Carb Tuning - I think I am close!

[scottiec]

Hey Gang,

I purchased the Innovate LM-2 with the exhaust clamp to read my AFR. I want to share my results and make sure I adjust my carb in the right direction. Here are the specs on my motor and the results of my AFR readings.

Specs:
302 with GT40P heads, B303, QuickFuel 600cfm carb with vacuum secondaries. My initial timing is 8*. My total timing is 33*. I know my initial seems a little low, but I set it there because I was more concerning about my total timing being safe while tuning. I plan to raise my initial to 10 or 12* and put limiters in my dizzy to keep it between 32 and 34*.

AFR readings:
Cruise: perfect. between 14.5 and 15.0

Part Throttle: Was seeing a lean condition. Started by spiking to 20.0ish when rolling into it, then settling around 16.0 (I already went up 2 jet sizes before doing this test because part throttle was driving like crap)

Full throttle: Pulls great, but is rich at 11.7

What to do: I plan to go down two jet sizes on my secondaries to lean out the wide open throttle. Shooting for 12.5. For my part throttle, this is where I am a little confused. Should I go up in main jet size? Or accelerator pump?

Thanks guys!

[Itchief]

Scottie

The cruise/part throttle seems lean to me I have my efi cruise set to about 13.4 and wot at 12.2


Rick

[scottiec]

For my part throttle I need to determine if the initial lean spike is caused by not enough accelerator pump or main jet size

[Driver_WT]

Cruise AFR looks bang on to me.

[johnnybgoode]

Good thread here that might help you. Scott
http://www.ffcars.com/forums/17-fact...g-101-a-2.html

[edwardb]

Good thread here that might help you. Scott
http://www.ffcars.com/forums/17-fact...g-101-a-2.html

Agree 100%. Wayne's Carb Tuning 101 thread is a great resource. This link starts on page 1: http://www.ffcars.com/forums/17-fact...ing-101-a.html. What this thread describes, and what I see missing from your work so far, is carb tuning is a methodical process that goes through each circuit in the proper sequence. This was a big point of understanding for me when I tried my hand at carb tuning. Each circuit builds on the previous. Swapping out main jets is near last in line. Your LM-2 (same thing I've used) makes the process pretty straightforward.

[scottiec]

Thank you guys! Much appreciated. I will be going through that thread.

[CraigS]

Both the spike to 20 and the settle to 16 are way lean. I'd maybe do the pump first mainly because you don't need to take much apart. But first verify that there is no play in the linkage. It must squirt w/ the tiniest throttle movement.
https://www.holley.com/blog/post/how...y_carburetors/
You may need a different cam too. but check which color is on there as some of them have two possible mounting positions which affect how much they squirt. The cams are cheap and common enough that you may be able to find them locally if there are any old school speed shops around. Also be aware that if you go up in nozzle size, w/ no other changes, you get a bigger squirt that doesn't last as long.