Hoping to get some guidance / opinions from some experienced carbureted LS folks on how to fix my lean tip-in / stumble off idle. My issue is that when I am cruising or at a stop, and punch the throttle, I see a lean spike for about 1-2 seconds where the AFR jumps to between 17-20:1 and then power comes on and the engine runs great. Here are the details on the setup:

2000' S10 with a 5.3 (L33), 799 heads, BTR Truck Norris Cam, MSD-6014, High Rise Single Plane (similar to Super Vic), Brawler 650CFM DP, using a wideband O2 and the MSD Box to read vacuum. Timing is at 26deg below 3000RPM and 32 deg above 3000RPM. Vacuum seems to be fine, between 12-15inHg idle & cruising, and 0inHg at wide open throttle. Running round (heat-sink) coils, and spark plug readings look great. AFRs usually stay between 12-15:1.

Here is what I have tried to do to fix it, with zero luck so far:

- Swapped accelerator pump nozzles progressively with no change (31s, 35s, 37s, and now running 45s).
- Swapped (primary) power valve from 6.5inHg to 8.5inHg with no change.
- Swapped stock pink accel pump cams to Orange (primary) and Red (secondary).
- Zero lash in accelerator pump linkage.

I realize my intake manifold is probably not ideal for the engine / cam combo, but I'm a huge fan of the high-rise single plane and really hope to keep it. Any ideas would be greatly appreciated! Seems like this is a common issue for us LS carb guys if I cant figure it out I might just pull the trigger on an AED Carb, I've heard alot of success with these. Pic of the combo below:

 

You went the wrong way with the power valve.

https://www.holley.com/blog/post/how...ifold%20vacuum.

 

The 650 Brawler DP I have also had a severe lean spot at tip in.

The great thing about these carbs is everything is adjustable. I went down 6 sizes on the IABs for the primary side and this resolved it.

I also had to really lean the idle mixture as it idled very rich. Pretty common on Holley's out of the box.

My 650 is 1/2 turn out on the idle mix and I went down 2 sizes on the Idle mix jets.

It now runs well. Slightly lean on a cold morning until fully warmed up.

 

That is usually false-lean. The carb is giving the amount of fuel to have a good mix, but the burn is incomplete and unburnt air makes the O2 sensor read lean.

Basically, not something to worry about.

 

Thanks for the explanation! With a 20 mile drive to work it's fine!