Edelbrock carb tuning

[c1500sbc]

Any Edelbrock gurus out there? I can’t seem to get rid of an off idle/acceleration lean on my 383. Edelbrock 750

Engine pulls right at 9-10” vacuum at idle. Fairly large camshaft. I’m running the stock jets with 70/37 rods and the pink 7” springs. I changed the accelerator pump to the .043 nozzle and I’m running the linkage in the bottom hole. It still stumbles off idle a little, sometimes going to full “LEAN” message on the AFR gauge and the same for a 3rd gear full throttle pull. It’ll go full “LEAN” on the gauge on initial acceleration before catching up and ending up in the 12’s at the upper rpm range.

The carb is fairly new. The plugs are certainly full of soot indicating it’s running really rich, but I was running 75/42 jets up until today and the lightest 8” spring so I think the rod was coming out of the jet at idle due to the low vacuum of the engine.

I’m using the lighter springs just in an attempt to fix the lean stumble.

Any thoughts?

[F-BIRD'88]

When the plugs are carbon fouled it will stumble when ever you stab the throttle, no matter the carb setup..
Your advance curvevis not right at all, also..
When it stumbles and missfires the AFR gauge shows lean...
New plugs, correct type and heat range.
Fix the advance curve.. Go back to stock default carb jetting.
71x47 metering rod .113 (pri) .107 (sec) main jets .025 shooter. 4" spring

Suggest a 10 degree advance curve 26 idle 36 max mech WOT.

What plugs type are correct for those heads?

What is your local elevation?

[c1500sbc]

When the plugs are carbon fouled it will stumble when ever you stab the throttle, no matter the carb setup..
Your advance curvevis not right at all, also..
When it stumbles and missfires the AFR gauge shows lean...
New plugs, correct type and heat range.
Fix the advance curve.. Go back to stock default carb jetting.
70x47 metering rod .113 (pri) .107 (sec) main jets .025 shooter. 4" spring

Suggest a 10 degree advance curve 26 idle 36 max mech WOT.

What plugs type are correct for those heads?

What is your local elevation?

I guess I just figured if the plugs are fouled it would be rich not lean? My advance curve is almost exactly what you described. I was able to limit mechanical to 8 degrees and I have idle set at 26 degrees to be safe.

.750 gasket set plugs are recommended.

I live in the Leander/Austin tx. About 500-1000 ft above sea level,

[F-BIRD'88]

Ignition stumble missfire shows as Lean on the AFR guage.
So your spark timing is 26 idle and 34 total max WOT now..

Get some new plugs .
6 heat range in NGK or 12 heat range in Champion.
Gap at .035"
Autolite #3924 Champion RC12YC ¾" reach 14mm ⅝ hex

[rebelrouser]

On your metering springs, I never go by the rated opening, with the engine warmed up and idling, loosen the cover for the metering piston, and move it to one side. slowly move the throttle, and watch, you want the pistons to rise as you move the throttle. simply stretch, a little at a time until, that happens. Or of course if the pistons jump up at idle, before the throttle is moved, they are too stiff.
I have been drilling and taping the idle air bleeds for a Holley air bleed jet, and getting big improvements in idle quality and part throttle response. Stock air bleeds are around .040, and most mild cams seem to like around .030 bleeds. And most of the time, I drill the accelerator squirters, a little bigger as well. And while the hole you put the pump arm in, is an adjustment, that can fix a lean stumble. You have to understand how the accelerator pump works as well. You can not compress a liquid, so when you smack the throttle wide open, you have to have some give, or you would just bend the linkage. The spring that sits on top of the accelerator piston, is the biggest tuning on the pump shot. When the linkage is at full closed, the spring compresses, and as that pressure pushes down on the plunger, is what determines to a large extent the volume of the pump shot. So again many times, a simple stretch of that spring to make it stiffer, will fix a lean off idle stumble.

[F-BIRD'88]

With that cam it will not respond from real low rpm.
It wants rpm.. Its not on the cam till 3000+/- rpm.
EG: if it was a auto trans car it would want a 3000-3500 rpm stall..

Keep that in mind when evaluating the throttle responce from low rpm..
Verify thevjets are stock default .113 pri .107 sec..
If the sec venturi cluster is the infamous restricted one as per TUNER, You need to correct that FIRST..
or you will never get the WOT jetting correct.
(It will tend to want huge rich primary jetting because the secondary never gets rich enough reguardless of sec main jet)
Not all the 750's have this fault but some do..

If your final best jetting metering rod combo is not close to stock default something is wrong... EG restricted seconday cluster ..

[F-BIRD'88]

You don't change the jets or rods to fix idle off idle tip in part throttle low speed tuning.
Initial throttle response issues are not the jetting.
Fouled plugs will never have sharp clean response and rarely recover.. Get new Start overwith all stock default carb metering.

If yiu want best low speed throttle response take the carb spacer off or get a 4 hole spacer or a combo spacer with divider.. Low speed response is best when the plenum is fully divided.. A open spacer can add a bit of top end power way up in the rpm range 5000+ rpm but the trade off is low speed throttle sharpness. You do 90% of street driving at low speed small throttle openings.. Especially critical on heavy vehicle with manual trans.

[c1500sbc]

Thank y’all for helping me. Can you explain why a worn out plug would come up as a lean spot on an AFR gauge rather that a rich condition? Wouldn’t there be a bunch of unburnt fuel being pushed out the motor down the exhaust?

Can you explain what to look for in the secondary Venturi cluster and what I can look for to see if it’s the restricted design?

I’ll definitely get some new plugs, and I’ll check the metering spring operation as described. I need to read more on transition circuits and how the all operate together. I wish the this was an AVS carb and I could adjust the secondary opening rate.

I’ve really thought hard about buying an EFI system. Holley sniper or the like. Reliability is a concern though. A custom carb is definitely an option too, but may be a little pricey for just a street driven truck.

[rebelrouser]

The way I understand it is the 02 measures un burned oxygen in the exhaust. Perfect combustion leaves little oxygen in the exhaust stream. When you have a missfire, the oxygen that should have been burnt in that cylinder, goes out the tail pipe, 02 picks that up as a lean condition. Notice we get maximum C02 at stoichiometric, 14.5 and very little unburned oxygen at max power 12.5

[c1500sbc]

The way I understand it is the 02 measures un burned oxygen in the exhaust. Perfect combustion leaves little oxygen in the exhaust stream. When you have a missfire, the oxygen that should have been burnt in that cylinder, goes out the tail pipe, 02 picks that up as a lean condition. Notice we get maximum C02 at stoichiometric, 14.5 and very little unburned oxygen at max power 12.5

Ahh, gotcha! My thinking was that it measures unburned fuel but in reality it’s measuring unburned oxygen...makes sense now.

Do y’all would recommend a resistor spark plug rather that a non resistor?

[NormS]

With the amount of intake manifold vacuum you have , I suspect that the primary butterflies are too far open at idle, exposing too much of the transition slots beneath them.This can make the idle mixture too rich, even with the mixture screws turned way in, and can cause an off idle stumble. You may need to drill some small air holes in the butterflies, so that you can get those primaries closed down, so that at idle, only about .030" of the bottom of the slots are showing below the butterfly's edges. This will give you more control over idle mixture, and can help eliminate your off idle stumble. I suggest starting with small holes drilled in the secondary butterflies, 1 in each, around 1/16" to 3/32". Check your primary butterfly position after drilling the holes, and enlarge the holes in small steps, if the primary butterflies are still too far open. You should make all your idle speed and idle mixture adjustments with the engine fully warmed up, and with your trans in gear, if you have an automatic. Norm/CFS

[Steve K]

Try moving the pump shot rod to the top hole, kinda lengths the time of the pump shot. I did this on my Tunnel Ram cars and it fixed a tip in stumble. Might work on your engine.

[c1500sbc]

I’ll check the transfer slot. Is it optimal to have none showing at all? I pulled all the plugs, and I don’t think that’s my issue. With better lighting, even the number one plug looks decent. I think it was some bad lighting and bad picture initially. Check these out. They are ngk heat range 6 but per gapped to .032. I will open that up a little bit, clean them up and I’m going to just reinstall them. I don’t see a reason to replace, unless y’all think otherwise?

[c1500sbc]

Took off the top of the carb again and figured out the jets have been changed. They are .107 primary and .101 secondary. Maybe this is why the tuning rods I thought would work aren’t. I believe as firebird88 pointed out the factory jets are .113 and .107.

I’ll change these back to stock and see what happens.

[F-BIRD'88]

Yup the jetting is way off.. Go back to default.
Get new plugs... Get resistor type plugs.. .035" gap.

71x47 rod 113/107 jets 4" spring .035 shooter.
Don't drill any holes in the throttles. Its not needed.
Now that the idle timing is 26 deg, the throttle transition slot exposure will be right.. (PCV connected and working)