG]

Lots of good info there I think I'll run to the hardware store and get some of those washers. I never considered changing the needle jet guess I should.

As for sensor placement the instructions said at least 18 inches from exhaust port and at least 36 inches if running turbo, race fuel,or extended high RPM running. I'm running the full length thunderheader and the sensor is approximately 18 inches from engine and 18 inches from collector basically in the middle of the front head pipe

[speedzter]

Using unleaded turbo blue ?

[Y2KRKNG]

Turbo,supercharger,race gas, Av gas,exhaust Temps over 800*, extended high rpms, etc

[Y2KRKNG]

Thanks for letting me hijack the end of this thread.  One more symptom i'm having is on a WOT run from say 3k rpm to redline my afr sets out at a decent 11.9-12.4, runs there up to about 4.5-5k then settles in at a rich 10.x till 6k

[HD/Wrench]

Drop the main then you may have to raise the needle to get you mid range back. BTW what is the light throttle cruise reading??   also how far are you running the pump stroke..

[Y2KRKNG]

Drop the main then you may have to raise the needle to get you mid range back. BTW what is the light throttle cruise reading??   also how far are you running the pump stroke..

Ok WOT is good now! im playing with the cruise now;165 main, 98 needle, stock needle jet, top notch with plastic and metal washers. This combo is almost there except for VERY lite throttle cruise, say like 1/16 throttle 50mph level ground no head wind im at high 15s afr. 65mph+ and its cool, low-mid 14s. I'm thinking ill move the c clip down 1 to the 2nd from top position and remove plastic washer leaving metal one.
Pump stroke is long due to the set screw falling out with the standard gap on the beginning of it.

[speedzter]

Your close. Try the 97 needle.
So are you using the unleaded version turbo  fuel ?, that was my question earlier.

[Y2KRKNG]

Your close. Try the 97 needle.
So are you using the unleaded version turbo  fuel ?, that was my question earlier.

I know I wasn't clear on the first post. I'm running just plain California 91. Used to run 20% 100ll Avgas but I don't want to f up my sensor anymore. It sure is good though and smelled good too.
I was referring to the sensor placement my instructions stated 18 inches from exhaust port for normal Motors but for anything extreme turbo race fuel extended high RPMs to run 36 inches from exhaust port.
I believe the 97 needle is more rich don't think that's what I need unless I change the needle jet also

[Y2KRKNG]

Oooooh! 97 dropped all the way down should give me a good lite cruise zone....hmmmm. I'll do it

[glens]

18 to 36 on a Harley, or is that perhaps in automotive terms?

[Y2KRKNG]

wasn't sure my self but they mentioned EGTs of over 800* needed to be further from ex port. The only negative I can think of being too far downstream would be fresh air influence from reversion messing with readings. Don't think this full length 2-1 will have a problem, but IDK.