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Engine performance troubleshooting help. Lean stumble when hovering just off idle.


Well-Known Member
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3:10 AM
Jul 15, 2017
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Nova Scotia, Canada
Hoping someone could offer some advice, I'm currently stumped. Thanks in advance.

I've provided a lot more engine config and setup info below for your reference. This is a new build, 3 months old; now have 3000kms on the engine. Note that I have an AEM x-series wideband o2 gauge and sensor installed. Engine idles great - right now set to be a little rich at 12-12.5 AFR. Under light and hard acceleration car pulls hard and performs very well.

The issue I'm having is when I apply a slight amount of pedal, I have a lean bog ... before I imagine the primary jets engage and all is okay. This lean bog shows up clearly on the AFR gauge - it'll jump from 12.5 to 15-18 and then go back to 11-13 under hard acceleration. This lean bog happens at a complete stop when moving through traffic or stop lights. It also happens when cruising at any speed and the throttle is at that slight open position. I want to be clear, when I stomp on the pedal, there is no bog - it responds great. Its when the throttle is "feathered" or hovering in that just slightly open position where it's lean and stumbles.

Something that confuses me is that at idle, I measure no vacuum from the manifold vacuum port (at the front of the carb, in the base plate). When I open the throttle, vacuum at this location jumps up and reads - 20+. Perhaps this is the cam profile doing this, not sure.

engine timing - initial at 15, total 30-32
Carb setup -
fuel bowls half way up the sight glass
four corner idle screws at 2.5 turns out
primary throttle blades set so I have almost a perfect square transfer slot showing
secondary throttle blades are adjusted from factory position to control idle (currently idles about 800rpm) any higher idle than that and I end up with dieseling when engine is heat soaked
primary jet - 80
power valve - 6.5
secondary jet - 88
accelerator pump (primary and secondary) are adjusted to near no slack; immediate engagement
accelerator pump cam - Red in there right now; have the kit of cams and have tried a variety of them - none of them affected the issue I'm having

Recent compression test on all cylinders is between 190 and 200psi
74 440 block, bored .030 over with 440source stroker complete kit to 505 cubic inch
10.547:1 compression ratio (link to calculation here with all engine measurements (Compression Ratio Calculator (Static and Dynamic))
Trickflow Powerport 240 CNC ported aluminum heads (TFS-61617802-C00)
Harland Sharp heavy duty adjustable roller rockers (S70015KE)
Trend Performance custom push rods; 9.200, .105 wall, 5/16"
Edelbrock Performer RPM dual plane aluminum intake
Xtreme Energy 242/248 hydraulic roller cam (Duration @ .050 (Int.Exh): 242/248, lift (Int.Exh): .549/.544, 110 lobe separation)
Edelbrock Performer RPM Street fuel pump
Holley Brawler carburetor, 850CFM
MSD 6AL2 Ignition control, MSD Blaster 2 coil, MSD wires, MSD Pro-Billet distributor (8546)
2.5 turns on all 4 mixture screws seem like a lot. could you be fouling your plugs idling too rich?
perhaps bigger accelerator pump shooters are needed to overcome what may be a lean stumble as you say?
The reason for the 2.5 turn out is probably because the IFR is too small, causing the lean condition on light throttle. Larger IFR, reduce the idle mixture screw turns which gives more fuel available in the transition. Could also reduce the IAB.

Make sure you have plenty initial ignition timing.
When you go off idle, it transitions to the main metering circuit. What is your AFR during this condition ( during the time the main metering circuit is supplying the fuel ) ?
When you go off idle, it transitions to the main metering circuit. What is your AFR during this condition ( during the time the main metering circuit is supplying the fuel ) ?

When I’m on the main circuit all is good, runs around 12 AFR. I have changed out the primary jets up and down and I can move the AFR around no problem there. Seems to me the problem is on the idle circuit but I’m not an expert at this.
I bet the sec blades are open too far. The engine needs a lot more idlke timing, maybe 30-35*. That will increase idle rpm [ turn dist while idling & see ], allowing the sec blades to be closed more. See below, the importance of idle timing....
Note that this timing advice comes from a carb book. Why would idle timing be mentioned in a carb book.....

Some good advice posted already. I agree that 2.5 turns out on the idle mixture screws seems like to much. All my carbs are set to near 1 turn out. Replacing the IFR's would be the first thing I would experiment with. Sometimes just by using the #2 setting on an accelerator pump cam will cure a lean blip like you have as well.
I also had this lean area at very slight throttle opening. A small spot of a very lean Afr that was constant if I held the throttle still.
....though no problem launching from a stand still or anywhere else for that matter.

The cause was the carb and intake combination (Classic holley and CH4B) that didnt move the air fast enough at that point in throttle.
I changed to a carb (Street demon 625) with smaller primaries and now the spot was rich instead. (Could not be tuned out.)
I changed intake (Eddy Performer) and the spot was gone.
Having to back the mixture screws out is an indicator idle jet/air bleed issues. Timing may be too slow. Bogs and flat spots can happen from high float levels/overly rich tuning as well as a lean condition. Tuning the accelerator pump action for street driving vs racing are two different things.
On the Holley Avenger series carbs, they have a "street" calibration, and I ended up enlarging the IFR (Idle Fuel Restrictor) to at least 0.030"+ (I think stock is 0.026 or 0.028"?)
12s on the AFR at idle is way too rich. You should find the sweet spot somewhere between 13.5-14.5 with your setup. The only time you should see anything in the 12s is at WOT. If you aren't cruising at mid 13s to mid 14s it's too rich at cruise also. We don't need/want a ton a fuel at a the lighter load cruise situations where vacuum is peaking.
Of course, always verify the gauge by inspecting the plugs.

Anyway, back on topic...as has been said the 2+ turns on the corner screws shows something is way off.
IMO you need to start from the beginning. Correct the idle timing. You shouldn't need crazy amounts like 30 or more with that cam and compression but I gurantee 15 isn't enough. Then work the idle, then the transition section (when the idle circuit still contributes to cruising) and then worry about the mains and WOT.
What's worked for me and other engines like yours I've tuned...set initial timing to 22-24 degrees to start. Set your secondary T-slots square, just like the primary side....don't trust any of the factory settings to be right for your specific combo. Closing the secondary T-slot to square and adding idle timing *should give you the desired idle speed.
Set all four corners at 1 to 1-1/4 turn out.
Start it up and when it's at a hot idle/operating temp give the primary pump arm a short, quick bump. If the idle speeds up, it wants a little more fuel from the corner adjustment. If it slows, it want a little less. Repeat for the secondary side.
I don't know about your exact carb, but mine is very sensitive here and responds to 1/16 turns of the screws, so take your time and go easy.

You may well need to get into changing the internal restrictions, getting a carb well-dialed usually does...(hopefully your model carb already has replaceable PVCR/IFR bleeds, if not it's easy to do with some brass set screws and small drill bits)....but you must correct the idle area first or you'll be chasing your tail.
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What is your idle speed?
What vacuum is it at idle?
What is vacuum at the lean spot? Idle rpm +200??
Vacuum at 1200 rpm?