Reversion?

[Restomod Z28]

I've had an off-idle stutter in my 10:1 383 SBC that has been present since I built the engine. I've tried tuning it out with timing, I've tried covering the "lean" spot with richer primaries (and pump shot and PV), and I've tried many combinations of both timing and fuel curve. While I have minimized it, still, my stutter remains. My wideband O2 will go from lean, to rich as this occurs.

I notice it occurring under slight load (going up a hill) when cruising between 2k and 2.4k RPM's. Once engine speed picks up, it is fine. I have a 230° @ 0.05" cam. Edelbrock RPM heads (70cc), RPM intake with the plenum divider notched in the middle, 1" open phenolic spacer, Holley 650HP carb, MSD pro billet distributor (18° initial, 18° mechanical in by 3k, 10° vacuum advance on manifold vacuum), Tremec TKO, and 3.73's. I've tried several carbs & an HEI setup and it still has the same problem.

I'll add that the vacuum is good - 13.5" at idle, 21" at cruise. I do notice that under light acceleration, the vacuum reading steadily & rapidly fluctuates +/- 1" of vacuum - which I'm thinking is the reversion. I just ordered a 1", 4-hole, spacer to replace my open spacer with the hope of boosting the signal over the boosters at low RPM's...

Short of putting in a cam with less overlap, is there a better solution? Any other ideas, or is a different spacer my only other hope?

EDIT: I should note that I'm running 1 5/8" primary headers, 2.5" exhaust w/ h-pipe, chambered mufflers out the rear...

[AM.MSCL]

Have you checked the Vacuum diaphragm?
I know Holley mentions this may need to be adjusted if you are experiencing a stutter or dead spot on acceleration.

[Restomod Z28]

On the vacuum secondaries? No, this is a mechanical secondary carb. The vacuum advance on the HEI and MSD both worked when tested (verified with dial back timing light at idle).

[Skip Fix]

The idle circuit on a Holley affects your AFR well into cruising and really affects tip in. I was having a similar issue , messing with squirters etc until I put my wide band on it. The convertor was tight enough that AFR was fine in neutral but once dropped into gear and loaded leaned to 16! So off idle was due to idle circuit being so lean, fattened it up and it went away.

[Restomod Z28]

Thanks Skip Fix. That's worth checking since I can't cover the lean spot with my primaries, PV, or Pump shot.

Interestingly enough, I had to close down the air bleeds 1 size because I was going lean at idle with the original Holley tune. It made a dramatic improvement. Perhaps I'll try going down another size with some spare air bleeds to richen up the mixture some more. Also, it seems like I'm either really lean or really rich with my primary jet choice - there is no in between.

[MrQuick]

what is your jets split front and rear? also are your plates drilled? 230 is pretty small for the standard idle holes Holley HP uses.

[Restomod Z28]

what is your jets split front and rear? also are your plates drilled? 230 is pretty small for the standard idle holes Holley HP uses.

67/76 jets. no holes.

[MrQuick]

just for fun, try 71's up front.

[Restomod Z28]

Thanks again for all the input. I put the 4-hole spacer on the engine and the stumble went away. I leaned out my primary jets and squirters and my a/f ratio looks great. The engine pulls well and I inadvertently shredded my tires at the first stop sign. I'm shocked that this solved the problem, but happy to finally have the stumble gone.

[MonzaRacer]

Yeah the 4 hole gave your boosters more signal to pull in more fuel. Most likely the emulsion holes in the metering block needed some opening up, not for a amateur.