Edelbrock 650 AVS2 going super lean at part throttle

[travis]

I’m trying to figure out why this thing is so lean with a load on it at light part throttle acceleration, especially when going up a small hill at low speeds.

Cruise afr is 13.8-14.2 range, WOT is 12.5-12.7. But, light acceleration (especially going up hills) goes as lean as 16.4-17.5 range, and I can’t get it back into a “normal” range without going super heavy into the throttle. Light throttle acceleration is very weak.

I’ve got the lightest step up spring I had available in it (8”) which “should” have helped, but didn’t really seem to make any difference at all. I’ve tried running it 1-2 steps richer in cruise mode but that doesn’t seem to help either.

WOT performance is pretty good, but normal drivability is still lacking. I’ve got some lighter advance springs to try but am cautious with it going so lean. I don’t want it to ping with hypereutectic pistons.

[novadude]

So you are saying driveabilty is an issue at 16.4-17.5? If it's not bucking / surging, I'd be real happy with those numbers. A properly functioning carb should go a little leaner under light load increases (pulling hills, etc) according to classic text book examples of proper carb behavior. If it's going so lean that you can feel it surging, that is an issue, but otherwise I would not worry about trying to bring the gauge readings into some other definition of "normal".

Also, I suspect that under the conditions you are describing, the idle circuit is doing most of the heavy lifting, and that is probably why jet and metering rod changes to the main circuit do not have any impact at all.

[F-BIRD'88]

Open the idle mix screws ¼ turn and retest at that drive point.

[F-BIRD'88]

The "econimizer" (idle feed restriction) in the primary clusters may be blocked with dirt or grit.
Blow out primary air bleeds with compressed air.
Flushout with carb cleaner.
The IFR can be probed thru the first idle air bleed with a small wire.. It is behind the idle bleed in the cluster.

What throttle position % and what manifold vacuum point is this occuring at? What road RPM?

Is this a actual drivability problem (surging, etc) or just afr guage observation?

Possible exhaust gasket leak. (it really throws off that guage.)

[1980RS]

I’m trying to figure out why this thing is so lean with a load on it at light part throttle acceleration, especially when going up a small hill at low speeds.

Cruise afr is 13.8-14.2 range, WOT is 12.5-12.7. But, light acceleration (especially going up hills) goes as lean as 16.4-17.5 range, and I can’t get it back into a “normal” range without going super heavy into the throttle. Light throttle acceleration is very weak.

I’ve got the lightest step up spring I had available in it (8”) which “should” have helped, but didn’t really seem to make any difference at all. I’ve tried running it 1-2 steps richer in cruise mode but that doesn’t seem to help either.

WOT performance is pretty good, but normal drivability is still lacking. I’ve got some lighter advance springs to try but am cautious with it going so lean. I don’t want it to ping with hypereutectic pistons.

I can feel for you, I have 2 of the 600 performer carbs that did the same thing and just off idle both are horrible on an engine with a stock cam but ever where else they ran great, never did find the problem as I suspect it's in the boosters myself. When retested with an engine that had a hotter cam they were better, did not understand that. Both customers put different carburetors on their engine and both ran perfect after that.

[F-BIRD'88]

Possible the PCV valve is wrong calibration for this engine/carb. Temp block it and re-drive test.

[F-BIRD'88]

Possible the float level is a bit too low.
(late primary mains startup)

[F-BIRD'88]

Can you re create this part throttle lean spot by hand in neutral in the driveway with tach, vacuum guage and afr guage? If so, get an assistant to hand modulate the idle air bleeds and main air bleeds by small wire probe or... to see the afr effect at that hand throttle point..
Note the vac guage.. Do the other stuff first.

[travis]

No drivability issues, just weak at light throttle. No surging at all.

Carb has been gone through, floats corrected (I always check edelbrocks right out of the box, as 3/4 of them I’ve dealt with was off right ootb). I’m using the stock truck PCV setup, but I’ve tried it blocked off as well. No real difference there either.

It’s funny, do a search on here and find nothing. Make a post, and similar topics come up with potentially lots of answers lol.

Similar posts have me thinking I may be looking in the wrong place for a solution. I’m going to try quickening up the advance curve and see if that helps. I’m wondering if maybe it needs more advance in light throttle/higher vacuum situations.

[F-BIRD'88]

Idle vacuum and rpm is?
Idle base timing is?
Cam is?
Vac adv ported or full manifold?
Yes weak advance progression will give weak throttle response.
Idle mix screws are xx turns out at base idle?
Initial base throttle open position at idle is? (t slot exposure at idle)
If too far open or too far closed at idle,, off idle mix will not be right.. Adjust idle initial base timing +/- to get correct idle throttle base opening at idle.. (t slot exposure)

If cammed more base idle timing is better.

Dirt infection in primary clusters.
Does blocking the pcv effect afrs +/-at all?

[F-BIRD'88]

Lean afr at part throttle "weak at light part throttle"
Leaner afrs want MORE TIMING as leaner afrs burn slower.
Especially if cammed (egr dilution effect at low speeds)

[F-BIRD'88]

Do a off idle low speed light load cylinder balance test.
Disable one cylinders spark at a time, on the diz cap, note vacuum guage/tach rpm drop effect.. The weak cylinder will drop vac/rpm the least..
Vacuum guage, tach, afr guage.. + assistant to disable spark.. Insulated plyers (wood or other non conductive)

[Tuner]

I’m trying to figure out why this thing is so lean with a load on it at light part throttle acceleration, especially when going up a small hill at low speeds.

Cruise afr is 13.8-14.2 range, WOT is 12.5-12.7. But, light acceleration (especially going up hills) goes as lean as 16.4-17.5 range, and I can’t get it back into a “normal” range without going super heavy into the throttle. Light throttle acceleration is very weak.

I’ve got the lightest step up spring I had available in it (8”) which “should” have helped, but didn’t really seem to make any difference at all. I’ve tried running it 1-2 steps richer in cruise mode but that doesn’t seem to help either.

WOT performance is pretty good, but normal drivability is still lacking. I’ve got some lighter advance springs to try but am cautious with it going so lean. I don’t want it to ping with hypereutectic pistons.

If it doesn't surge and gets good mileage, leave it alone. A clue is how well it runs on a cold start and how long it takes to warm up until it runs good.

Does your intake have exhaust heat?

Do you have small drill bits or pin gauges to measure idle jets and air bleeds? What size are the idle jets at the tip of the small diameter tubes in the primary metering clusters that project down from the bottom of the cluster.

I recall a few years back going through this problem with an Edelbrock carb in this forum with someone who fixed it by installing 6-32 setscrews in the idle air bleeds in the clusters. I recall the cure was a smaller 1st AB.

There are two bleeds in the idle circuit. The idle fuel flow path is through the idle jet at the tip of the tube, up to the 1st IAB (the one in the side) where it mixes with air, then passes horizontally through the economizer, then mixes with more air from the 2nd IAB (the one in the top) and then goes down into the carb body to the T-slots and mixture screws.

As mentioned above, it is possible that raising float level will help.

A common problem I see in the edelbrock carbs is the idle jets and main air bleeds (the tube projecting up vertically from the cluster is the main air bleed) are not always the same size and sometimes odd sizes that make no sense, too big or too small.

[travis]

I’m not going to be able to get back to this anytime real soon, as other pressing matters came up.

But...to answer some of y’all’s questions...

.030” over 351w, 10:1 compression, Jones 264/268@.006 hydraulic roller (218/222@.050, .544/.544 lift, 112 lsa/109 ica), 180cc aluminum heads w/2.02-1.60 valves, 1 5/8” long tube headers, weiand stealth intake, etc. Timing is 8* initial, 32* total all in at 3000 rpms, plus 15* vacuum advance. Before anybody says it’s not enough timing, this is what it likes. It starts instantly, pulls 17” vacuum at a 700 rpm idle, and pulls cleanly to 5800. More timing makes it harder to start when hot and cuts top end power noticeably.

From dead cold, it starts instantly and will idle on it’s own within 1-2 minutes, with ambient temps anywhere from 50 to 90F. No exhaust heat in the intake.

I do not have any pin drill bits, but can get some.

I’ve got a 670 Holley street avenger carb, that runs stronger everywhere EXCEPT for this massive off idle lean spot that I have not been able to fix. From doing a bunch of research on this apparently common issue, it’s seems that the problem is something screwy with the primary metering block. I’ve got another block to try now...just haven’t had time to mess with it.

[travis]

Watching some dyno comparisons between a performer rpm and the stealth intake on a 351w similar to mine, it looks like the stealth is kind of a turd under 4500 rpms, so an intake change may be in order. I know on a very mild 351w, the stock iron 4bbl intake is far better at any usable rpm than the stealth