Enriching low end and leaning top end

JohnJL

Member
Hi all, long time reader, first time poster. I hope to contribute and benefit from the time here.

I am o2 logging my son's lo206 on a red slide and in certain conditions want to enrich the 2000 - 4000 rpm range, while leaning out the 4000 -6000 range. Could I;

1. lower the float level (less fuel in the bowl) and lower the needle clip or the opposite;

2. Raise the float level (more fuel in the bowl) and raise the needle clip?

Thanks!
 
I would think either of two options . Lower the fuel level and raise the needle .
Or raise the fuel and lower the needle .
 
Analytically, raising the float level enriches entire range.
Lowering float level leans entire range.

Since you want to richen low end, and lean out on top, raising the fuel level, and raising the clip seems like the right answer. However, that is predicated on throttle position matching rpm, which is likely not the case.

Lowering float level likely leans the top end more than low end. So, lowering the clip richens top end back up.

Now, you need to get the correct mixture @ whatever rpm helps net the lowest lap times.

Richer mixture generally helps torque at the expense of top end power.
Where does the engine spend most of it's time during a lap of your track?
Enhancing that part of the lap will most likely overcome the difference made during the shortest portion of the lap.

As flattop stated, either could work.
Stop watch will tell the story.
 
I would think either of two options . Lower the fuel level and raise the needle .
Or raise the fuel and lower the needle .
I would think one of these would be right and one of these would be wrong? I have limited runs tomorrow and want to test other things so hoping to get guidance to shorten my cycles.
 
Analytically, raising the float level enriches entire range.
Lowering float level leans entire range.

Since you want to richen low end, and lean out on top, raising the fuel level, and raising the clip seems like the right answer. However, that is predicated on throttle position matching rpm, which is likely not the case.

Lowering float level likely leans the top end more than low end. So, lowering the clip richens top end back up.

Now, you need to get the correct mixture @ whatever rpm helps net the lowest lap times.

Richer mixture generally helps torque at the expense of top end power.
Where does the engine spend most of it's time during a lap of your track?
Enhancing that part of the lap will most likely overcome the difference made during the shortest portion of the lap.

As flattop stated, either could work.
Stop watch will tell the story.
Thanks Shaw. My track is technical with a 900m kinked straight that generally gives lowest lap times when hitting the rev limiter 3/4 the way down the straight. On say a 20 (~75 F?) degree day with 94% air density on .885 float height the 2nd clip from the top gives ~14 afr up to 4000 rpm and then 12.5 AFR up to 6000 rpm. I am trying to reverse that AFR curve so its richer down low and leaner up top. Tomorrow's practice and race is on an unusual layout emphasizing low speed corners and acceleration.

I have the O2 logger on and have left the float level and clip at a known baseline to see how the track's changed layout will look. From there I am hoping someone says "oh, just lower the float height (lower the fuel in the bowl) and lower the clip to richen up the low end and lean the top end .

Thanks to all of you for your help!
 
Considering what Mr . Shaw stated and the added facts presented . I would be richer mid range let the high be where it ends up .
Hopefully Brian Carlson chimes in on this .
 
Not sure I can contribute much more than what's already been shared -- O2 sensors can give you some great data, but interpreting that data can be somewhat of a challenge. 12.5 afr from 4000 to 6000 would be the "prescribed" target afr for a gas engine, BUT you may be leaving power on the table by tuning by that alone. As you suggested, for a more technical course, you want more fuel on bottom end to help with torque. Personally, I'd sacrifice a bit on the top end by richening the entire carb. Even if your AFR goes rich across the board, I think you'll find better lap times. I have spent entire days testing on practice days only to find myself back at my original bench settings on the carb. Not all the testing was a waste of time, mind you, but the data can sometimes get your head in the matrix instead of focused on the reality of simply laying down better lap times.

I'd say you are very close on tune with your current settings. We've had good success with .900" height and 3rd clip settings too. Be sure to reset your air-bleed each time you make these adjustments. Having a couple extra sets of pre-set floats and needles already made up will help the tuning day go quicker/easier.


-----
🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
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www.CarlsonMotorsports.com
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Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
One other influence that could effect the results would be throttle positioning during braking and acceleration events.
 
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