Edelbrock 650 AVS2 going super lean at part throttle

[Tuner]

Your OP says 625 Ededbrock AVS2, but now you flipped to a 670 Holley street avenger. To quote R. Lee Ermey, "What is your major malfunction?"

[travis]

Your OP says 625 Ededbrock AVS2, but now you flipped to a 670 Holley street avenger. To quote R. Lee Ermey, "What is your major malfunction?"

I have both. The Holley is sitting on the shelf, the edelbrock is on the truck

[F-BIRD'88]

Recurve the diz for 16 initial and 32 total. 16+16=32

Not unusual for your cam pick to want up to 24 ° base initial ignition timimg.. 8° is lame lame on this.

No wonder its lame at low speed.. Advance the cam to 106 in c/L too.. Not familiar with the Stealth 351w intake but if not full plenum divide its an issue.

Ford Duraspark II ignition has a built in cranking start retard function . (Its the white wire)
Use it.. A plenum heated manifold makes all the difference in the world.. Thats why every oem carbed intake ever has plenum heat.. Fix that.

There is nothing wrong with that carb.

[F-BIRD'88]

The smooth plenum floor on the stealth (especially no heat) is a mishap.. The sbc eddy performer suffers the same thingie. All the well developed Street friendly 4 bbl intakes have a agressive rib/ X or what ever pattern on the plenum floor AND plenum heat too.

[Tuner]

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

Sorry you have 'pressing matters', we all know how life does that.

Your OP says 625 Ededbrock AVS2, but now you flipped to a 670 Holley street avenger. To quote R. Lee Ermey, "What is your major malfunction?"

I have both. The Holley is sitting on the shelf, the edelbrock is on the truck

So, the Edelbrock is the subject and this means perhaps we will revisit this thread to sort it out in the future?

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

You are not the first or the only to realize for street use the OEM 351 intake is superior to the aftermarket. The OEM intakes are engineered with A/F distribution as a primary consideration.

If you can use exhaust heat it is the right thing to do. If you have experienced the same engine with and without exhaust heated intake, isn't it amazing how much leaner and smoother it will run at a steady state cruise and part throttle. Whoever buys your gas will thank you for the improved fuel economy.

[travis]

Unfortunately there is no way to run exhaust heat...there is no crossover ports in these heads. And I don’t remember now, but I don’t think there is a crossover in the intake either. I’m in Texas...does it really need the exhaust crossover most of the time anyway?

Is there a way to quickly and temporarily shorten the advance curve on an GM style aftermarket HEI? This truck originally came with a Duraspark III system...which is a weird California only system for this era of truck (thus the switch to HEI).

[F-BIRD'88]

A low profile machine screw installed in the advance plate below the weights limits the advance travel. The diameter of the head of the screw adjusts the adv travel limit.
Not hard to try out.

The machine screw head has to be low enough to not snag the advance weights.
Yes the stealth intake has ex heat. you could plumb it for hot water. The smooth plenum floor is a bummer. IMHO.
A hot water jacketed carb spacer may help.
Don't know anything about the duraspark iii box.
Somebody smart will.
A non heated intake will definatly be hungry for generous initial base spark timing.

[rebelrouser]

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.

You said you had the lightest spring, what is the vacuum of the engine when it is giving you problems? If you want it to richen quickly at load, you need a stiffer spring. Vacuum pulls the piston down, against spring pressure. I set my springs the old dog way, let it idle, loosen the cover plate, hold you finger over the hole so the spring does not fly out and slowly play with the throttle. I simply stretch the spring until when I move the throttle the piston starts to rise. Of course you want it to remain seated at idle, so if it has a big cam you have to split the difference.

[F-BIRD'88]

When fuel in the manifold does vapourize it must absorb heat to do that. this cools th

[F-BIRD'88]

When fuel in the manifold does vapourize it must absorb heat to do that. this cools things. Without added heat (plenum heat) vapourization quality and quanity suffers as you drive along at part throttle . Poor vapourization = poor engine performance. Then you need more spark advance and richer carb Off idle low speed cruise setup to compensate.
Plenum heat is important.
The plenum cools off pretty fast when at WOT cause so much fuel is vapourizing . Part of the reason WOT AFR hass to be richer than 14.7:1.

[F-BIRD'88]

Some Aluminum heads can be drilled fromthe intake flange toa exhaust port bowl to add a exhaust heat path to the intake manifolds under plenum heat passage.

[travis]

I finally got to play with this a bit today, and made some positive improvements.

I slightly tightened up the secondary air valve, which helped a bit by itself. I also put a much more aggressive curve in the distributor...it’s still 8* initial, but now it’s 32* total all in by 2200, instead of 3000...plus another 15* vacuum advance on manifold vacuum. More tweaks of the idle mix screws and idle speed added another 1-1.5” vacuum at 700 rpm idle, so now it’s 18-18.5” vacuum at idle. Idle afr is 12.6:1 now...but with these timing settings this seems to be what it likes.

I tried 16* initial...it flat will not start with that much initial when at operating temps. Is there any way to do a start retard with an HEI?

Drivability is definitely better now, and it lights the right rear tire up easier than ever.

[F-BIRD'88]

The 4 PIN 80's version of the GM HEI module has a retard function. Its either 5° or 10° retard.. You can also use a interupt switch while hot cranking... I bet the starter motor is weak too. Clean all connections. Starter motor needs a heat shield added.

[F-BIRD'88]

MSD did make 2 different model timing control boxes (with dash control dial) that did work directly with HEI. They can be wired to add a switched cranking timing retard too.
No MSD 6 CD box required.
Out of production but good used will do.
#8682 piggybacks (module in) #8782 "Blue Ribbon" deletes the gm module (its own internal inductive amplifier is included).
Mallory 818-1 is another option.

You need to address the cold manifold with smooth plenum floor.

[F-BIRD'88]

The reason you don't see these simple timing controls for oem ignitions eg: HEI is they may be considered as emissions control device tampering. Not worth bringing to market in the U.S. even thou they'd be popular with hotrodders.